Voltmeter with hold reading

[mg]

Hello All,

I posted a few months ago about building a discharger for a DeWalt 18v
battery. Thanks to everyone who contributed suggestions and ideas. We
have our first one built and working.

A bit of background - We have a DeWalt 18v recip saw on our rescue
van. We have used it during extrications where we have a great deal of
water (i.e. a stream). We use our corded recip saw in less hazardous
environments. We have three DeWalt batteries. Each battery rotates
from 1) charger to 2) saw to 3) spare every week. It's not unusual for
this recip saw to go unused for almost a year at a time. We needed
some way to check the batteries. So, we decided to build a discharge
unit to determine the condition of the batteries.

We would like to add one additional feature to our discharger. Some
sort of voltmeter that we could hold the last reading. This would
allow us to double check that we stop the discharge process at 15v.

We have a DPDT relay that we use to control a timer. This allows us to
monitor how much time it takes for each battery to drop to 15v. We
hope that this will allow to determine when we need to replace a
battery.

Does anyone know of a voltmeter that would allow us to hold a reading
via some sort of external relay or contact?

TIA,
Mark
Firefighter Sky Valley / Scaly Mountain Volunteer Fire & Rescue
--
Email hint - Everything after the @ is spelled backwards.

There are only 10 types of people in the world:
those that understand binary, and
those that don't.

 

[Rich Grise]

On Mon, 20 Jun 2005 14:26:33 +0000, mg wrote:

Hello All,

I posted a few months ago about building a discharger for a DeWalt 18v
battery. Thanks to everyone who contributed suggestions and ideas. We
have our first one built and working.

A bit of background - We have a DeWalt 18v recip saw on our rescue
van. We have used it during extrications where we have a great deal of
water (i.e. a stream). We use our corded recip saw in less hazardous
environments. We have three DeWalt batteries. Each battery rotates
from 1) charger to 2) saw to 3) spare every week. It's not unusual for
this recip saw to go unused for almost a year at a time. We needed
some way to check the batteries. So, we decided to build a discharge
unit to determine the condition of the batteries.

We would like to add one additional feature to our discharger. Some
sort of voltmeter that we could hold the last reading. This would
allow us to double check that we stop the discharge process at 15v.

We have a DPDT relay that we use to control a timer. This allows us to
monitor how much time it takes for each battery to drop to 15v. We
hope that this will allow to determine when we need to replace a
battery.

Does anyone know of a voltmeter that would allow us to hold a reading
via some sort of external relay or contact?

Wouldn't it make more sense to, rather than just try to see what the
volts were when the timer timed out, monitor the volts to see when
the discharger's done, and see how long it takes? You could use an
ordinary (analog) electric clock - when your controller says, "15v!" you
shut down and the clock stops. You don't even have to set it to "real"
time - set it for noon, or something, start your discharger, and read
the hours right off the clock face at your leisure.

Good Luck!
Rich

 

[John Fields]

On Mon, 20 Jun 2005 14:26:33 GMT, mg <mark_gramlich@oohay.moc> wrote:

Hello All,

I posted a few months ago about building a discharger for a DeWalt 18v
battery. Thanks to everyone who contributed suggestions and ideas. We
have our first one built and working.

A bit of background - We have a DeWalt 18v recip saw on our rescue
van. We have used it during extrications where we have a great deal of
water (i.e. a stream). We use our corded recip saw in less hazardous
environments. We have three DeWalt batteries. Each battery rotates
from 1) charger to 2) saw to 3) spare every week. It's not unusual for
this recip saw to go unused for almost a year at a time. We needed
some way to check the batteries. So, we decided to build a discharge
unit to determine the condition of the batteries.

We would like to add one additional feature to our discharger. Some
sort of voltmeter that we could hold the last reading. This would
allow us to double check that we stop the discharge process at 15v.

We have a DPDT relay that we use to control a timer. This allows us to
monitor how much time it takes for each battery to drop to 15v. We
hope that this will allow to determine when we need to replace a
battery.

Does anyone know of a voltmeter that would allow us to hold a reading
via some sort of external relay or contact?

