Say for a NiCad or NiMH battery, how is battery capacity calculated?
Say I put a resistor across the battery and measured the voltage
periodically. Is it the area of the curve above 1.1 volts, 0.9 volts,
or what?

Thanks for your help.

You can measure it either in watt-hours or in ampere-hours. The first is
the true energy delivered, and would be the area under a voltage-vs-time
graph at constant discharge current. If current isn't constant, you
would have to measure the voltage and current at each time interval to
be rigorous, plot the product of V and I vs time, and integrate that
function.

However, capacity of NiCd and NiMH cells is just about always specified
in ampere-hours, or milliampere-hours, since the discharge voltage is
fairly constant anyway. That can be measured by simply discharging the
battery at constant current and multiplying by the discharge time. If
the current isn't constant during discharge and you wanted to be
accurate, you'd have to measure the current at various time intervals,
plot that against time, and integrate the result. Of course, a simple
rectangular or triangular integration would be simple to do even with a
spreadsheet, or a very simple program in the language of your choice,
and would be entirely adequate for the job.

But because a NiCd or NiMH cell voltage stays pretty constant between
1.2 and 1.25 volts during the majority of the discharge period, you
could also discharge it with a resistor, then estimate the average
current by assuming a voltage midway between those values, and simply
multiply by the discharge time. That would be close enough for most
purposes.

1.0 volts is the usually specified cutoff for NiCd and NiMH cells. When
the cell voltage reaches that value, there's very little energy left, so
the voltage falls very rapidly beyond that. There's actually very little
energy left at 1.1 volts with a normal cell, but one suffering from
voltage depression (the so-called "memory" effect that's cured by
discharge to 1.0 volt) can deliver quite a bit of energy at 1.1 volt.

Roy Lewallen, W7EL

Bruce W.1 wrote:
Say for a NiCad or NiMH battery, how is battery capacity calculated?
Say I put a resistor across the battery and measured the voltage
periodically. Is it the area of the curve above 1.1 volts, 0.9 volts,
or what?

Thanks for your help.

Roy Lewallen wrote:

You can measure it either in watt-hours or in ampere-hours. The first is
the true energy delivered, and would be the area under a voltage-vs-time
graph at constant discharge current. If current isn't constant, you
would have to measure the voltage and current at each time interval to
be rigorous, plot the product of V and I vs time, and integrate that
function.

However, capacity of NiCd and NiMH cells is just about always specified
in ampere-hours, or milliampere-hours, since the discharge voltage is
fairly constant anyway. That can be measured by simply discharging the
battery at constant current and multiplying by the discharge time. If
the current isn't constant during discharge and you wanted to be
accurate, you'd have to measure the current at various time intervals,
plot that against time, and integrate the result. Of course, a simple
rectangular or triangular integration would be simple to do even with a
spreadsheet, or a very simple program in the language of your choice,
and would be entirely adequate for the job.

But because a NiCd or NiMH cell voltage stays pretty constant between
1.2 and 1.25 volts during the majority of the discharge period, you
could also discharge it with a resistor, then estimate the average
current by assuming a voltage midway between those values, and simply
multiply by the discharge time. That would be close enough for most
purposes.

1.0 volts is the usually specified cutoff for NiCd and NiMH cells. When
the cell voltage reaches that value, there's very little energy left, so
the voltage falls very rapidly beyond that. There's actually very little
energy left at 1.1 volts with a normal cell, but one suffering from
voltage depression (the so-called "memory" effect that's cured by
discharge to 1.0 volt) can deliver quite a bit of energy at 1.1 volt.

Roy Lewallen, W7EL

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

Thanks Roy.

I found a good way to measure the capacity of a single cell. This Radio
Shack multimeter:
http://www.radioshack.com/product.as...%5Fid=22%2D812

It logs voltage (or current) and its software can output the log to a
text file. Now all I have to to is write a little computer program to
calculate the capacity.

"Bruce W.1" wrote in message ...


I found a good way to measure the capacity of a single cell. This Radio
Shack multimeter:
http://www.radioshack.com/product.as...%5Fid=22%2D812

It logs voltage (or current) and its software can output the log to a
text file. Now all I have to to is write a little computer program to
calculate the capacity.

I have a similar meter that seems pretty accurate, and because the
voltage is around half of one of the full-scale ranges, you don't
sacrifice much because of poor resolution (as you would at, say,
2.1V).

An easier way (for those of us who don't want to deal with programming
access to the info) than writing a program is just to import the text
file to a spreadsheet. You then have a column of voltages at uniform
time intervals. If you know the discharge resistance (load
resistance), then I=V/R and you can make a column of that value. The
power at each interval is just V*I -- or just go to that directly as
V^2/R. Then the total energy is the integral of the power over
time...in watt-seconds, just the sum of the power column, if your time
interval is one second. Divide by 3600 seconds/hour to get
watt-hours. Sum the amps column to get amp-seconds and divide by 3600
to get amp-hours.

I've done exactly this sort of thing with my RS-232-interface
voltmeter. Works fine.

Cheers,
Tom

On 19 Aug 2003 11:36:37 -0700, (Tom Bruhns) wrote:

"Bruce W.1" wrote in message ...


I found a good way to measure the capacity of a single cell. This Radio
Shack multimeter:
http://www.radioshack.com/product.as...%5Fid=22%2D812

It logs voltage (or current) and its software can output the log to a
text file. Now all I have to to is write a little computer program to
calculate the capacity.

I have a similar meter that seems pretty accurate, and because the
voltage is around half of one of the full-scale ranges, you don't
sacrifice much because of poor resolution (as you would at, say,
2.1V).

