How to measure battery capacity?

[Bruce W.1]

I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.

I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.

I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.

This could be electronic, if it's inexpensive and I can get the parts at
Radio Shack. But I'm thinking more along the lines of a mechanical
device to drain the battery in a constant current fashion. An electric
motor would just slow down, so that wouln't work. What type of
mechanical device pulls a constant current regardless of input voltage?

Does anyone have any suggestions on how to do this?

Thanks for your help.

[John D. Adamson]

Put an AMP meter across the battery. A "AA" should be good
for about 1.2 amps or so. Make sure the multi meter is in
the 10 amp position before you do this!

Good luck, John w5cgu

Post by Bruce W.1
I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.
I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.
I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.
This could be electronic, if it's inexpensive and I can get the parts at
Radio Shack. But I'm thinking more along the lines of a mechanical
device to drain the battery in a constant current fashion. An electric
motor would just slow down, so that wouln't work. What type of
mechanical device pulls a constant current regardless of input voltage?
Does anyone have any suggestions on how to do this?
Thanks for your help.

[budgie]

Post by Bruce W.1
I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.
I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.
I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.
This could be electronic, if it's inexpensive and I can get the parts at
Radio Shack. But I'm thinking more along the lines of a mechanical
device to drain the battery in a constant current fashion. An electric
motor would just slow down, so that wouln't work. What type of
mechanical device pulls a constant current regardless of input voltage?
Does anyone have any suggestions on how to do this?

You can use a 3 terminal voltage regulator and a resistor or two. They can
be wired for a constant current generator instead of constant voltage.

That was going to be my suggestion, for a split second. Then I
realised the Vin-Vout differential wouldn't let this work on a single
AA.

[budgie]

Post by Bruce W.1
I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.
I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.
I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.

You don't need a constant current sink to determine "true" capacity.
If your load resistor is suitable rated and reasonably low tempco,
then monitoring the voltage tells you the current to a fair degree of
accuracy, so you can even work out how many coulombs are extracted
before a chosen end voltage is reached.

Besides, how many loads in your assorted appliances are constant
current?

We do discharge testing (as part of evaluating Li-Ion charging
systems) using precision logging equipment monitoring voltage while
discharging into resistive loads of known characteristics. The only
limit is the accuracy you require, and in your application a couple of
percent is probably the spec.

Post by Bruce W.1
This could be electronic, if it's inexpensive and I can get the parts at
Radio Shack. But I'm thinking more along the lines of a mechanical
device to drain the battery in a constant current fashion. An electric
motor would just slow down, so that wouln't work. What type of
mechanical device pulls a constant current regardless of input voltage?
Does anyone have any suggestions on how to do this?
Thanks for your help.

[Bruce W.1]

Post by budgie

Post by Bruce W.1
I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.
I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.
I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.

You don't need a constant current sink to determine "true" capacity.
If your load resistor is suitable rated and reasonably low tempco,
then monitoring the voltage tells you the current to a fair degree of
accuracy, so you can even work out how many coulombs are extracted
before a chosen end voltage is reached.
Besides, how many loads in your assorted appliances are constant
current?
We do discharge testing (as part of evaluating Li-Ion charging
systems) using precision logging equipment monitoring voltage while
discharging into resistive loads of known characteristics. The only
limit is the accuracy you require, and in your application a couple of
percent is probably the spec.

====================================================

I could monitor and log the voltage across a fixed resistance, draw a
curve, calculate the area... This would be a real pain in the rear for
the number of batteries I want to test. So I'd really like to find a
simple constant current device.

This Radio Shack digital multimeter does voltage logging on a PC:
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F008%5F002%5F000&product%5Fid=22%2D812
The details on the software are vague. Does anyone have one of these?

Here's a cool device that would do the job, and then some. But I'd like
to keep it just a bit simpler:
http://www.yoredale.uklinux.net/electronics/capacity1cell.htm

[Paul Burridge]

Post by Bruce W.1
I could monitor and log the voltage across a fixed resistance, draw a
curve, calculate the area... This would be a real pain in the rear for
the number of batteries I want to test.

How about using a storage scope to plot the discharge curves and a
computer program to work out the areas under the curves?
--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill

[budgie]

Post by Bruce W.1

Post by budgie

Post by Bruce W.1
I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.
I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.
I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.

