# How is the discharge of a battery calculated

Discussion in 'Power Electronics' started by chrischrischris, Oct 5, 2016.

1. ### chrischrischris Thread Starter Member

Feb 18, 2012
287
10
If an NiMh rechargeable AA battery is rated at 2550mAh and can charge to 1.44V, how can I calculate hold long it would take to drop to 3V under a load of say 50mA continuous draw?

I assume if the "capacity" is 2550mAh, at 50mA draw it could last 2550/50 = 51 hours. But what voltage would that bring it to? Is this theoretical 0V? So for 1.44V drop (to 3V) would I interpolate the value. I assume it must not be linear. Sorry if this is a silly question.

2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,581
389
h c,
Look at this PDF for guidance,
It will depend upon the age of the battery and the number of charge/discharge cycles .... etc
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3. ### chrischrischris Thread Starter Member

Feb 18, 2012
287
10
Thanks Eric. There's alot of facts on that pdf, but not really a clear answer for me. I do realise that if the battery is old and has been charged many times, it's duration will decrease. But I'm still not understanding how to calculate the discharge. It does state:

"For nickel-metal hydride batteries, the rated capacity is normally determined at a discharge rate that fully depletes the battery in five hours". It then shows a graph in "figure 5" showing something about "C/3 discharge IR test (750mA / 10mA pulse)".

If this is the graph to look at, then it seems that from 1.44 to 1.3V is about 10% of the time (so 10% x 5 hours = 1/2 hour time to reach this lower voltage). Then to 1.3V to 1.2V is about 60% of the total discharge time (3h).

I just don't understand the "C/3". are they saying the 750mA discharge is C/3? Does that mean the C (battery capacity?) is infact 3 x 750 = a 2250mAh battery?

Jul 11, 2016
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5. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,581
389
Hi c,
It means the rated AmpHr of the battery divided by 3, so the C/3 of a 2.55AHr battery is 0.75Amps.
This means that if the battery is in a 100% 'condition' and fully charged it will deliver 0.75Amps for for 3 hours, at which time it should be disconnected from the load and recharged.
Be aware that at different C/x rates the discharge curve and discharge time will not be the same.

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6. ### wayneh Expert

Sep 9, 2010
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3,224
No, a "discharged" battery is not taken to zero voltage, since this is very damaging to most battery chemistries. For instance a car battery is considered fully discharged at, as I recall, ~11.5V (depending on load and temperature). Obviously there is still some charge in there.

The link in #2 seems to suggest that 0.9V is a good stopping point for NiMH chemistry.

Last edited: Oct 5, 2016
7. ### chrischrischris Thread Starter Member

Feb 18, 2012
287
10
Thanks Ci for that link. Yes, it does seem a bit vague.

Eric, nice graph. I'll try to interpolate from there.

Wayne, yes I agree. It seems like the lowest "recommended" discharge voltage is 0.9V per cell, even though it can go to zero. But there it seems to affect the cell chemistry adversely. I know my charger doesn't want to charge a cell lower than a certain voltage (I think lower that 1V).

I think the best here will be for me to do a trial discharge with an Arduino connected just sitting there and plot the voltages. Thanks everyone for your help.