Circuit to choose/correct battery polarity

I would load the battery to at least 25 mA when testing.

It would probably be better trying to measure the IR drop or the resistance of the battery. Not sure exactly how to do that except measure V at two different loads and look at (delta V)/(Delta I). Re-arranging the equations should work out using two loads too.

The Effective Series Resistance is a better parameter to look at.


Joined Mar 14, 2008
My experience has been that the open-circuit voltage of an alkaline battery is a fairly good indicator of its capacity remaining.
> 1.5V is near full capacity, 1.2V-1.3V or so is about 50% capacity and 1V is near dead.
I often pull batteries from devices I have that stop working at about 1.2-1.3V battery voltage and put them in clocks or my mouse, which often go down to <1V before they die.
My mouse will still typically operate for several months and the clocks typically for 6 months or more.

Thread Starter


Joined Nov 25, 2018
Are you planning on loading the battery during the test?
If so, how much current?
Thank you. The testing of the voltage is one phase. This is for a battery charging project (grouping several used batteries of close capacity/voltage to charge a larger battery). So, if this charging happens during the voltage test phase, then after the test is complete, the battery would be loaded in order to charge.

I hope this makes sense. Thank you!

ian field

Joined Oct 27, 2012
If voltages are not large, I'd use transistors instead of relays. Faster, no spark issue (so safe anywhere), no snubber needed, and no time delays due to EM field charge/discharge time.
TV horizontal output transistors could be found with ratings up to about 2kV - so transistors don't have to be low voltage. But we're talking power transistors with gain figures right down to 2. Nixie driver discrete transistors usually weigh in somewhere around 400V. Cfls for UK mains are about the same, many lower power units have TO92 packages.


Joined Mar 14, 2008
How are you going to measure the battery voltage?
Note that the scheme I proposed needs the circuit power ground, isolated from the relay output measurement ground.