---
from your description, it doesn't sound like you need to hold a
voltmeter reading as much as it does to determine the amount of time
elapsed between when you started discharging the battery and when its
voltage, under discharge, fell to 15V. An easy way to do that would
be to start a clock when the discharge process started and then to
stop it when the battery voltage fell to 15V. A simple comparator
working against a voltage reference, a counter with a 1 second time
base, and a seven-segment display frozen at the time accumulated when
the 15V discharge point was reached would do it.

--
John Fields
Professional Circuit Designer

 

[Roy Lewallen]

John Fields wrote:

---
from your description, it doesn't sound like you need to hold a
voltmeter reading as much as it does to determine the amount of time
elapsed between when you started discharging the battery and when its
voltage, under discharge, fell to 15V. An easy way to do that would
be to start a clock when the discharge process started and then to
stop it when the battery voltage fell to 15V. A simple comparator
working against a voltage reference, a counter with a 1 second time
base, and a seven-segment display frozen at the time accumulated when
the 15V discharge point was reached would do it.

I've used an even simpler method for many years. The comparator opens a
relay which removes power to an old-fashioned analog electric clock. The
clock is set to 12:00 at the beginning of the test, and shows the
elapsed time at the end.

Roy Lewallen

 

[Brian]

www.fluke.com


"mg" <mark_gramlich@oohay.moc> wrote in message news:fhhdb1dch1t0dtn9nn964ivng5ccpoa54s@4ax.com...
Hello All,

I posted a few months ago about building a discharger for a DeWalt 18v
battery. Thanks to everyone who contributed suggestions and ideas. We
have our first one built and working.

A bit of background - We have a DeWalt 18v recip saw on our rescue
van. We have used it during extrications where we have a great deal of
water (i.e. a stream). We use our corded recip saw in less hazardous
environments. We have three DeWalt batteries. Each battery rotates
from 1) charger to 2) saw to 3) spare every week. It's not unusual for
this recip saw to go unused for almost a year at a time. We needed
some way to check the batteries. So, we decided to build a discharge
unit to determine the condition of the batteries.

We would like to add one additional feature to our discharger. Some
sort of voltmeter that we could hold the last reading. This would
allow us to double check that we stop the discharge process at 15v.

We have a DPDT relay that we use to control a timer. This allows us to
monitor how much time it takes for each battery to drop to 15v. We
hope that this will allow to determine when we need to replace a
battery.

Does anyone know of a voltmeter that would allow us to hold a reading
via some sort of external relay or contact?

TIA,
Mark
Firefighter Sky Valley / Scaly Mountain Volunteer Fire & Rescue
--
Email hint - Everything after the @ is spelled backwards.

There are only 10 types of people in the world:
those that understand binary, and
those that don't.

 

[mg]

Just to clarify a few points.

We have the elapsed timer working well.

We discharge the battery very, very slowly. The elapsed times are
running between 5.5 and 6 hours. Because of the long elapse time, we
can't just have someone hang around or remember to return to the
station at the correct time to check the voltage.

What we want to do is record or hold the ending voltage somehow. As
this is our first attempt at a discharger, we want some way to double
check that we don't drop below 15v. We are pretty green at putting
this together. And let's just say we aren't 100% confident that we
have everything right.

Our first thought would be to use some sort of voltmeter that had an
external method of triggering a "hold". We are open to suggestions.

TIA,
Mark
--
Email hint - Everything after the @ is spelled backwards.

There are only 10 types of people in the world:
those that understand binary, and
those that don't.

 

[ehsjr]

mg wrote:

Just to clarify a few points.

We have the elapsed timer working well.

We discharge the battery very, very slowly. The elapsed times are
running between 5.5 and 6 hours. Because of the long elapse time, we
can't just have someone hang around or remember to return to the
station at the correct time to check the voltage.

What we want to do is record or hold the ending voltage somehow. As
this is our first attempt at a discharger, we want some way to double
check that we don't drop below 15v. We are pretty green at putting
this together. And let's just say we aren't 100% confident that we
have everything right.

Our first thought would be to use some sort of voltmeter that had an
external method of triggering a "hold". We are open to suggestions.

TIA,
Mark

Mark,

It makes more sense to test the discharge circuit than to
monitor it, in this case. Connect it to a variable power
supply set to say 17 volts, and start the discharger.
Slowly reduce the voltage from the supply, while monitoring
with the DMM, until the discharger turns off. Do that
a few times to make sure you get the same reading, and
you will know what the discharger cut off point is.
You can repeat that test whenever you want.

