Em 8/2/2011 16:50, alan smith escreveu:
> So in general, given a NiHM cell, the mAh capacity is the amount of curre=
nt it can deliver over a given time into a set load, correct?

Correct.

> I have some unmarked AAA cells, so I charged them with my commercial char=
ger and set up a fixed load (power resistor 75ohm 5W)


It would be better to use a constant current sink to do such measurement.
Cell efficiency varies with the discharge rate. In your case you had a
high discharge rate at first that gradually diminished with time.

The cells deliver more total charge when discharged more slowly.

You could measure the total charge at several discharge currents for
comparison purposes.

With the constant current sink you just need to measure the time until
the cell voltage drops to a certain set-point (usually 1V/cell or
0.9V/cell). The total charge is the product of the time by the constant
current.


>  and then proceeded to log the current it was delivering over time.  The =
curve was not really what I expected,

Did you integrated the curve to obtain the total charge?


>  it showed a slow discharge for about an hour and then a very sharp drop =
to where it then flattened out.


That's exactly the behavior of most batteries.


> Am I measuring this incorrectly to find the mAh capacity of the cell?  Fr=
om what I can see, it tells me that I have a 275mAh to 310mAh cell (those a=
re the starting points to where it dropped off sharply)


You must measure the charge until the voltage drops below the set-point.


Isaac

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