Best Method to Slow Charge NiMH Batteries

Hi,
Since I created a NiMH battery pack using recently bought NiMH "C"
batteries, I am in search of knowledge for the best way to slow charge these
1500 MAH "C" cells in series. I prefer using a timer and a constant current
charger (I made) that supplies a constant 150MA.

There is a lot of information on the web, but one site says one thing,
and another says something else.

www.batteryprice.com/batterycareguide.aspx provided the kind of
information I was looking for:

(Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
charge). Note: Batteries "fully discharged" to 1V each cell.

In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
to me, but I wonder if anyone sees a flaw.

Thank You in advance, John

 

[mike]

On 12/23/2012 1:53 PM, jaugustine@verizon.net wrote:

Hi,
Since I created a NiMH battery pack using recently bought NiMH "C"
batteries, I am in search of knowledge for the best way to slow charge these
1500 MAH "C" cells in series. I prefer using a timer and a constant current
charger (I made) that supplies a constant 150MA.

There is a lot of information on the web, but one site says one thing,
and another says something else.

www.batteryprice.com/batterycareguide.aspx provided the kind of
information I was looking for:

(Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
charge). Note: Batteries "fully discharged" to 1V each cell.

In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
to me, but I wonder if anyone sees a flaw.

Thank You in advance, John


What does it say on the manufacturer's technical datasheet or charging

application note?

If they were NiCd, you'd have no problem.
The consensus appears to be that NiMH don't tolerate long-term overcharge.
Opinions vary on short-term overcharge. I expect you're doing about
the best you can do with simple schemes.

The difficulty is not in the equation. The difficulty is in determining
full discharge. How do you determine that?
Do you measure the voltage? under what conditions? OR do you just
charge it when the performance of your device drops? OR at random?
The range between 1.1V and 0.9V is huge depending on the load current
you use to test. And the device probably quits long before that.
I know because I have a computer hooked to a programmable power supply
and load fixture and I've tested it on a many different cells with widely
varying history.

One example application is the emergency flashlight.
You can't just charge it when it goes dark. Even in the best of conditions,
when it's starting to dim, you know you should charge it IMMEDIATELY...
but you've only got one more thing to do...so you do it.
More batteries are damaged from overdischarge than anything else.
For it's intended use, the emergency flashlight needs to be at FULL
charge all the time. So does your cell phone and your car battery and
your electric drill and...
Even low-drain applications are not immune. Do you want your alarm
clock to go dead at 4AM on the morning of your big presentation.

Bottom line is that you expect your device to perform for the entire
duration of the need...even when you have no idea how long that is.
That requirement often leads to systematic overcharge.

But:
I expect you're doing about the best you can do with simple schemes.

If you've done something like solder on the cells, you can expect
some variations in self-discharge and capacity. Some amount of overcharge
tends to keep them equalized.

Are we having fun yet?

 

[Jeff Liebermann]

On Sun, 23 Dec 2012 16:53:34 -0500, jaugustine@verizon.net wrote:

Hi,
Since I created a NiMH battery pack using recently bought NiMH "C"
batteries, I am in search of knowledge for the best way to slow charge these
1500 MAH "C" cells in series. I prefer using a timer and a constant current
charger (I made) that supplies a constant 150MA.

There is a lot of information on the web, but one site says one thing,
and another says something else.

www.batteryprice.com/batterycareguide.aspx provided the kind of
information I was looking for:

(Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
charge). Note: Batteries "fully discharged" to 1V each cell.

In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
to me, but I wonder if anyone sees a flaw.

Thank You in advance, John

In the dark ages, I've charged NiCd and NiMH batteries with a timer.
It's a good way *IF* you know the capacity of the battery and the
state of charge. Capacity is easy as it's in the spec sheet. However,
a given battery can have radically different capacity values at
different charge/discharge rates. To make the numbers look huge,
manufacturers use a slow discharge rate. The battery also has a
charging efficiency, where it takes more coulombs to charge the
battery, than it can deliver. I use 75% for most batteries. Once you
know all that, and the state of charge, you can calculate how long you
need to charge the battery. If the battery gets warm at the end,
you're overcharging and have probably killed the battery.

The catch is that if you forget to discharge to a known setpoint (i.e.
1V) you could easily overcharge the battery. Also, if you use a
timer, and the power fails during the charge, you could also charge
for longer than anticipated. Rather than go through this ordeal,
technology exists to detect the end of charge. You can get the chips
and make your own, or just buy a commercial NiMH charger.

More detail:
<http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride>
Note the comments on difficulties with trickle charging NiMH
batteries.
<http://www.buchmann.ca/article18-page1.asp>
(Note: Isador Buchmann is the founder of Cadex and author of the
BatteryUniversity.com web site).


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[William Sommerwerck]

In the dark ages, I've charged NiCd and NiMH batteries with a timer.
It's a good way *IF* you know the capacity of the battery and the
state of charge. Capacity is easy as it's in the spec sheet. However,
a given battery can have radically different capacity values at
different charge/discharge rates. To make the numbers look huge,
manufacturers use a slow discharge rate. The battery also has a
charging efficiency, where it takes more coulombs to charge the
battery, than it can deliver. I use 75% for most batteries. Once you
know all that, and the state of charge, you can calculate how long
you need to charge the battery. If the battery gets warm at the end,
you're overcharging and have probably killed the battery.

This is one of those rare cases where I have to disagree with Mr Lieberman.
How warm is warm? "Getting warm" is an indication that charging is complete,
or near-complete.

Many NiMH manufacturers claim their cells need to be "slammed" to get maximum
charge. I'm reluctant to do this. However, I've charged NiMH cells in those
"15-minute" chargers, and though the cells got hot, they were not destroyed.
(I no longer do this. I have about 20 cells of 2500mAh and higher capacity, so
I never need a quick charge.)