An easier way (for those of us who don't want to deal with programming
access to the info) than writing a program is just to import the text
file to a spreadsheet. You then have a column of voltages at uniform
time intervals. If you know the discharge resistance (load
resistance), then I=V/R and you can make a column of that value. The
power at each interval is just V*I -- or just go to that directly as
V^2/R. Then the total energy is the integral of the power over
time...in watt-seconds, just the sum of the power column, if your time
interval is one second. Divide by 3600 seconds/hour to get
watt-hours. Sum the amps column to get amp-seconds and divide by 3600
to get amp-hours.

That is *exactly* what I suggested in another version of this thread.
(Not claiming anything except that at least TWO ppl find that way
easy).

I've done exactly this sort of thing with my RS-232-interface
voltmeter. Works fine.

Cheers,
Tom

On 19 Aug 2003 11:36:37 -0700, (Tom Bruhns) wrote:

"Bruce W.1" wrote in message ...


I found a good way to measure the capacity of a single cell. This Radio
Shack multimeter:
http://www.radioshack.com/product.as...%5Fid=22%2D812

It logs voltage (or current) and its software can output the log to a
text file. Now all I have to to is write a little computer program to
calculate the capacity.

I have a similar meter that seems pretty accurate, and because the
voltage is around half of one of the full-scale ranges, you don't
sacrifice much because of poor resolution (as you would at, say,
2.1V).

An easier way (for those of us who don't want to deal with programming
access to the info) than writing a program is just to import the text
file to a spreadsheet. You then have a column of voltages at uniform
time intervals. If you know the discharge resistance (load
resistance), then I=V/R and you can make a column of that value. The
power at each interval is just V*I -- or just go to that directly as
V^2/R. Then the total energy is the integral of the power over
time...in watt-seconds, just the sum of the power column, if your time
interval is one second. Divide by 3600 seconds/hour to get
watt-hours. Sum the amps column to get amp-seconds and divide by 3600
to get amp-hours.

That is *exactly* what I suggested in another version of this thread.
(Not claiming anything except that at least TWO ppl find that way
easy).

I've done exactly this sort of thing with my RS-232-interface
voltmeter. Works fine.

Cheers,
Tom

"Bruce W.1" wrote in message ...


I found a good way to measure the capacity of a single cell. This Radio
Shack multimeter:
http://www.radioshack.com/product.as...%5Fid=22%2D812

It logs voltage (or current) and its software can output the log to a
text file. Now all I have to to is write a little computer program to
calculate the capacity.

I have a similar meter that seems pretty accurate, and because the
voltage is around half of one of the full-scale ranges, you don't
sacrifice much because of poor resolution (as you would at, say,
2.1V).

An easier way (for those of us who don't want to deal with programming
access to the info) than writing a program is just to import the text
file to a spreadsheet. You then have a column of voltages at uniform
time intervals. If you know the discharge resistance (load
resistance), then I=V/R and you can make a column of that value. The
power at each interval is just V*I -- or just go to that directly as
V^2/R. Then the total energy is the integral of the power over
time...in watt-seconds, just the sum of the power column, if your time
interval is one second. Divide by 3600 seconds/hour to get
watt-hours. Sum the amps column to get amp-seconds and divide by 3600
to get amp-hours.

I've done exactly this sort of thing with my RS-232-interface
voltmeter. Works fine.

Cheers,
Tom

Now, that looks like a handy little gadget. Thanks for bringing it to
our attention. Should do the job, all right.

Roy Lewallen, W7EL

Bruce W.1 wrote:
Thanks Roy.

I found a good way to measure the capacity of a single cell. This Radio
Shack multimeter:
http://www.radioshack.com/product.as...%5Fid=22%2D812

It logs voltage (or current) and its software can output the log to a
text file. Now all I have to to is write a little computer program to
calculate the capacity.

Roy Lewallen wrote:

Now, that looks like a handy little gadget. Thanks for bringing it to
our attention. Should do the job, all right.

Roy Lewallen, W7EL

Bruce W.1 wrote:
Thanks Roy.

I found a good way to measure the capacity of a single cell. This Radio
Shack multimeter:
http://www.radioshack.com/product.as...%5Fid=22%2D812

It logs voltage (or current) and its software can output the log to a
text file. Now all I have to to is write a little computer program to
calculate the capacity.

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

I wrote the program and will make it available to anyone that wants it.
I runs on Windows and the .NET Framework. Works fine. The only problem
I have is remembering to disconnect the battery from the resistor after
it hits one volt, Doh! Some of them were pulled down to 0.4 volts
(under load) but they recover to about a volt at rest.

Yet one question still lingers in my mind. Is a discharge voltage to,
say, 1.1 volts under load or at rest?

The end discharge voltage (generally 1.0 volt per cell for NiCd and NiMH
cells) is measured under load. After disconnecting the load, the voltage
will typically rise substantially, but its value isn't of any significance.

It's usually not recommended to discharge below 1.0 volt, but it doesn't
seem to cause any harm, at least if it's not done really often. I think
a cell is more likely to grow dendrites and short if it's left in an
extreme discharged state for an extended period, so it's probably a good
idea to put at least some charge back in before too awfully long if
you've discharged it particularly deeply. What is harmful is reverse
charging of the cell. But that happens only when you have an external
source of current, like other cells in a series connected battery.

Roy Lewallen, W7EL

Bruce W.1 wrote:

I wrote the program and will make it available to anyone that wants it.
I runs on Windows and the .NET Framework. Works fine. The only problem
I have is remembering to disconnect the battery from the resistor after
it hits one volt, Doh! Some of them were pulled down to 0.4 volts
(under load) but they recover to about a volt at rest.

Yet one question still lingers in my mind. Is a discharge voltage to,
say, 1.1 volts under load or at rest?