If you don't have one, you can build a variable supply for
less than 10 dollars worth of parts - a 24 volt dc wall wart
from Allelectronics costs $4.50, and you need an LM317 and
the circuit on page 9 of
http://cache.national.com/ds/LM/LM117.pdf

That will give you a regulated variable supply capable of
600 mA. I don't know what the discharger load draws,
but if it is taking 5.5 to 6 hours to discharge the
packs, 600 mA is more than enough.

I don't remember your discharger circuit - was it the TL431
driving a transistor that operated a relay?

Ed

 

[Palindr☝me]

mg wrote:

Just to clarify a few points.

We have the elapsed timer working well.

We discharge the battery very, very slowly. The elapsed times are
running between 5.5 and 6 hours. Because of the long elapse time, we
can't just have someone hang around or remember to return to the
station at the correct time to check the voltage.

What we want to do is record or hold the ending voltage somehow. As
this is our first attempt at a discharger, we want some way to double
check that we don't drop below 15v. We are pretty green at putting
this together. And let's just say we aren't 100% confident that we
have everything right.

Our first thought would be to use some sort of voltmeter that had an
external method of triggering a "hold". We are open to suggestions.

So, why not try my idea of using a capacitor to store the
value - together with a relay contact that will isolate the
capacitor at the end of the dischage phase?

From the above, I guess that you want to hold the value for
a few hours, so an electrolytic isn't going to manage - so
use a polypropylene instead.

Try this as a quick test: Get say a 10uF polypropylene
capacitor, shove it across a battery to "sample" the battery
voltage - disconnect it and leave it for whatever time you
think you need. Then stick it across a multimeter and take
the immediate* reading and compare it to what the battery
voltage was when its voltage was "sampled". You may find
that the readings are near enough for what you need.

*The voltage on the capacitor will start falling quite
quickly - once the multimeter is connected - so you do have
to take an instant reading.

--

Sue

 

[Dimitrios Tzortzakakis]

I don't know if that will work with the capacitor.However, I have a nicd
battery charger/discharger for my digital camera.The digital camera uses
only partially the charge of the nicd battery, so they need to be discharged
first.mg however wants to find if the batteries are ok, AFAIK nicd batteries
last up to 1000 charge-discharge cycles, and I think IMHO is a good idea to
run the saw from time to time so that condensed humidity will evaporate.The
nicd batteries according to my experience can keep their charge for months,
as I have a pocket light with nicds.

--
Tzortzakakis Dimitrios
major in electrical engineering, freelance electrician
FH von Iraklion-Kreta, freiberuflicher Elektriker
dimtzort AT otenet DOT gr
? "Palindrâ~ťme" <sb382638@hotmail.com.invalid> ?????? ??? ??????
news:11bfutn6tp104a1@corp.supernews.com...

mg wrote:
Just to clarify a few points.

We have the elapsed timer working well.

We discharge the battery very, very slowly. The elapsed times are
running between 5.5 and 6 hours. Because of the long elapse time, we
can't just have someone hang around or remember to return to the
station at the correct time to check the voltage.

What we want to do is record or hold the ending voltage somehow. As
this is our first attempt at a discharger, we want some way to double
check that we don't drop below 15v. We are pretty green at putting
this together. And let's just say we aren't 100% confident that we
have everything right.

Our first thought would be to use some sort of voltmeter that had an
external method of triggering a "hold". We are open to suggestions.


So, why not try my idea of using a capacitor to store the
value - together with a relay contact that will isolate the
capacitor at the end of the dischage phase?

From the above, I guess that you want to hold the value for
a few hours, so an electrolytic isn't going to manage - so
use a polypropylene instead.

Try this as a quick test: Get say a 10uF polypropylene
capacitor, shove it across a battery to "sample" the battery
voltage - disconnect it and leave it for whatever time you
think you need. Then stick it across a multimeter and take
the immediate* reading and compare it to what the battery
voltage was when its voltage was "sampled". You may find
that the readings are near enough for what you need.

*The voltage on the capacitor will start falling quite
quickly - once the multimeter is connected - so you do have
to take an instant reading.