You don't need a constant current sink to determine "true" capacity.
If your load resistor is suitable rated and reasonably low tempco,
then monitoring the voltage tells you the current to a fair degree of
accuracy, so you can even work out how many coulombs are extracted
before a chosen end voltage is reached.
Besides, how many loads in your assorted appliances are constant
current?
We do discharge testing (as part of evaluating Li-Ion charging
systems) using precision logging equipment monitoring voltage while
discharging into resistive loads of known characteristics. The only
limit is the accuracy you require, and in your application a couple of
percent is probably the spec.

====================================================
I could monitor and log the voltage across a fixed resistance, draw a
curve, calculate the area... This would be a real pain in the rear for
the number of batteries I want to test. So I'd really like to find a
simple constant current device.

As I alluded to in my previous, I suspect you will find simple
constant current and 1v5 are relatively mutually exclusive unless you
want to go to low voltage op amps and roll your own.

I don't know what you are using to post your n/g messages, but I use a
PC. They DO have other capabilities besides internet connectivity.
It takes but a few lines of code - even in humble QBasic - to log the
voltages at set intervals, calculate current, and do time integration
to obtain joule or coulomb figures. Having a constant current device
as your load won't make this task any easier in reality.

Post by Bruce W.1
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F008%5F002%5F000&product%5Fid=22%2D812
The details on the software are vague. Does anyone have one of these?

The average "affordable" RS-232 equipped multimeter seems to
invariably have a poor overall accuracy spec. This won't affect
repeatability or the validity of comparisons, but does obviously
impact on absolute numbers you obtain.

Post by Bruce W.1
Here's a cool device that would do the job, and then some. But I'd like
http://www.yoredale.uklinux.net/electronics/capacity1cell.htm

[Roy Lewallen]

Post by budgie
As I alluded to in my previous, I suspect you will find simple
constant current and 1v5 are relatively mutually exclusive unless you
want to go to low voltage op amps and roll your own.
. . .

It's not difficult. I use an supply to power an op amp which senses the
current across a small resistor and drives a power FET. It does fine
down to less than one volt. In fact, it was actually designed to use as
a constant current source for charging a NiCd battery from a solar
panel, and it does double duty as a constant current load for testing
battery and cell capacity.

Roy Lewallen, W7EL

[budgie]

Post by Roy Lewallen

Post by budgie
As I alluded to in my previous, I suspect you will find simple
constant current and 1v5 are relatively mutually exclusive unless you
want to go to low voltage op amps and roll your own.
. . .

It's not difficult. I use an supply to power an op amp which senses the
current across a small resistor and drives a power FET. It does fine
down to less than one volt. In fact, it was actually designed to use as
a constant current source for charging a NiCd battery from a solar
panel, and it does double duty as a constant current load for testing
battery and cell capacity.

i.e. roll your own, as I indicated.

[Dave Platt]

Post by Bruce W.1
I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.
This could be electronic, if it's inexpensive and I can get the parts at
Radio Shack.

One possibility is an LM317 voltage regulator, wired up with one
resistor between Vout and Vadj to create a current source (see the
data sheet).

Another possibility is a JFET, with source and gate shorted together.

Problem with both of these approaches is, they probably need several
volts of "headroom" before they begin acting as "stiff"
constant-current sources. If you're hoping to run an individual
1.2-volt cell down to, say, 1.0 volts to measure its useful capacity,
neither of these will work.

One thing you could do is simply discharge through a fixed resistor,
measure the voltage periodically, and use Ohm's Law to calculate the
current being discharged at each measurement time.

[Doug McLaren]

In article <***@NoDirectMail.com>,
Bruce W.1 <***@NoDirectMail.com> wrote:

| I'm trying to find a way to measure the capacity of single -- mainly AA
| -- batteries. Some of my battery packs don't last as long as they
| should because of mismatches in cell capacity.

Well, if you don't mind spending some money --

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXCJG7&P=7

This is a great charger. Not only does it do a great job with my R/C
plane batteries, but it does my ham batteries too. It can charge or
discharge 1-24 cells, and can do NiCd, NiMH, Li-Ion, Li-Poly or lead
acid bateries.

For one battery, you just charge the battery, then set it to discharge
at perhaps 1/4 C, and tell it to cut off at 0.9 volts. Once done,
you'll know exactly what capacity it has.

As another poster mentioned, the R/C communities know all about
batteries. Read their stuff on it, you'll learn a lot. :)

Oh, if you do use a charger like this to charge a battery, be sure to
make sure you've got a direct path to the battery itself -- trying to
push 1 A through a charge circuit that lights up a LED and such is a
great way to let out the magic smoke -- even through the battery can
handle it no problem.