The best way to charge batteries is with a charger that lets you set the
charge rate, and watches for the signs that indicate the cell is charged, such
as the MAHA C9000. I generally charge at 0.3C or 0.4C, which is considered on
the low side for NiMH cells. Contrary to what Battery University claims, I've
never had problems with the cells overheating, or the charger failing to halt
at around 1.42 to 1.45 volts. (Note that his gripes are principally directed
at "consumer" chargers.)

The C9000 displays the battery voltage, so I can choose to stop charger at
whatever voltage I feel comfortable with. I'm not stupid enough to let any
charger operate without checking it occasionally. If you like, stick a timer
on the charger to shut it off.

 

[Jeff Liebermann]

On Mon, 24 Dec 2012 04:32:24 -0800, "William Sommerwerck"
<grizzledgeezer@comcast.net> wrote:

In the dark ages, I've charged NiCd and NiMH batteries with a timer.
It's a good way *IF* you know the capacity of the battery and the
state of charge. Capacity is easy as it's in the spec sheet. However,
a given battery can have radically different capacity values at
different charge/discharge rates. To make the numbers look huge,
manufacturers use a slow discharge rate. The battery also has a
charging efficiency, where it takes more coulombs to charge the
battery, than it can deliver. I use 75% for most batteries. Once you
know all that, and the state of charge, you can calculate how long
you need to charge the battery. If the battery gets warm at the end,
you're overcharging and have probably killed the battery.

This is one of those rare cases where I have to disagree with Mr Lieberman.
How warm is warm? "Getting warm" is an indication that charging is complete,
or near-complete.

That's easy. This is Figure 1:
<http://www.buchmann.ca/Article18_files/Figure1.gif>
from this article:
<http://www.buchmann.ca/article18-page1.asp>
The graph is for NiCd but NiMH is similar. I've also created my own
temperature curves for my experiments in extremely rapid NiCd
charging. Not that the temperature starts to rise at about 75% of
full charge and has risen 45C (113F) above ambient at 100%. That's
more than "getting warm". That's hot.

Many NiMH manufacturers claim their cells need to be "slammed" to get maximum
charge.

I believe I demonstrated this in the past. See:
<http://802.11junk.com/jeffl/NiMH/>
It seems that you have to charge cycle a NiMH battery about 3 times
before it will deliver full capacity. However, the difference is
small. For example:
<http://802.11junk.com/jeffl/NiMH/Duracelll-NiMH-2050.jpg>
shows that the first charge cycle results in a 1180mA-hr capacity,
while the third charge cycle increased it to 1360mA-hr or about 15%
increase. Is that what you mean by "slammed"?

I'm reluctant to do this. However, I've charged NiMH cells in those
"15-minute" chargers, and though the cells got hot, they were not destroyed.

My favorite NiMH charger is a Radio Shack 23-1305 30 minute NiMH
charger. The cells get warm at the end of charge. I haven't bothered
to measure the temperature. Again, if you look at the previous
article showing the temperature graph, the cell should be 113F at full
charge. Hopefully, the charger slows down the rate of charge before
reaching 100% or it probably will get that hot.

(I no longer do this. I have about 20 cells of 2500mAh and higher capacity, so
I never need a quick charge.)

I've noticed that my NiMH battery collection (flashlights, cameras,
GPS, radios, etc) spent much of their time in the charger simply
recovering from self discharge. So, I invested in some Sanyo Eneloop
batteries, which have a much lower self discharge rate. I took some
photos yesterday with a camera that has been bouncing around my car
for about 5 months. That battery indicator showed about 3/4 charge.
Had this been the older batteries, they would have been almost totally
discharged.

The best way to charge batteries is with a charger that lets you set the
charge rate, and watches for the signs that indicate the cell is charged, such
as the MAHA C9000. I generally charge at 0.3C or 0.4C, which is considered on
the low side for NiMH cells. Contrary to what Battery University claims, I've
never had problems with the cells overheating, or the charger failing to halt
at around 1.42 to 1.45 volts. (Note that his gripes are principally directed
at "consumer" chargers.)

<http://www.mahaenergy.com/mh-c9000/>
Nice charger/analyzer. I dunno the break-in and battery-forming
features. Stressing the battery doesn't seem like a good way to
increase battery life though it might produce rated capacity earlier.
However, it's a much better charger than the common consumer junk
chargers, and would probably be a good charger for the OP.

The C9000 displays the battery voltage, so I can choose to stop charger at
whatever voltage I feel comfortable with. I'm not stupid enough to let any
charger operate without checking it occasionally. If you like, stick a timer
on the charger to shut it off.

Have you tested your battery for rated capacity after one of your
charge cycles? The MH-C9000 will run a discharge mode test and
display battery capacity.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[William Sommerwerck]

<http://802.11junk.com/jeffl/NiMH/Duracelll-NiMH-2050.jpg>
shows that the first charge cycle results in a 1180mA-hr capacity,
while the third charge cycle increased it to 1360mA-hr or about 15%
increase. Is that what you mean by "slammed"?

No. It means hitting it hard with a high charge current.

I've noticed that my NiMH battery collection (flashlights, cameras,
GPS, radios, etc) spent much of their time in the charger simply
recovering from self discharge.

I've never had that problem -- as far as I can tell. I've had cells that sat
around for the better part of year deliver 20 or 30 flashes, with rapid
recycling.

Most of my cells have been close to rated capacity. In one case, a cell was
about 20% low, and MAHA replaced it.