--

Sue

 

[mg]

On Tue, 21 Jun 2005 04:59:53 GMT, ehsjr <ehsjr@bellatlantic.net>
wrote:

Mark,

It makes more sense to test the discharge circuit than to
monitor it, in this case. Connect it to a variable power
supply set to say 17 volts, and start the discharger.
Slowly reduce the voltage from the supply, while monitoring
with the DMM, until the discharger turns off. Do that
a few times to make sure you get the same reading, and
you will know what the discharger cut off point is.
You can repeat that test whenever you want.

If you don't have one, you can build a variable supply for
less than 10 dollars worth of parts - a 24 volt dc wall wart
from Allelectronics costs $4.50, and you need an LM317 and
the circuit on page 9 of
http://cache.national.com/ds/LM/LM117.pdf

That will give you a regulated variable supply capable of
600 mA. I don't know what the discharger load draws,
but if it is taking 5.5 to 6 hours to discharge the
packs, 600 mA is more than enough.

I don't remember your discharger circuit - was it the TL431
driving a transistor that operated a relay?

Ed

Ed,

A very good point about testing the discharger. I'll mention this to
some of the crew tonight (we do training every Tuesday evening).

Whoa.... The circuit on page 9 went way over my head. I'm always
willing to learn something. Can someone translate the circuit into
simple terms?

We used a DPDT relay that looped back to keep itself energized. We
have 660 ohm resistor and an adjustable potentiometer in series with
relay. We used the adjustable potentiometer to set the relay to drop
out at 15v. I don't understand enough electronics to use a TL431. From
the research I did, a TL431 seemed like something we could use. I just
didn't understand it enough. Something like a DPDT relay and resistors
- I can understand. A TL431 seemed more exact than resistors. Again,
if someone can explain using a TL431 in simple terms, I'm all ears.

Just so I don't forget
THANKS to everyone for the suggestions.

I hope we don't have to use our recip saw on a vehicle extrication.
But, thanks to the generosity of folks here, we'll have full use of
the saw and batteries.

Mark
--
Email hint - Everything after the @ is spelled backwards.

There are only 10 types of people in the world:
those that understand binary, and
those that don't.

 

[John Fields]

On Mon, 20 Jun 2005 16:37:51 -0700, Roy Lewallen <w7el@eznec.com>
wrote:

John Fields wrote:
---
from your description, it doesn't sound like you need to hold a
voltmeter reading as much as it does to determine the amount of time
elapsed between when you started discharging the battery and when its
voltage, under discharge, fell to 15V. An easy way to do that would
be to start a clock when the discharge process started and then to
stop it when the battery voltage fell to 15V. A simple comparator
working against a voltage reference, a counter with a 1 second time
base, and a seven-segment display frozen at the time accumulated when
the 15V discharge point was reached would do it.


I've used an even simpler method for many years. The comparator opens a
relay which removes power to an old-fashioned analog electric clock. The
clock is set to 12:00 at the beginning of the test, and shows the
elapsed time at the end.

---
Sounds good to me!

--
John Fields
Professional Circuit Designer

 

[Palindr☝me]

Dimitrios Tzortzakakis wrote:

I don't know if that will work with the capacitor.<snip

Any particular reason why it shouldn't be worth trying?

The 10uF polypropylene capacitor will, of course, start to
discharge once removed from the supply - but the internal
leakage resistance will typically be of the order of greater
than 1,000,000,000,000 ohms (from a spec.sheet) - so it will
take a while (over a year to fall 2%, if that was the only
leakage). There will be surface leakage as well, but that
shouldn't be too great - it isn't as if it is mounted on a
pcb and will need guard rings and the rest. Its voltage
isn't being constantly monitored, so won't be loaded by that
circuitry. So, it should hold the voltage for quite a few
hours, without difficulty.

Of course, once the meter is put across it, it will start to
discharge into the meter input impedance. However, even if
you take 2 seconds to read the meter, the time constant is
such that the value will still be within 2% (and the meter
is probably only accurate to that..). If you take 5 seconds
to read the meter, the value will still be within 5%.

If anyone has a 10uF polypropylene capacitor, a 15 volt
supply and a test meter, perhaps they would give this a try
and report back?

