# How to estimate battery capacity?

Discussion in 'General Electronics Chat' started by moeburn, Aug 25, 2013.

1. ### moeburn Thread Starter Member

Aug 16, 2013
31
0
Can I estimate my NiMH battery capacity (at specific load and environment, of course, because I know they have different capacities depending on the load current and air temperature and such) by discharging it at a set rate and measuring the time it takes to get to 0v (assuming that the rated capacity is based on using every last little millivolt, and not just discharging to 1.0v, which is usually considered 'dead' by most applications)?

So if I discharged my batteries at a rate of 1 amp, and it took 2.3 hours for them to reach 0v, this would mean that they have a capacity of 2300mAh (when discharging at 1 amp, at room temperature), right?

How would I go about discharging them at a specific rate, that varies only a little? Would I just use a high wattage resistor, or a lightbulb, or what?

PS I'm well aware that this won't be very exact, but it would be nice to have an estimate, for the purposes of comparing one set/brand of batteries to another.

2. ### paulktreg Distinguished Member

Jun 2, 2008
672
138
Can you not find the datasheets online for your batteries?

3. ### #12 Expert

Nov 30, 2010
17,899
9,318
First, you do not want to take any rechargeable battery to zero volts. You will make it very sick. The published tests stop at a volt or more for a very good reason.

You must declare a smaller range of voltages. For best precision, you would want a constant current load, but it is rather difficult to make them work at only 1 volt.

The compromise position is a resistor, and it doesn't have to be awfully large for a battery that only has about 1.2 volts on a good day. Go to Radio Shack and buy a pair of 1 ohm, 10 watt resistors and keep a log of the discharge voltage and time to get pretty good accuracy.

4. ### moeburn Thread Starter Member

Aug 16, 2013
31
0
Sure I can, but I would never trust the datasheets for Energizer (I'm testing Energizer's new LSD NiMHs), considering that their old 2500mAh batteries are world-renowned for being the worst NiMH on the planet.

I figured that if you dropped the voltage too low, they would die, since that's what happens to Li-ions. One of my circuits (AA->USB charger) drops them to 0.2v-0.3v, is that really bad too?

I happen to have four 0.75ohm, 5 watt resistors lying around, can I use those, log the voltage, and then apply ohm's law to get the current, and then multiply by time to get the mAh estimate?

And thanks for that page, it seems very informative, pretty much all the information I could ever want to know!

5. ### #12 Expert

Nov 30, 2010
17,899
9,318
Yes, and Yes.

Aug 29, 2013
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