<http://www.mahaenergy.com/mh-c9000/>
Nice charger/analyzer. I dunno the break-in and battery-forming
features. Stressing the battery doesn't seem like a good way to
increase battery life though it might produce rated capacity earlier.
However, it's a much better charger than the common consumer junk
chargers, and would probably be a good charger for the OP.

Agreed. Thomas sometimes sold the C9000 for ~ $40, but those days seem over. I
have two, so I don't have to wait if a lot of cells need charging.

 

[Jeff Liebermann]

On Mon, 24 Dec 2012 11:28:48 -0800, "William Sommerwerck"
<grizzledgeezer@comcast.net> wrote:

http://802.11junk.com/jeffl/NiMH/Duracelll-NiMH-2050.jpg
shows that the first charge cycle results in a 1180mA-hr capacity,
while the third charge cycle increased it to 1360mA-hr or about 15%
increase. Is that what you mean by "slammed"?

No. It means hitting it hard with a high charge current.

Yuck. Why would I want to do that? If there were dendrites in NiMH
batteries like in NiCd, then perhaps it would make sense. Besides if
such a high current blast was necessary to produce a proper battery,
or to produce bigger numbers on the data sheet, the manufacturers
would already be doing it.

Where did you see this recommendation? I couldn't find anything with
Google under "NiMH slamming" variations, except under National
Institute of Mental Health. I did find this blurb that recommends
against the practice:
<http://forums.dpreview.com/forums/post/25581821>

I've noticed that my NiMH battery collection (flashlights, cameras,
GPS, radios, etc) spent much of their time in the charger simply
recovering from self discharge.

I've never had that problem -- as far as I can tell. I've had cells that sat
around for the better part of year deliver 20 or 30 flashes, with rapid
recycling.

Perhaps my cameras and GPS require a higher minimum state of charge
than your flash? When I used to leave my conventional NiMH batteries
in the camera for a few months, it would usually be good for one or
two photos before complaining. I always kept a spare set of charged
NiMH with the camera in case I run out. They were also only good for
a few shots.

<http://www.stefanv.com/electronics/sanyo_eneloop.html>
Scroll down to the two pink graphs near the bottom of the article. The
conventional battery is down to half capacity in 75 days, and 1/4
capacity in 150 days.

Incidentally, this article:
<http://www.stefanv.com/electronics/using_nimh.html>
covers quite a bit of ground on dealing with NiMH batteries, including
trickle charging, and a home made NiMH computah controlled charger:
<http://www.stefanv.com/electronics/battman2.html>
and a simplified USB powered charger:
<http://www.stefanv.com/electronics/usb_charger.html>
I'm tempted...

Most of my cells have been close to rated capacity. In one case, a cell was
about 20% low, and MAHA replaced it.

I didn't know that Maha made batteries:
<http://www.mahaenergy.com/batteries/>
$4.25/ea for AA in a 4 pack is not cheap. Somewhat more expensive
than Sanyo Eneloop. I dunno.

There are other low self discharge NiMH batteries available.
Duracell Pre Charged
<http://www.duracell.com/en-US/product/rechargeable-staycharged-batteries.jspx>
Rayovac Hybrid or Platinum:
<http://www.rayovac.com/Products/Rechargeable-Batteries-and-Chargers/Everyday-Use-best-battery-charger.aspx>
Kodak Pre-Charged
<http://store.kodak.com/store/ekconsus/en_US/pd/ThemeID.3925700/Ni-MH_Pre-Charged_Rechargeable_Batteries_AA/productID.158853000>

http://www.mahaenergy.com/mh-c9000/
Nice charger/analyzer. I dunno the break-in and battery-forming
features. Stressing the battery doesn't seem like a good way to
increase battery life though it might produce rated capacity earlier.
However, it's a much better charger than the common consumer junk
chargers, and would probably be a good charger for the OP.

Agreed. Thomas sometimes sold the C9000 for ~ $40, but those days seem over. I
have two, so I don't have to wait if a lot of cells need charging.

I'm finding prices from $50 to $140 (including tax and shipping).
<http://www.ebay.com/sch/i.html?_nkw=maha%20mh-c9000&_sop=15>


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[William Sommerwerck]

"Jeff Liebermann" wrote in message
news:edihd8h7ce3v4uvcjhqtd7s97ll9irjr43@4ax.com...
On Mon, 24 Dec 2012 11:28:48 -0800, "William Sommerwerck"
<grizzledgeezer@comcast.net> wrote:

http://802.11junk.com/jeffl/NiMH/Duracelll-NiMH-2050.jpg
shows that the first charge cycle results in a 1180mA-hr capacity,
while the third charge cycle increased it to 1360mA-hr or about 15%
increase. Is that what you mean by "slammed"?

No. It means hitting it hard with a high charge current.

Yuck. Why would I want to do that? If there were dendrites in NiMH
batteries like in NiCd, then perhaps it would make sense. Besides if
such a high current blast was necessary to produce a proper battery,
or to produce bigger numbers on the data sheet, the manufacturers
would already be doing it.

Where did you see this recommendation? I couldn't find anything with
Google under "NiMH slamming" variations, except under National
Institute of Mental Health. I did find this blurb that recommends
against the practice:
<http://forums.dpreview.com/forums/post/25581821>


"Slam" is my choice of words.

I can't give you any references, but I've repeatedly seen in the documentation
for NiMH cells, that hitting them hard is necessary to get a "full" charge.
You are repeatedly warned NOT to charge them below 0.3C, and higher values are
preferable.

I've noticed that my NiMH battery collection (flashlights, cameras,
GPS, radios, etc) spent much of their time in the charger simply
recovering from self discharge.

I've never had that problem -- as far as I can tell. I've had cells that
sat
around for the better part of year deliver 20 or 30 flashes, with rapid
recycling.

