Li-ion Battery Capacity Testing

Well I’m just getting into the wonderful world of li-ion batteries – cheers Chris!

The trouble is, I’m not sure where some of the older, recovered, laptop batteries I’ve acquired are at. Some of them really don’t seem to last as long as I expect, but then I don’t know what the capacity is supposed to be so…

Yesterday, I ended up watching a Dave Jones EEVblog video on “Constant Current Dummy Load for Power Supply and Battery Testing“. Well, it seemed to fit the bill 🙂

This circuit was shown…

What it says on the tin

What it says on the tin

So, I’ve got a pile of op-amps which I don’t know how to use. I’ve got N-channel MOSFETs, which I don’t know how to use…

I think I may have watched another Dave Jones video on the basics of op-amps but, whatever, this looked like a chance to test some batteries.

I had a bag of TS-358 single supply, dual op-amps which seemed to be close enough to what Dave used. I also had a FQP30N06L MOSFET and, again, that seemed “close enough”. So, I gave it a go starting with everything on a breadboard. That mostly worked, although I couldn’t get the current to be more than about 0.75A and it was obvious that the MOSFET was getting pretty toasty – fair enough, it should in this configuration. Since it seemed “close enough”, I chucked it all on some proto board…

Dummy load and friends…

As you can see, I added the biggest heatsink I had available and a 5V fan. Everything runs much better away from the breadboard and I can dial this up to a load of 1100mA. It’s all mounted on a Proto-Pic Arduino and Breadboard Baseplate that I had lying around – they’re not cheap at around £8 so that’s easily the most expensive part of this.

I want to add an LCD display so I used an “Arduino Uno” – well, it’s a cheap, as in $2.88 delivererd cheap, chinese version from here. (Shock horror! They’re now back up to $3.10, I must have been lucky 🙂 ). I’ve enjoyed making my own arduino clones on proto board but the damn proto board costs more than one of these things!

The scary yellow wire isn’t connected yet. That’s for sensing the battery/PSU voltage – only needed for info and to switch off at some predefined voltage if I decide to implement that. I probably will.

There’s a (yet to be) home made LCD shield to go on top of the arduino and I need to put a voltage reference on there as well as a couple of voltage dividers. I may sneak a transistor on the proto board so I can switch off the circuit if I want – that should be fairly straightforward.

So, not finished yet and there’s some calibration to do as that 1Ω resistor isn’t 1Ω. Calculations from the breadboard suggest it’s more like 1.6Ω but I’ll measure again since I really don’t trust breadboards with op-amps after what I’ve seen today.

All looking good, just need to get it finished and test a few batteries.


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