Testing batteries by measuring their short circuit current, why it doesn't work?

Question

I've experimented a little bit with some of my old, rechargeable batteries. (They "slept" around 5 years in a cabinet. I've long forgot their previous charged status.) What I did:

1. I've measured their voltage. It was okay, maybe a little bit lower as their nominal voltage.
2. I recharged them: I gave them around 12 hours in a battery charger. Unfortunately, this battery charger didn't give me any information, with the exception of a led showing, some type of charging actually happens.
3. I've measured their voltage again. This time, it was better.

Earlier I experienced that completely (or near completely) exhausted batteries give their nominal voltage measured by a multimeter, but it doesn't mean, that they could produce usable power on load.

My idea would be, that if measuring the voltage doesn't show anything about their power, maybe measuring their short circuit current would. Yes, I know it harms the battery, but I think, for the some seconds while I read the current on the display of my multimeter, it is practically negligible. And knowing the voltage, and their short circuit current will show, if they are really usable or not.

One of my friends (professional electric engineer) said, it is a bad idea, but he didn't explain clearly, why.

So, why is it a bad idea? How could I test a battery, if I have only a multimeter?

(P.s. they are NiMH batteries, if it matters.)

Answer 1

It's generally a bad idea, especially if you don't know with 100% certainty a lower bound on the internal resistance of the battery. As a secondary concern it may damage the battery.

That said, I do this myself briefly with alkaline batteries because I know that the cell or battery can produce only a few amperes into the 10A multimeter input. I would not do it with any other kind of battery, including NiMH.

Some battery types can produce much larger currents- enough to damage themselves (heat up, vent electrolyte, explode, even), melt the test leads causing severe burns etc.

The proper way would be to apply a reasonable load (reasonable depends on the battery chemistry, size, design etc) and measure the voltage. NiMH is a bit thorny since it's difficult to determine the charge condition from the terminal voltage. It's better to discharge with a moderate load to a known discharge condition (perhaps 1V per cell or a bit less) then charge from there. Overcharging NiMH batteries causes irreversible cumulative damage.

Once the battery has charged properly you can discharge it into a reasonable load and measure how long it lasts (taking care not to discharge it too much, which can also cause permanent damage).

Answer 2

Short circuit test of batteries is a bad idea because it can damage the batteries. Lithium polymer battery have a large discharge current on short circuit it may explode. You can discharge the battery using a proper dummy load for testing the capacity of the battery.

Answer 3

The proper way is to measure internal resistance.

You do that by drawing an AC current, which can be small. Simply switch a resistor across the battery with a FET, or use a switchable current source...

Now, an AC current will cause an AC voltage to appear on the battery, depending on its internal resistance, U=RI after all.

If your ADC has tons of bits you can acquire this voltage directly. Or you can use a cap to AC-couple the battery voltage, then amplify the AC part with an opamp, and acquire it with a standard microcontroller ADC.

Synchronizing the ADC with the current pulses will give you two voltages (with and without current applied) and the difference will give you internal resistance. You can average over several cycles to reduce noise if needed.

Answer 4

A few seconds can cause irrecoverable harm depending on the cell type/chemistry. Using a reasonable load (power resistor) is a more appropriate approach. Stepping the DC load (eg, 1A -> 2A) is a common method for gaining insight into cell health and determining equivalent DC resistance of the cell.

Answer 5

If you really want to delve into how good your battery is working, then you need to monitor the voltage and current over time and find the charging cycle and then compare it to curves for healthy or bad cells.

Comparison of the discharge voltage of an alkaline battery (red) and a NiMH battery (blue). The green line is the voltage at which the battery is considered dead.

Source: Using Nimh (This link will tell you that you might need a better charger)

You could do this with a multi meter and a variable power supply. However it would require you to log the voltage and current every so often and put it in a table and then graph it out.