Perhaps my cameras and GPS require a higher minimum state of charge
than your flash? When I used to leave my conventional NiMH batteries
in the camera for a few months, it would usually be good for one or
two photos before complaining. I always kept a spare set of charged
NiMH with the camera in case I run out. They were also only good for
a few shots.

Most of my flashes are 500-series Canons, which are hardly wimpy li'l
critters.


<http://www.stefanv.com/electronics/sanyo_eneloop.html>
Scroll down to the two pink graphs near the bottom of the article. The
conventional battery is down to half capacity in 75 days, and 1/4
capacity in 150 days.

That's not what I would consider "rapid" self-discharge -- other than,
perhaps, compared to a lead-acid battery.



Incidentally, this article:
<http://www.stefanv.com/electronics/using_nimh.html>
covers quite a bit of ground on dealing with NiMH batteries, including
trickle charging, and a home made NiMH computah controlled charger:
<http://www.stefanv.com/electronics/battman2.html>
and a simplified USB powered charger:
<http://www.stefanv.com/electronics/usb_charger.html>
I'm tempted...

Most of my cells have been close to rated capacity. In one case, a cell was
about 20% low, and MAHA replaced it.

I didn't know that Maha made batteries:
http://www.mahaenergy.com/batteries/

I don't know if they make them, but they definitely sell them under the
PowerEx brand. Thomas sells four for about $12. Considering that a charged
2500mAh cell gives about as much runtime as an alkaline cell, it doesn't take
long to amortize the cost.

Thomas sometimes sold the C9000 for ~ $40, but those days seem over.
I have two, so I don't have to wait if a lot of cells need charging.

I'm finding prices from $50 to $140 (including tax and shipping).
<http://www.ebay.com/sch/i.html?_nkw=maha%20mh-c9000&_sop=15>

The eBay prices are nuts.

I was wrong. Thomas's holiday deal is the charger, a generic carrying case,
four Immedion AA cells and a plastic case for the latter, for about $52. Given
the cost of the "accessories", the charger nets at $40. This would be a very
good time to grab a C9000 from Thomas Distributing.

 

[Jeff Liebermann]

On Mon, 24 Dec 2012 14:39:56 -0800, "William Sommerwerck"
<grizzledgeezer@comcast.net> wrote:

"Slam" is my choice of words.

I can't give you any references, but I've repeatedly seen in the documentation
for NiMH cells, that hitting them hard is necessary to get a "full" charge.
You are repeatedly warned NOT to charge them below 0.3C, and higher values are
preferable.

I think I've decoded the effect. It's a cure for "voltage
depression", where the NiMH battery decides to operate fairly
normally, but at a somewhat lower voltage. The usual culprit is over
charging. I've had it happen to me and almost tossed a set of good
batteries. The recommended fix is to cycle charge the batteries a few
times and it should recover. However, the not so graceful fix is to
discharge the battery, and then to a radical quick charge, being
careful not to overcharge it again. Allegedly, this avoids having to
cycle charge the battery several times, probably at the expense of
battery life.

<http://en.wikipedia.org/wiki/Memory_effect#Voltage_depression_due_to_long-term_over-charging>

See voltage depression section near the bottom of the page:
<http://www.stefanv.com/electronics/using_nimh.html>

<http://www.greenbatteries.com/nibafa.html#NiMH%20memory>

Most of my flashes are 500-series Canons, which are hardly wimpy li'l
critters.

I'm not familiar with the flash, but I suspect is does not have the
continuous high current load of a camera with it's focus and aperture
motors grinding away for many minutes.

http://www.stefanv.com/electronics/sanyo_eneloop.html
Scroll down to the two pink graphs near the bottom of the article. The
conventional battery is down to half capacity in 75 days, and 1/4
capacity in 150 days.

That's not what I would consider "rapid" self-discharge -- other than,
perhaps, compared to a lead-acid battery.

Well, it's rapid enough that I was recharging almost everything I
owned every 3 months or so. If it were only one device, I wouldn't
complain, but it was a mix of GPS, flashlights, cameras, phones,
walkie-talkies, FRS radios, clocks, etc.

I don't know if they make them, but they definitely sell them under the
PowerEx brand. Thomas sells four for about $12. Considering that a charged
2500mAh cell gives about as much runtime as an alkaline cell, it doesn't take
long to amortize the cost.

Well, the plan was to save money by using rechargeable batteries. I
usually buy Costco Kirland AA batteries:
<http://www2.costco.com/Browse/Product.aspx?prodid=11285539&whse=BD_115&topnav=bdoff&cat=21810&hierPath=68474*91765*&lang=en-US>
at about $0.30 each. Sanyo Eneloop NiMH batteries are costing me
about $3.00 each. So, if the NiMH battery lasts 10 charge cycles, I
break even. So far, that's been the case, so for cost savings,
rechargeable wins.

However, that has NOT been the case for convenience. I can leave an
alkaline AA battery in my various devices for many months, and not
have a self discharge problem. I find myself carrying spare alkaline
batteries for these devices just in case the battery decides to
discharge itself. Extra points for devices that suck power when
turned off. Therefore, for convenience, conventional rechargeable
NiMH is a problem. The low self discharge batteries are a big
improvement and may just solve the problem. I'm not keeping score,
but I'm definitely not recharging anywhere near as often as when I was
using conventional NiMH batteries.

I was wrong. Thomas's holiday deal is the charger, a generic carrying case,
four Immedion AA cells and a plastic case for the latter, for about $52. Given
the cost of the "accessories", the charger nets at $40. This would be a very
good time to grab a C9000 from Thomas Distributing.

