Last day off for a while so I’ve mostly been “chilling” whilst the dummy load and a little bit of firmware did the hard work. The results I have are interesting but I need more data to draw any conclusions. The firmware (+ any plotting software I may write) will appear here at some point.
The scores on the doors are:
There are 2 different batteries involved.
The “flat” batteries are a bunch of old laptop battery cells that Chris gave me – there are no markings on them so it’s not clear what they are supposed to be. From the above, I’m guessing that they are nominally 1400mAh or 1500mAh but who knows if they used to be anything more. This is the same battery and it was “fully charged” in both tests. My default firmware has a 3V0 cutoff and that didn’t seem to be enough for these cells – that’s why I recharged and ran again with a 2V6 cutoff. So, “fully charged” means the battery charging circuit thinks it’s done – but if you discharge it and charge it again you get a fair bit more in there. Useful to know.
The 18650 batteries came from a netbook (I prefer the term “craptop” over “netbook” to be honest – it gives you a better idea about what you’re dealing with 🙂 ) and are 2.2Ah Samsung cells. Again, I started with a top up of a “fully charged” cell and ran it with the dummy load set at 1A. It didn’t seem to quite get to 2200mAh. Since one of the things I’m currently interested in making is a cheap 1W LED light (using 1W LEDs, obviously, plastic vitamin pill pots and 18650 batteries) then I set the dummy load to 350mA, recharged and ran again. Once more, this “charge after discharge” gives a higher starting voltage and gets much closer to the published limit. I can also see from the data that I get 6 hours of full brightness in this configuration which isn’t to be sniffed at given “full brightness” is bugger bright.
So, early days yet, but it looks like how/when you charge these cells is really important. Unless there’s some kind of “memory effect” (which I’ve never heard of in relation to this kind of battery) then you need to discharge before you charge to get the most out of the batteries.
I’ll push the firmware to GitHub soon but I’ve already got plans to improve it. I’d like to be able to capture the data more easily and provide metadata for each “experiment”.
It would also be useful to override the battery cutoff voltage since these “flat” cells seem to be happy going lower. Unless they’re just fsck’d of course.
The hardware also has limitations. If I supplied 12V to the op amps then I could easily get a voltage of 5V0 from the pot which would allow me to build a 5A version – with a chunkier MOSFET and heatsink. The pot is also too “twitchy” since I didn’t have any “10 turn” ones. Building a “new improved” version will happen but this gives me a chance to play with the software and to do useful (for what I need just now) tests on various batteries. I’m no battery expert – more a battery numpty to be honest – so this should help me learn some stuff.