I’ve been attracted to the unbelievably low prices of the Aldi “Active Energy” rechargeable brand for a while. $6 for 2100 mAh? Sounds too good to be true!

But….most things that sound too good to be true; are too good to be true. The only way I can find out if I’m getting a good deal is to test the actual capacity of the cells to find out what I am really paying for.

To test the capacity of the battery we need to be able to discharge it at a constant current. I built a simple circuit out of an LM358 opamp to do this:

Here the LM358 is used to compare a reference voltage (V2) against an measured voltage (R1):

If voltage at R1 is greater than V2, the LM358 output voltage will decrease.

than V2, the LM358 output voltage will decrease. If voltage R1 is lower than V2, the LM358 output voltage will increase.

This setup will mean that 0.1V will be maintained across R1, from ohm’s law, we can calculate there must be 0.1A of current flowing through R1 as well.

I connected VBat and IBat to a raspberry pi using an adafruit ADS1115 analogue to digital converter and logged the voltage and current every 30 seconds. Once I had gathered enough data I was able to produce a graph of the Activ Energy cell’s discharge curve:

Well, it looks like I didn’t get my full 2100mAh. It would be more accurate to rate this cell at 1600mAh. In this instance, it seems I got what I paid for.

Admittedly, this is only a sample size of one; so it’s possible that I was unlucky and received a bad cell. In the future I’ll test a cell from another batch, perhaps Aldi will be able to redeem themselves in that test.