Good price, but the web pile is back to $52 (including shipping) and
without the AA cells and plastic case:
<http://www.ecrater.com/p/11476696/maha-powerex-c9000-aa-aaa>

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[mike]

On 12/24/2012 2:10 PM, Jeff Liebermann wrote:

On Mon, 24 Dec 2012 11:28:48 -0800, "William Sommerwerck"
grizzledgeezer@comcast.net> wrote:

http://802.11junk.com/jeffl/NiMH/Duracelll-NiMH-2050.jpg
shows that the first charge cycle results in a 1180mA-hr capacity,
while the third charge cycle increased it to 1360mA-hr or about 15%
increase. Is that what you mean by "slammed"?

No. It means hitting it hard with a high charge current.

Yuck. Why would I want to do that? If there were dendrites in NiMH
batteries like in NiCd, then perhaps it would make sense. Besides if
such a high current blast was necessary to produce a proper battery,
or to produce bigger numbers on the data sheet, the manufacturers
would already be doing it.

Where did you see this recommendation? I couldn't find anything with
Google under "NiMH slamming" variations, except under National
Institute of Mental Health. I did find this blurb that recommends
against the practice:
http://forums.dpreview.com/forums/post/25581821

I've noticed that my NiMH battery collection (flashlights, cameras,
GPS, radios, etc) spent much of their time in the charger simply
recovering from self discharge.

I've never had that problem -- as far as I can tell. I've had cells that sat
around for the better part of year deliver 20 or 30 flashes, with rapid
recycling.

Perhaps my cameras and GPS require a higher minimum state of charge
than your flash? When I used to leave my conventional NiMH batteries
in the camera for a few months, it would usually be good for one or
two photos before complaining. I always kept a spare set of charged
NiMH with the camera in case I run out. They were also only good for
a few shots.

http://www.stefanv.com/electronics/sanyo_eneloop.html
Scroll down to the two pink graphs near the bottom of the article. The
conventional battery is down to half capacity in 75 days, and 1/4
capacity in 150 days.

Incidentally, this article:
http://www.stefanv.com/electronics/using_nimh.html
covers quite a bit of ground on dealing with NiMH batteries, including
trickle charging, and a home made NiMH computah controlled charger:
http://www.stefanv.com/electronics/battman2.html
and a simplified USB powered charger:
http://www.stefanv.com/electronics/usb_charger.html
I'm tempted...

Most of my cells have been close to rated capacity. In one case, a cell was
about 20% low, and MAHA replaced it.

I didn't know that Maha made batteries:
http://www.mahaenergy.com/batteries/
$4.25/ea for AA in a 4 pack is not cheap. Somewhat more expensive
than Sanyo Eneloop. I dunno.

There are other low self discharge NiMH batteries available.
Duracell Pre Charged
http://www.duracell.com/en-US/product/rechargeable-staycharged-batteries.jspx
Rayovac Hybrid or Platinum:
http://www.rayovac.com/Products/Rechargeable-Batteries-and-Chargers/Everyday-Use-best-battery-charger.aspx
Kodak Pre-Charged
http://store.kodak.com/store/ekconsus/en_US/pd/ThemeID.3925700/Ni-MH_Pre-Charged_Rechargeable_Batteries_AA/productID.158853000

http://www.mahaenergy.com/mh-c9000/
Nice charger/analyzer. I dunno the break-in and battery-forming
features. Stressing the battery doesn't seem like a good way to
increase battery life though it might produce rated capacity earlier.
However, it's a much better charger than the common consumer junk
chargers, and would probably be a good charger for the OP.

Agreed. Thomas sometimes sold the C9000 for ~ $40, but those days seem over. I
have two, so I don't have to wait if a lot of cells need charging.

I'm finding prices from $50 to $140 (including tax and shipping).
http://www.ebay.com/sch/i.html?_nkw=maha%20mh-c9000&_sop=15


+1 on the c9000

I've had good results with the older
Enloop charger...the one with the single led that can
charge an odd number of cells.

There are two categories of dendrites.
Dendrites that have already shorted need to be vaporized with
short-duration
high-current. That clears the short and blasts a bigger hole in the
separator.
Causes higher self-discharge and faster dendrite regrowth.
Back in the day when NiCds were expensive and shorted ones were free,
it made a lot of sense for hobby projects.

The second class is those that have not yet pierced the separator.
For those, I'm a fan of burp charging. Same process you'd use
for plating when you want a smooth surface.
I did some experiments, but I had nowhere enough data to draw a
conclusion. And I concluded that the cost of powering the computer
and the programmable power supply and load probably exceeded the benefit.

The biggest problem with charging batteries is determining when to
stop.

I had a friend who did radio repair for the city.
Back in the day, police radios were terminated by a thermal switch.
That worked surprisingly well, but only because the battery was
predictably depleted during the shift.
If you stuck a charged radio into the charger, you did some damage.
He had a lot of failed batteries from people who left the radio off
all day and stuck it back in the charger.

There's so much variability in temperature, thermal time constant,
voltage gradient, etc. that it's best to measure change than
absolute value.
I'm a fan of 0deltaV termination with timer and temperature fail-safe
for NiMH cells.

 

[William Sommerwerck]

Most of my flashes are 500-series Canons, which are hardly
wimpy li'l critters.

I'm not familiar with the flash, but I suspect is does not have
the continuous high current load of a camera with it's focus
and aperture motors grinding away for many minutes.

Most DSLRs use lithium-ion batteries. I've never measured the current drain,
but I doubt it approaches what's required to charge an electronic flash from a
dead start. And unless you're using the focus-track feature, auto-focus
operation is intermittent.