--

Sue

 

[ehsjr]

mg wrote:

On Tue, 21 Jun 2005 04:59:53 GMT, ehsjr <ehsjr@bellatlantic.net
wrote:


Mark,

It makes more sense to test the discharge circuit than to
monitor it, in this case. Connect it to a variable power
supply set to say 17 volts, and start the discharger.
Slowly reduce the voltage from the supply, while monitoring
with the DMM, until the discharger turns off. Do that
a few times to make sure you get the same reading, and
you will know what the discharger cut off point is.
You can repeat that test whenever you want.

If you don't have one, you can build a variable supply for
less than 10 dollars worth of parts - a 24 volt dc wall wart

from Allelectronics costs $4.50, and you need an LM317 and

the circuit on page 9 of
http://cache.national.com/ds/LM/LM117.pdf

That will give you a regulated variable supply capable of
600 mA. I don't know what the discharger load draws,
but if it is taking 5.5 to 6 hours to discharge the
packs, 600 mA is more than enough.

I don't remember your discharger circuit - was it the TL431
driving a transistor that operated a relay?

Ed


Ed,

A very good point about testing the discharger. I'll mention this to
some of the crew tonight (we do training every Tuesday evening).

Whoa.... The circuit on page 9 went way over my head. I'm always
willing to learn something. Can someone translate the circuit into
simple terms?

I'll try a diagram:
D1
+---------|<----------+
| ----------- |
| | LM317 | |
Vin-----+----|Vin Vout|----+--------+--------+---Vout
| Adj | | | |
----------- --- / |
| / \ D2 \ R1 --- C1
| ----- / ---
| | | |
+-----+----------+--------+ |
| | | Gnd
| / | +
+---->\ --- C2
/ R2 ---
\ |
| |
Gnd Gnd


Ok, first D1 and D2 are diodes that are used to protect
the LM317. The symbol ---|<--- shows an "arrow" ( < )
pointing to a vertical line. The actual component is
a little cylindrical thing with one wire sticking out
from each end, and with a white band "painted" on one end.
The vertical line in the symbol represents the banded end
of the actual diode. In your case, you could probably
omit D1 and D2.

Next, C1 and C2 are capacitors that tend to stabilize
circuit operation and minimize ripple voltage. They
can also be removed for your purpose. That reduces
the circuit to just three components, and I'll draw it below:

-----------
| LM317 |
Vin----------|Vin Vout|----+------------Vout
| Adj | |
----------- /
| \ R1
| /
| |
+-----+----------+
| |
| /
+---->\
/ R2
\
|
Gnd

The LM317 has 3 leads, called Vin, Adj and Vout. If
you look at page 2 on the site, you will see which
physical lead corresponds to which place on the
diagram. (I'll draw it below, anyway.) You are
already familiar with resistors and potientiometers -
R1 (240 ohms) and R2 (5K pot), respectively, in the
diagram.

Vin refers to + input voltage. It is the place where
you connect the positive wire from the 24 volt wall
wart. (Allelectronics part #DCTX2460) The other
wire from the wall wart connects to Gnd.
Vout refers to the output voltage (positive). You
connect your discharger to Vout and Gnd.

You adjust the output voltage (Vout) by turning
the pot (R2)
________
| O |
|________|
/_______ /|
| | |
| | |
| |/
-------
| | |
| | |
A V V
d o i
j u n
t

We used a DPDT relay that looped back to keep itself energized. We
have 660 ohm resistor and an adjustable potentiometer in series with
relay. We used the adjustable potentiometer to set the relay to drop
out at 15v. I don't understand enough electronics to use a TL431. From
the research I did, a TL431 seemed like something we could use. I just
didn't understand it enough. Something like a DPDT relay and resistors
- I can understand. A TL431 seemed more exact than resistors. Again,
if someone can explain using a TL431 in simple terms, I'm all ears.

Just so I don't forget
THANKS to everyone for the suggestions.

I hope we don't have to use our recip saw on a vehicle extrication.
But, thanks to the generosity of folks here, we'll have full use of
the saw and batteries.

