Best Method to Slow Charge NiMH Batteries

[mike]

On 12/26/2012 5:52 AM, jaugustine@verizon.net wrote:

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.

I've never tried any of these. I've read reviews that suggest that

HF sells cells that were swept off the floor at the place that dug
rejects out of someone else's dumpster.

I do have some experience with drills bought from HF.
Those batteries were uniformly horrible. And I bought some HF
closeout tool batteries with the intention of using the cells elsewhere.
No go...all crap...six cells for 99-cents and it still was a bad deal.
I was never able to quick charge them and slow charging only worked a few
times.

There exist real rechargeable C cells...
http://www.batteryspace.com/nimh-rechargeable-cell-c-size-1.2-v-5000-mah-with-button-tabs.aspx
is the first google hit. Looks like just over $4 each. Have no opinion
on that brand. Twice the cost of HF. But more than 3x the RATED
capacity, and I'd bet $$$ that they're actually usable in a real
application.

Most end-user grade C-size rechargeable batteries are a smaller,
often much smaller cell embedded
inside. You can tell by the weight.
At 1500 mAh, if you don't need the form factor, you're probably better
off with a quality
AA cell.

 

[Jeff Liebermann]

On Wed, 26 Dec 2012 11:27:56 +0000,
adrian@poppyrecords.invalid.invalid (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?

Thermal lag. It takes a while for the battery to become warm to the
touch. If the -deltaV sensor says it's done at a conservative point,
you'll never feel the heat (which is a good thing). With a fast
charge, you could easily overcharge before the internal heating has
time to make it to the surface. We're also not talking about much
heat. 0.5C charge into 1.35V is only about 1.4 watts. Heating
shouldn't be a problem with a slow charge, which might simply radiate
the heat before it gets excessively too hot. Extra credit for
manufacturers that include small fans in their chargers which insures
that the temperature sensors don't work.

However, I've never seen a cold and overcharged battery. With
reasonably fast charging, the batteries I've played with all get warm
or hot as they approach full charge. The battery manufacturers are
making sure that the battery gets warm by playing with the ma-hr
capacity number. For example, a 2300 ma-hr NiMH battery, may achieve
the rated capacity at 0.1C discharge. However at 1C discharge, it's
only good for 2000 ma-hr. Less for higher discharge rates. Old
batteries also lose surface area, and therefore lose capacity. Trying
to charge a moving target like with a fixed timer charger, is just not
going to work. It will overcharge.

Now, if you really want entertainment value, try putting a sealed
zip-lock plastic bag over the charger and give it a nice hefty
overcharge. The bag will inflate slightly with even a modest
overcharge. Where does the gas come from? It's from blowing the vent
seal on the battery and boiling off electrolyte. Note the pressure
curve:
<http://www.buchmann.ca/Article18_files/Figure1.gif>
The safety vent opens at about 130 psi and stays open forever. The
gas is mostly hydrogen, so you probably make a small bang. You can
sometimes see condensed electrolyte on the bag surface. The potassium
hydroxide can be detected with ph test strips. Do that a few times
and there will be little electrolyte left in the battery. I
"rejuvenated" at few NiCd cells by filing off a corner of the cell,
and using a hypodermic to inject KOH back into the cell. I'm not
certain how well it worked, because there may have been other damage
inside, but it worked well enough to run an HT for a few months.

I guess that's one advantage to 0.1C charging. It will never get hot
enough to vent.


--
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 Wed, 26 Dec 2012 08:52:11 -0500, jaugustine@verizon.net wrote:

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.

Links would be helpful.

<http://www.harborfreight.com/pack-of-2-nimh-rechargeable-c-batteries-90149.html>
Those are AA cells wrapped in excess cardboard in order to fit in a C
cell package. You can tell with a magnet. If it sticks to the
outside of the cell, it's a real C cell and I'm wrong. If it doesn't
stick, it's cardboard. Welcome to Harbor Fright.

<http://www.harborfreight.com/pack-of-2-high-capacity-nimh-rechargeable-c-batteries-97864.html>
2500 ma-hr is probably a real C cell. However, it's a the low end of
the capacity scale for an NiMH C cell, which should be 3000 to 5000
mah-hr. I would be concerned about quality.

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

Between the changing capacity with age, the unknown state of charge,
and the rather odd cell capacity specification, you have a moving
target.

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

Forget everything you know about charging NiCd and NiMH. Lithium Ion
charging is very different and difficult. To do it right, you need a
coulomb counter fuel gauge, that counts electrons going in and out,
and adjusts the charge accordingly. You might be able to get away
with something less (as the RC model people suggest). Light reading:
<http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries>
Note that a warm and fully charged Li-Ion battery self-deteriorates.
Spend some time reading here before diving in:
<http://www.candlepowerforums.com/vb/forum.php>
<http://www.candlepowerforums.com/vb/forumdisplay.php?107-Smoke-and-Fire-Hot-Cells-and-Close-Calls-The-dangerous-side-of-batteries>
How to make a 18650 battery and flashlight into an accidental rocket:
<http://www.candlepowerforums.com/vb/showthread.php?280909-Ultrafire-18650-3000mA-exploded>
I've had an Ultrafire (cheap brand available on eBay) go bad on me,
but not explode. That's because there's a mess of electronics under
the negative terminal in the battery that's suppose to prevent shorts
and bangs.

--
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

 

[who where]

On Wed, 26 Dec 2012 09:11:27 -0800, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Wed, 26 Dec 2012 08:52:11 -0500, jaugustine@verizon.net wrote:

(snip)

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

Forget everything you know about charging NiCd and NiMH. Lithium Ion
charging is very different and difficult. To do it right, you need a
coulomb counter fuel gauge, that counts electrons going in and out,
and adjusts the charge accordingly.

Not necessarily difficult, but a world away from Ni-xx. The primary
ingredients for Li-Ion charging are a current-limited constant voltage
source that has good voltage control. Near enough is not near good
enough. We have used bench PSU's on occasion as part of out testing.

While John is reading up on the material you linked, he could also
check out the available documentation from some charger-controller
chip makers. Back half a lifetime ago I designed a commercial charger
using the MAX1737, and the data sheet included a lot of good info for
(Li-Ion) beginners on care and feeding requirements,

http://datasheets.maximintegrated.com/en/ds/MAX1737.pdf