--> discussion of rechargeable versus disposable batteries snipped <--

I use rechargeables on any device where I can easily swap the batteries.
Disposables are okay for certain kinds of toys and LED flashlights. (Most take
AAA cells, and I have a huge box of Polaroid AAA alkalines I got cheap.)

 

On Sun, 23 Dec 2012 16:53:34 -0500, in sci.electronics.repair you wrote:

Hi,
Since I created a NiMH battery pack using recently bought NiMH "C"
batteries, I am in search of knowledge for the best way to slow charge these
1500 MAH "C" cells in series. I prefer using a timer and a constant current
charger (I made) that supplies a constant 150MA.

There is a lot of information on the web, but one site says one thing,
and another says something else.

www.batteryprice.com/batterycareguide.aspx provided the kind of
information I was looking for:

(Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
charge). Note: Batteries "fully discharged" to 1V each cell.

In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
to me, but I wonder if anyone sees a flaw.

How about getting back to the original subject, "Best Method to Slow

Charge..."

Here is additional information that seems to support that formula:

I just bought a pair of 2500 MHA NiMH "C" cells that state on the
package, "Recharge in just 2.5 hours with 1250 mAh rapid charger or
15 hours with a 250 mAh standard charger". Note: There was no
charging information on the 1500 MHA NiMH "C" cells I used in
my battery pack project. The 1500mAh is a different brand than the
2500mAh.
John

 

[William Sommerwerck]

How about getting back to the original subject,
"Best Method to Slow Charge..."

If you believe most of what the manufacturers say, there is no "best"
slow-charging method. Slow charging is deprecated.

 

[Jeff Liebermann]

On Tue, 25 Dec 2012 09:53:49 -0500, jaugustine@verizon.net wrote:

On Sun, 23 Dec 2012 16:53:34 -0500, in sci.electronics.repair you wrote:
Hi,
Since I created a NiMH battery pack using recently bought NiMH "C"
batteries, I am in search of knowledge for the best way to slow charge these
1500 MAH "C" cells in series. I prefer using a timer and a constant current
charger (I made) that supplies a constant 150MA.

Perhaps it might be useful if you would disclose the brand and model
number of these "C" cells? The typical "C" NiMH battery has a
capacity of 3000 to 5000 ma-hrs, not 1500, which sounds more like a
"AA" battery or possibly a sub-C size battery.

There is a lot of information on the web, but one site says one thing,
and another says something else.

www.batteryprice.com/batterycareguide.aspx provided the kind of
information I was looking for:

(Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
charge). Note: Batteries "fully discharged" to 1V each cell.

In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
to me, but I wonder if anyone sees a flaw.

How about getting back to the original subject, "Best Method to Slow
Charge..."

The 1.4 factor is:
1 / 1.4 = 0.7 = 70% charging efficiency.
That's a reasonable number.

Using a timer is fine, if you're very careful. However, I guarantee
that it will shorten the life of your battery. The object of the
exercise is to charge the battery without going into overcharge.
That's easy with a new battery, where the aformentioned timer formula
works just fine. However, as the battery ages, it's capacity also
decreases. Charging at the same 0.1C rate for the same amount of time
is a guaranteed overcharge. I have some experience with that, having
successfully trashed some rather expensive radio battery packs with
timer controlled charging. In order to do it correctly, you would
need to ocassional measure or estimate the battery pack capacity, and
adjust the timer according to the reduced capacity. You would also
need to know the state of charge, or discharge the battery to a known
charge point.

<http://www.buchmann.ca/article18-page1.asp>
"Harmful overcharge can occur if a partially or
fully charged battery is charged with a fixed timer. The
same occurs if the battery has aged and can only hold 50
instead of 100 percent charge. Overcharge could occur
even though the NiMH battery feels cool to the touch."

In other words, to make the timer method work, you would need to know
the present capacity and state of charge of each battery at the time
of charge. If you keep detailed records and make accurate
measurements, that's possible. I heard a talk by someone from NBC who
did exactly that for maintaining the battery packs for portable
cameras and recorders. Serial numbers on every battery pack, regular
testing, and detailed records were required.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Jeff Liebermann]

On Mon, 24 Dec 2012 17:20:32 -0800, mike <ham789@netzero.net> wrote:

+1 on the c9000

Sigh. I've got an itchy mouse finger hovering over the "buy" button.

There are two categories of dendrites.
Dendrites that have already shorted need to be vaporized with
short-duration
high-current. That clears the short and blasts a bigger hole in the
separator.
Causes higher self-discharge and faster dendrite regrowth.
Back in the day when NiCds were expensive and shorted ones were free,
it made a lot of sense for hobby projects.

I've done my share of NiCd cells directly across a 12v car battery for
a few fractions of a second. No explosions, and a rather minimal
success rate. The ones that worked didn't last very long.

The second class is those that have not yet pierced the separator.
For those, I'm a fan of burp charging. Same process you'd use
for plating when you want a smooth surface.
I did some experiments, but I had nowhere enough data to draw a
conclusion. And I concluded that the cost of powering the computer
and the programmable power supply and load probably exceeded the benefit.

Also known as "negative pulse charging" or "reflex charging". There
are commercial chargers that do this.
Pulse-Power:
<http://www.fm2way.com/batterycharger/technica.htm>
"Negative Pulse Charging Myths and Facts"
<http://www.powerdesigners.com/pdf/Tech%20Brief%20Negative%20Pulse%20Charging%20Techniques%20Myths%20&%20Facts%20-%20Final.pdf>
I'm not convinced, but then I haven't tried one of these chargers.

The biggest problem with charging batteries is determining when to
stop.

Yep. Kinda like filling the gas tank with no gas gauge or automatic
pump shutoff.