Mark

Here's the TL431 circuit - 3 resistors, a pot, a relay
a diode, and a momentary pushbutton switch:

Relay
Contact2
/
+---------------------o o---------------+
| |
| Relay |
| Contact1 |
| / D1 /
+----o o---+-----|<--------+ \ Load resistor
| | | /
| __ | | \
+----o o---+---RelayCoil---+ |
| PB | | |
| R1 R4 |
+ | | | |
Battery | ___|__/ |
| \ / / \ |
| R2 /<----------->/ \ TL431 |
| \ ----- |
| | | |
| R3 | |
| | | |
+-----------+---------------+-------------+

D1 is a 1N4001 diode
PB is a pusbutton switch (eg MPB-1 from Allelectronics)
R1 is 2.2K
R2 is a 50 K pot
R3 is 2.2K
R4 is 1K Ohms
Relay - # RLY-622 from Allelectronics

Load resistor - this depends on the rate at
which you want to discharge the battery.
It must be at least 20 ohms, and at least 5 watts.
Part # 50-25 from Allelectronics will discharge
your battery at a rate of about 330 mA.

Ed

 

[Dimitrios Tzortzakakis]

Yes, of course, but I mean it's not a *standard* industrial methode.Of
course, anyone can try whatever he likes, particularly in low voltage
circuits, which are a nice field of experimentation.Mains voltage is a
different story.AFAIK, nicd batteries need discharging in a discharger only
if they are partially used in the appliance before charging them, due to
memory effect.

--
Tzortzakakis Dimitrios
major in electrical engineering, freelance electrician
FH von Iraklion-Kreta, freiberuflicher Elektriker
dimtzort AT otenet DOT gr
? "Palindrâ~ťme" <sb382638@hotmail.com.invalid> ?????? ??? ??????
news:11bh4ksqmujvg87@corp.supernews.com...

Dimitrios Tzortzakakis wrote:
I don't know if that will work with the capacitor.<snip

Any particular reason why it shouldn't be worth trying?

The 10uF polypropylene capacitor will, of course, start to
discharge once removed from the supply - but the internal
leakage resistance will typically be of the order of greater
than 1,000,000,000,000 ohms (from a spec.sheet) - so it will
take a while (over a year to fall 2%, if that was the only
leakage). There will be surface leakage as well, but that
shouldn't be too great - it isn't as if it is mounted on a
pcb and will need guard rings and the rest. Its voltage
isn't being constantly monitored, so won't be loaded by that
circuitry. So, it should hold the voltage for quite a few
hours, without difficulty.

Of course, once the meter is put across it, it will start to
discharge into the meter input impedance. However, even if
you take 2 seconds to read the meter, the time constant is
such that the value will still be within 2% (and the meter
is probably only accurate to that..). If you take 5 seconds
to read the meter, the value will still be within 5%.

If anyone has a 10uF polypropylene capacitor, a 15 volt
supply and a test meter, perhaps they would give this a try
and report back?

--

Sue

 

[ehsjr]

ehsjr wrote:

Vin refers to + input voltage. It is the place where
you connect the positive wire from the 24 volt wall
wart. (Allelectronics part #DCTX2460) The other
Make that DCTX-2460

^

 

[John Savage]

mg <mark_gramlich@oohay.moc> writes:

We discharge the battery very, very slowly. The elapsed times are
running between 5.5 and 6 hours. Because of the long elapse time, we
can't just have someone hang around or remember to return to the
station at the correct time to check the voltage.

What we want to do is record or hold the ending voltage somehow. As

I risk showing my ignorance here, but amongst the bewidering myriad
functions on a digital camera's menu, isn't there one where you can set
a time delay of 5.5 hours? If so, just photograph your voltmeter!
--
John Savage (my news address is not valid for email)

 

[ehsjr]

dmartin@newarts.com wrote:

How about just measuring the battery voltage when you get around to it
after the test?

Once the relay opens the battery should stop discharging.

Of course it'll slowly discharge but if it is any good at all a few
hours won't make any meaningful difference in the voltage you measure.

Say that you load the battery for 5.5 hours (no comparator circuit,
just a timer) and come back to measure its voltage 8 or some other
number of hours later. If the voltage is still above 15 you are sure
that it at least met the 15V after 5.5 hours of load criterion.

If the voltage is below 15, you know it either failed the test at 5.5
hours or was close to failure and self-discharged to an unacceptable
level thereafter. Either way it is no good.

