A self-powered battery tester with an EFM32

in measurement by DP | 3 comments


A self-powered battery tester with an EFM32 by Hlipka:

So the idea that came to my mind was: why not build a proper battery meter which can give real and accurate numbers. That can’t be so complicated! And since the EFM32 is so low-power, why not make it powered from the battery I’m currently measuring? Drawing some additional microamps from it can harldy change the results…

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  1. KH says:

    For useful results, I recommend a multi-load tester. I built a simple one using fixed resistor loads and NFETs as switches, just enough to generate useful information. For example, most new and in-use AA batteries loaded at 1K5 resistor level (about 1mA) are within 0.01-0.03V of the no-load voltage. Dying batteries are much worse, over 0.2V difference. Voltages at different loads can be all over the place while the chemistry struggle to work. So what do we really want to know about the battery? Weak batteries will often drop below 1.0V at only 10mA while giving 1.3V at no load, which is just misleading if we know only the higher number. So if your battery project uses a bit of load, then testing at no load just seems… inadequate.

  2. KH says:

    Ah, TFA does have switched loads. The boost converter thing is troublesome though.

  3. hli says:

    Yes, there are two different loads (though they differ with the battery voltage – its difficult to get a constant current source at that voltage level). The start of my article just doesn’t talk about that…

    I cared about the no-load scenario insofar as that, when the battery is nearly dead, its voltage might drop enough to leave the circuit without power. So the goal was to avoid that.

    The boost converter draws about 300µA, which not too bad IMHO.

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