I had a friend who did radio repair for the city.
Back in the day, police radios were terminated by a thermal switch.
That worked surprisingly well, but only because the battery was
predictably depleted during the shift.
If you stuck a charged radio into the charger, you did some damage.
He had a lot of failed batteries from people who left the radio off
all day and stuck it back in the charger.

I know it well. The Motorola HT600 and similar "brick" radios. As an
added bonus, Motorola may have designed the charger to destroy the
battery packs.
<http://www.repeater-builder.com/motorola/genesis/ht600-charger-mod.html>
They also have to be modified to work with NiMH packs:
<http://www.repeater-builder.com/motorola/genesis/nimh-charger-mod.html>

However, my favorite was a Kenwood something HT, that had small a red
plastic window in the bottom of the battery pack. The charger had a
corresponding window with a IR photo transistor in the charger base.
This was suppose to measure the cell temperature and stop charging
when it became warm. Unfortunately, a little dirt on either window
turned the charger into a battery overcharger.

There's so much variability in temperature, thermal time constant,
voltage gradient, etc. that it's best to measure change than
absolute value.
I'm a fan of 0deltaV termination with timer and temperature fail-safe
for NiMH cells.

For NiMH, that's a problem. Trickle charging an NiMH battery makes
the drop in terminal voltage very difficult to see. It's in the low
millivolt region. I think (not sure) that Maha recommends something
like 0.5C charge on their Eneloop just so that deltaV can be detected.
Here's a hint, without detail:
<http://www.flickr.com/groups/strobist/discuss/72157625332323687/>
"manufacturers recommend 0.5C to 1C to (1A to 2A on eneloop)
to make negative delta voltage detection more "detectable"."

I did some testing of temperature detections on individual NiCd cells
when I was doing my ultra-rapid charging experiments. I stuck 8
thermistors on various parts of the cell, and proceeded to
intentionally overcharge it. The mass of the cell was sufficiently
high that the thermal lag was substantial. That won't make much
difference for a slow charge, but can easily be catastrophic with
ultra-rapid charge, where seconds count. The best sensor location
appeared to be in the middle of the cell. Unfortunately, the plastic
or paper wrap around the battery is a good thermal insulator. I had
to scrape off the coating and connect directly to the metal case.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[William Sommerwerck]

MAHA has told me it will never provide a C or D adapter for the C9000. Not
wanting to figure out how to reliably attach wires to the charger's terminals,
I charge my C NiMH cells * in an Eveready charger with a NiCd / NiMH switch on
it. I monitor the charge with my hand. When the cell gets a bit warm, I pull
it.

* I use C cells in a potato-masher flash and a classic Sony FM radio.

Also known as "negative pulse charging" or "reflex charging".
There are commercial chargers that do this.
Pulse-Power:
http://www.fm2way.com/batterycharger/technica.htm
"Negative Pulse Charging Myths and Facts"
http://www.powerdesigners.com/pdf/Tech%20Brief%20Negative%20Pulse%20Charging%20Techniques%20Myths%20&%20Facts%20-%20Final.pdf
I'm not convinced, but then I haven't tried one of these chargers.

This principle isn't new. The basic system dates back to the late 60s.
Motorola made a "pulse-discharge" charger for Honeywell flashes. The claim was
not only that it could quickly recharge the power pack, but that it would
often "heal" a badly performing or even "dead" pack.

"Modern Photography" gave the system a rave review, confirming that it worked
as claimed. Several years later I asked Burt Keppler what happened to the
system. He said Honeywell pulled it because there were too many instances of
the packs exploding. When I asked him why he never reported this in "Modern",
he had no answer.

 

[mike]

On 12/25/2012 9:55 AM, Jeff Liebermann wrote:

On Mon, 24 Dec 2012 17:20:32 -0800, mike<ham789@netzero.net> wrote:

+1 on the c9000

Sigh. I've got an itchy mouse finger hovering over the "buy" button.

There are two categories of dendrites.
Dendrites that have already shorted need to be vaporized with
short-duration
high-current. That clears the short and blasts a bigger hole in the
separator.
Causes higher self-discharge and faster dendrite regrowth.
Back in the day when NiCds were expensive and shorted ones were free,
it made a lot of sense for hobby projects.

I've done my share of NiCd cells directly across a 12v car battery for
a few fractions of a second. No explosions, and a rather minimal
success rate. The ones that worked didn't last very long.

You need better control than that. I've had better luck discharging
a big cap into the cell.

The second class is those that have not yet pierced the separator.
For those, I'm a fan of burp charging. Same process you'd use
for plating when you want a smooth surface.
I did some experiments, but I had nowhere enough data to draw a
conclusion. And I concluded that the cost of powering the computer
and the programmable power supply and load probably exceeded the benefit.

Also known as "negative pulse charging" or "reflex charging". There
are commercial chargers that do this.
Pulse-Power:
http://www.fm2way.com/batterycharger/technica.htm
"Negative Pulse Charging Myths and Facts"
http://www.powerdesigners.com/pdf/Tech%20Brief%20Negative%20Pulse%20Charging%20Techniques%20Myths%20&%20Facts%20-%20Final.pdf
I'm not convinced, but then I haven't tried one of these chargers.

The biggest problem with charging batteries is determining when to
stop.

Yep. Kinda like filling the gas tank with no gas gauge or automatic
pump shutoff.

I had a friend who did radio repair for the city.
Back in the day, police radios were terminated by a thermal switch.
That worked surprisingly well, but only because the battery was
predictably depleted during the shift.
If you stuck a charged radio into the charger, you did some damage.
He had a lot of failed batteries from people who left the radio off
all day and stuck it back in the charger.

