Unique Li-Ion Battery Tester


[moris_zen] found himself with a hand full of Li-Ion batteries and no good way to see what their capacities were. He built a this unique tester really quick to get the job done. He’s using off the shelf components and a cheap pocket watch which he bought in Taiwan for $1. You hook the circuit up to your battery, click the button and the watch starts working. When it stops, you multiply that number by .38 to get your amp/hour results. Sure, he could have just used a counter, but this is an interesting approach. All he needs to do now is make a nice container to hide all of the circuitry. How could he change the circuit to make the end multiplication unnecessary?

16 thoughts on “Unique Li-Ion Battery Tester

  1. i saw something like this on Sam Goldwasser’s RepairFAQ, he was using NiCd’s though. Yet another use for those cheap Pound World/etc clocks :)

    Also, another approach would be a “step” timer with the input hooked up to a 555 timer set to pulse once every n seconds, so you can read the capacity in Ah directly off the clock’s dial.

  2. @pdx: A little of both. Yes, you can go through a resistor, but this way you can reliably time the discharge with a given current draw. Otherwise, you’d have to sit there and watch your meter the whole time, noting down when the battery died.


  3. It would be simple to measure the time with any other device, but the circuit itself is simplicity at it’s best. Though if the “true” mAh rating is what you are after I don’t think you are going to get it with this since the “true” rating is usually done at a 1/5 C discharge rate. By “true” I mean what the factory puts on the label… you know, marketing crap.

    @till: a pseudo deep-discharge is already in place: the white led. Of course you will still have leakage current through the PNP transistor, but that should be negligible unless you leave this thing hooked up for weeks.

    As for making the calculation unnecessary, make it draw 1mA (yes, impossible with this circuit). Then see the number of hours it runs. :D You could make it easier though: change the total resistance (including parallel relay coil) to about 37.5ohms (drawing 100mA) or 3.7ohms (drawing 1A). A 1A draw should be well within the 1C discharge limit of most LiION laptop cells and give more of a “real world” capacity, just make sure your resistor(s) can dissipate that kind of power.

  4. Just glad to see that it doesn’t use an arduino in a switch simulating role. This is a pretty clever way to find the amp hour rating. Might be a bit nicer to change the current draw to simplify the calculation, but the principle is sound and the use of the pocket watch is clever.


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