How is the discharge of a battery calculated

Thread Starter

chrischrischris

Joined Feb 18, 2012
313
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.
 

Thread Starter

chrischrischris

Joined Feb 18, 2012
313
Look at this PDF for guidance
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?
 

ericgibbs

Joined Jan 29, 2010
18,766
I just don't understand the "C/3"
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.

E
 

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wayneh

Joined Sep 9, 2010
17,496
But what voltage would that bring it to? Is this theoretical 0V?
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.
 
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Thread Starter

chrischrischris

Joined Feb 18, 2012
313
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.
 

ci139

Joined Jul 11, 2016
1,898
some time ago i (i guess -- but i gess it's a near correct memory) came over a PDF (likely) that discussed the topic . . .

?? the cell is empty NOT when the voltage is Zero but .... ??? when the further drop would start to cause the irreversible changes to a battery chemistry

... had to do with the Li-Ion electrode degradation/efficiency . . . i guess
 

MrAl

Joined Jun 17, 2014
11,389
Hi,

It's hard to predict this stuff the best you can do is look at the manufacturers data sheets which often gives curves.
There are estimates too, but you can also do your own tracking by testing the battery yourself and logging the results. Then you know what to expect next time you run it down.

Yes some batteries are damaged by taking the voltage down too low. You have to know when to stop draining it.
 

ci139

Joined Jul 11, 2016
1,898
... some while ago i kept logging the AA AAA 6LF22 Alkalines there seems to be correlation between mass/capacity and some generic formula about their charge . . .

my formula was fairly simple but thanks to over 100 item stat. avg. it allowed
iterativly to restore(read predict the most likely state for) each of the individual 6 cells in the 6LF22 only by knowing it's internal resistance and terminal voltage . . .

. . . as it came out (by mathematical prediction) the 5 of the cells were near their average capacity but one was about 10% of it's full (100%) capacity -- made the battery useless -- you can examine it for used batteries by braking them apart and measuring the voltage of each of it's cells . . . you will note " " some of them could still be used " " (if it were any practical)
 

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debe

Joined Sep 21, 2010
1,389
These are cheap on Ebay. I use it to test the capacity of batterys mainly to see if they are still usable. You set the required minimum discharge voltage, it then discharges the battery to that point & gives the capacity that was avalable..VOLTS.2.JPG VOLTS.JPG
 
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