I know it well. The Motorola HT600 and similar "brick" radios. As an
added bonus, Motorola may have designed the charger to destroy the
battery packs.
http://www.repeater-builder.com/motorola/genesis/ht600-charger-mod.html
They also have to be modified to work with NiMH packs:
http://www.repeater-builder.com/motorola/genesis/nimh-charger-mod.html

However, my favorite was a Kenwood something HT, that had small a red
plastic window in the bottom of the battery pack. The charger had a
corresponding window with a IR photo transistor in the charger base.
This was suppose to measure the cell temperature and stop charging
when it became warm. Unfortunately, a little dirt on either window
turned the charger into a battery overcharger.

There's so much variability in temperature, thermal time constant,
voltage gradient, etc. that it's best to measure change than
absolute value.
I'm a fan of 0deltaV termination with timer and temperature fail-safe
for NiMH cells.

For NiMH, that's a problem. Trickle charging an NiMH battery makes
the drop in terminal voltage very difficult to see.

Agreed. Trickle charging is for sissies. ;-) And only makes sense when
you have no reasonable termination system...and then, it still makes no
sense.

That's also my complaint about the C9000.
If your cells have slightly high internal resistance, it refuses
to charge them at rated current. If you crank the current down till
they charge, you risk failure of charge termination.

It's in the low

millivolt region. I think (not sure) that Maha recommends something
like 0.5C charge on their Eneloop just so that deltaV can be detected.
Here's a hint, without detail:
http://www.flickr.com/groups/strobist/discuss/72157625332323687/
"manufacturers recommend 0.5C to 1C to (1A to 2A on eneloop)
to make negative delta voltage detection more "detectable"."

I did some testing of temperature detections on individual NiCd cells
when I was doing my ultra-rapid charging experiments. I stuck 8
thermistors on various parts of the cell, and proceeded to
intentionally overcharge it. The mass of the cell was sufficiently
high that the thermal lag was substantial. That won't make much
difference for a slow charge, but can easily be catastrophic with
ultra-rapid charge, where seconds count. The best sensor location
appeared to be in the middle of the cell. Unfortunately, the plastic
or paper wrap around the battery is a good thermal insulator. I had
to scrape off the coating and connect directly to the metal case.

 

[Adrian Tuddenham]

Jeff Liebermann <jeffl@cruzio.com> wrote:

[...]

http://www.buchmann.ca/article18-page1.asp

[..]

Overcharge could occur
even though the NiMH battery feels cool to the touch."

Where does the excess energy go if it doesn't finish up as heat in the
battery?



--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

 

[mike]

On 12/26/2012 3:27 AM, Adrian Tuddenham wrote:

Jeff Liebermann<jeffl@cruzio.com> wrote:

[...]
http://www.buchmann.ca/article18-page1.asp

[..]
Overcharge could occur
even though the NiMH battery feels cool to the touch."

Where does the excess energy go if it doesn't finish up as heat in the
battery?



Cool to the touch is not the same as exactly at ambient.

 

On Tue, 25 Dec 2012 09:24:20 -0800, in sci.electronics.repair you wrote:

Hi,
Since I created a NiMH battery pack using recently bought NiMH "C"
batteries, I am in search of knowledge for the best way to slow charge these
1500 MAH "C" cells in series. I prefer using a timer and a constant current
charger (I made) that supplies a constant 150MA.

Perhaps it might be useful if you would disclose the brand and model
number of these "C" cells? The typical "C" NiMH battery has a
capacity of 3000 to 5000 ma-hrs, not 1500, which sounds more like a
"AA" battery or possibly a sub-C size battery.

Hi Jeff,

These I bought at Harbor Freight. 1500mAh are Chicago Electric Power
Systems (in my battery pack) item # 90149, and the 2500mAh (I just bought) are
Thunderbolt Magnum item # 97864.

www.batteryprice.com/batterycareguide.aspx provided the kind of
information I was looking for:

(Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
charge). Note: Batteries "fully discharged" to 1V each cell.

In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
to me, but I wonder if anyone sees a flaw.


The 1.4 factor is:
1 / 1.4 = 0.7 = 70% charging efficiency.
That's a reasonable number.

Using a timer is fine, if you're very careful. However, I guarantee
that it will shorten the life of your battery. The object of the
exercise is to charge the battery without going into overcharge.
That's easy with a new battery, where the aformentioned timer formula
works just fine. However, as the battery ages, it's capacity also
decreases. Charging at the same 0.1C rate for the same amount of time
is a guaranteed overcharge. I have some experience with that, having
successfully trashed some rather expensive radio battery packs with
timer controlled charging. In order to do it correctly, you would
need to ocassional measure or estimate the battery pack capacity, and
adjust the timer according to the reduced capacity. You would also
need to know the state of charge, or discharge the battery to a known
charge point.

http://www.buchmann.ca/article18-page1.asp
"Harmful overcharge can occur if a partially or
fully charged battery is charged with a fixed timer. The
same occurs if the battery has aged and can only hold 50
instead of 100 percent charge. Overcharge could occur
even though the NiMH battery feels cool to the touch."

I am aware of partially charged batteries and not to charge
them at the same length of time as charging fully discharged batteries.

In other words, to make the timer method work, you would need to know
the present capacity and state of charge of each battery at the time
of charge. If you keep detailed records and make accurate
measurements, that's possible. I heard a talk by someone from NBC who
did exactly that for maintaining the battery packs for portable
cameras and recorders. Serial numbers on every battery pack, regular
testing, and detailed records were required.

You made very good points regarding aging battery's capacity going down and
I will certainly keep that in mind.

I may consider rechargeable lithium batteries in the future, but I will do
some research first to learn about their issues.

Thanks again, John