If this is too much for you, there are other alternatives, but few are
so flexible for this `little' money.

[Roy Lewallen]

Post by Doug McLaren
. . .
As another poster mentioned, the R/C communities know all about
batteries. Read their stuff on it, you'll learn a lot. :)

But be aware that the way they're using batteries is very different from
the way you're likely to. So some of the characteristics important to
them might not be important to you, and vice versa.

Roy Lewallen, W7EL

[Tarmo Tammaru]

Bruce,

Here is a crude way to do unattended tests. There are any number of cheap
battery powered clocks that run off 1 or 2 AA cells; you would use external
power instead. Put one of these in parallel with your load, and see how long
the clock runs. You could add an opamp to set the trip point to a particular
voltage. Use an opamp whose common mode range includes the negative rail.

Tam/WB2TT

Post by Bruce W.1
I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.
I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.
I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.
This could be electronic, if it's inexpensive and I can get the parts at
Radio Shack. But I'm thinking more along the lines of a mechanical
device to drain the battery in a constant current fashion. An electric
motor would just slow down, so that wouln't work. What type of
mechanical device pulls a constant current regardless of input voltage?
Does anyone have any suggestions on how to do this?
Thanks for your help.

[Roy Lewallen]

That's a great idea.

It wouldn't really be important to trip at a particular voltage if
you're interested in finding out the capacity, provided that the clock
will run at one volt (which any device designed to run from an alkaline
cell certainly should)(*). There's very little energy left when a NiCd
or NiMH cell reaches one volt, and the cell voltage will drop from there
to the point where the clock shuts off in a very short time compared to
the total discharge time.

Roy Lewallen, W7EL

(*)Unlike NiCd and NiMH cells, an alkaline cell still has a subtantial
amount of energy left when the cell voltage is one volt, assuming a low
or moderate load. Well designed devices running from alkaline cells
should operate down to about 0.9 volt per cell at low or moderate drain,
or about 0.8 volt at heavy drain, in order to extract nearly all the
cell's energy. Ideally, a NiCd or NiMH powered device should disconnect
from the battery at one volt/cell to prevent reverse charging of a weak
cell. Even that isn't sure-fire if there are more than about 5 cells in
series, and if one is considerbly weaker than the others.

Post by Tarmo Tammaru
Bruce,
Here is a crude way to do unattended tests. There are any number of cheap
battery powered clocks that run off 1 or 2 AA cells; you would use external
power instead. Put one of these in parallel with your load, and see how long
the clock runs. You could add an opamp to set the trip point to a particular
voltage. Use an opamp whose common mode range includes the negative rail.
Tam/WB2TT

[Tarmo Tammaru]

Popular Photography tested AA alk batteries by repeatedly firing off the
flash until the recharge time became 10 seconds, or something like that.

Tam/WB2TT

[d***@gmail.com]

Post by Bruce W.1
I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.
I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.
I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.
This could be electronic, if it's inexpensive and I can get the parts at
Radio Shack. But I'm thinking more along the lines of a mechanical
device to drain the battery in a constant current fashion. An electric
motor would just slow down, so that wouln't work. What type of
mechanical device pulls a constant current regardless of input voltage?
Does anyone have any suggestions on how to do this?
Thanks for your help.

[highlandham]

Post by Bruce W.1
I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.
I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.

=======================================
Are you sure ? A Maha MH-C777 I have does single 1.2 V cells.

Frank KN6WH / GM0CSZ

[d***@hotmail.com]

Post by Bruce W.1
I'm trying to find a way to measure the capacity of single -- mainly AA
-- batteries. Some of my battery packs don't last as long as they
should because of mismatches in cell capacity.
I've got a good assortment of electronic equipment, variable power
supplies, high power resistors, etc. I've even got a Maha MH-C777PLUS
charger/discharger. But this doesn't let you do single cells.
I can come up with some -- not so great -- ways to measure capacity,
like discharging thru a resistor while monitoring voltage. This would
allow me to at least match batteries somewhat but not really measure
true capacity. For true capacity I need a constant current device, and
that's where I'm stumped.
This could be electronic, if it's inexpensive and I can get the parts at
Radio Shack. But I'm thinking more along the lines of a mechanical
device to drain the battery in a constant current fashion. An electric
motor would just slow down, so that wouln't work. What type of
mechanical device pulls a constant current regardless of input voltage?
Does anyone have any suggestions on how to do this?
Thanks for your help.