Dave

Suppose he measures the voltage of the battery, about 8
hours after starting. What do each of the following
possible readings tell him?
1) 15.5V
2) 15.4V
2) 15.3V
4) 15.2V
5) 15.1V
6) 15.0V
7) 14.9V
8) 14.8V
9) 14.7V
10) 14.6V
11) 14.5V

I think he wants to eliminate uncertainty.

Ed

 

[Rich Grise]

On Tue, 21 Jun 2005 01:52:00 +0000, mg wrote:

Just to clarify a few points.

We have the elapsed timer working well.

We discharge the battery very, very slowly. The elapsed times are
running between 5.5 and 6 hours. Because of the long elapse time, we
can't just have someone hang around or remember to return to the
station at the correct time to check the voltage.

What we want to do is record or hold the ending voltage somehow. As
this is our first attempt at a discharger, we want some way to double
check that we don't drop below 15v.

You need to let go of this mindset.

Throw the timer away. (or put it back into stock ;-) ) Turn off the
discharger based on voltage, not time. _That_ will ensure that you
don't drop below 15V, _and_ it will ensure that you _do_ _reach_ 15V.

Use the circuit that ehsjr suggested (or any threshold comparator-
type circuit) to turn off the discharger _when the battery voltage
decreases to 15 volts_. That way, you're assured that the discharge
stops when the battery is at 15 volts. It doesn't matter how long it
takes, and in fact, shouldn't be expected to take any particular amount
of time, because it will vary depending on the condition of the
batteries when they're mounted.

Now, if you think it's important, you could put an ordinary line-
operated, motorized electric clock on another set of relay contacts.
Set it for, say, 12:00, turn on the discharger, and go home. When
you come back the next morning, the time when the clock stopped
will be "how long it took for this battery to discharge to 15 V."
(or that plus 12 hours ;-P ) (don't set the clock to the actual
time of day, unless you want to write it down and do arithmetic in
hours & minutes.)

And, of course, you can check the battery voltage at your leisure -
that should hold up better than a capacitor. ;-)

Cheers!
Rich

 

[John Fields]

On Fri, 24 Jun 2005 21:37:15 GMT, Rich Grise <richgrise@example.net>
wrote:

On Tue, 21 Jun 2005 01:52:00 +0000, mg wrote:

Just to clarify a few points.

We have the elapsed timer working well.

We discharge the battery very, very slowly. The elapsed times are
running between 5.5 and 6 hours. Because of the long elapse time, we
can't just have someone hang around or remember to return to the
station at the correct time to check the voltage.

What we want to do is record or hold the ending voltage somehow. As
this is our first attempt at a discharger, we want some way to double
check that we don't drop below 15v.

You need to let go of this mindset.

Throw the timer away. (or put it back into stock ;-) ) Turn off the
discharger based on voltage, not time. _That_ will ensure that you
don't drop below 15V, _and_ it will ensure that you _do_ _reach_ 15V.

---
But he won't know how long it took to get there, so won't know
anything about the "goodness" of the battery.
---

Now, if you think it's important, you could put an ordinary line-
operated, motorized electric clock on another set of relay contacts.

---
Yes, that's already been suggested.

--
John Fields
Professional Circuit Designer

 

[ehsjr]

dmartin@newarts.com wrote:

If I understand the criterion correctly, everything between 1)-6) must
have passed the test.; i.e a known load for 5.5 hours did not discharge
below 15.0V. All others failed. Consider result 7), at best the loaded
battery dropped to 15.0V in 5.5 hours then lost an additional 0.1V with
no load on it in 8 hours. Clearly this is not a battery for long term
standby application.

I agree that he wants to minimize uncetrainty. The test I outlined is
not very uncertain regarding ability to deliver a particular charge in
a particular time and having a low self-discharge rate.

Ok, readings 1) through 6) range from 15.5 to 15.0

So he measures at ~8 hours and his battery reads 15.5.
How does that answer his post:
"We would like to add one additional feature to our discharger. Some
sort of voltmeter that we could hold the last reading. This would
allow us to double check that we stop the discharge process at 15v."

Does the 15.5 volt reading mean that: a) the discharger was not
finished, or b) the discharger was finished, but shut off at
other than 15 volts or c) the discharger shut off at 15 volts,
but battery voltage without load rose to 15.5?

The same questions apply for the other voltages posted.
He ends up where he started - uncertain as to whether the
discharger is turning off at 15 volts.

Ed