How is battery capacity calculated?
 

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.

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.

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

Usually they pick a voltage that lets you get some large percentage of
the total available energy. I think 1.0V is OK for a NiCd, though you
could be more conservative and use 1.1V. There probably isn't much
energy left by the time you reach a volt, at least at modest current.
It also makes a difference what rate you discharge. So for a 500mA-H
cell, if you discharge at 50mA, that's a "ten hour rate" and would be
a common way to rate the cell. If your application draws heavier
current, you'd be advised to test at that current, and you will find
the mA-H value to be lower than at the lower rate.

I'd think you could find lots of info on the web about this, since
it's a common topic. I've seen quite a few articles in various trade
journals about it.

Cheers,
Tom

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

Usually they pick a voltage that lets you get some large percentage of
the total available energy. I think 1.0V is OK for a NiCd, though you
could be more conservative and use 1.1V. There probably isn't much
energy left by the time you reach a volt, at least at modest current.
It also makes a difference what rate you discharge. So for a 500mA-H
cell, if you discharge at 50mA, that's a "ten hour rate" and would be
a common way to rate the cell. If your application draws heavier
current, you'd be advised to test at that current, and you will find
the mA-H value to be lower than at the lower rate.

I'd think you could find lots of info on the web about this, since
it's a common topic. I've seen quite a few articles in various trade
journals about it.

Cheers,
Tom

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?
======
Discharge the fully charged battery at a constant current until the voltage
has dropped to 1.0 volt per cell
A simple constant current drain can be made with a LM317 voltage regulator
used as a constant current regulator up to 1 ampere ,by tying the reference
leg 'downstream' of a resistor connected to the output leg.
Since the LM317 needs some 'head voltage' ,this system works well as from 3
cells in series , hence 3.6 V.
For 12 V and higher battery packs an additional resistor is advisable to
dissipate part of the discharged energy , alternatively you can use a LM
7805 voltage regulator in the same way, without an additional resistor.

Frank GM0CSZ /KN6WH

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?
======
Discharge the fully charged battery at a constant current until the voltage
has dropped to 1.0 volt per cell
A simple constant current drain can be made with a LM317 voltage regulator
used as a constant current regulator up to 1 ampere ,by tying the reference
leg 'downstream' of a resistor connected to the output leg.
Since the LM317 needs some 'head voltage' ,this system works well as from 3
cells in series , hence 3.6 V.
For 12 V and higher battery packs an additional resistor is advisable to
dissipate part of the discharged energy , alternatively you can use a LM
7805 voltage regulator in the same way, without an additional resistor.

Frank GM0CSZ /KN6WH

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.

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