Thoughts on battery testing

Batteries are handy things and these new-fangled šŸ™‚ Li-ion ones seem to be fairly useful. I dismantled another unwanted netbook (aka “craptop”) power pack yesterday and acquired another 6 x 18650 cells. Some of these don’t seem to be up to much but I’ve found it difficult to keep track of what I’ve done. That’s been bugging me as it doesn’t make sense to test stuff if you’re relying on your memory to tell you which batteries are “better” than others. More on that later as my ideas start to come together.

I had been wondering about how accurate the dummy load circuit was. Well, why not test it? I put theĀ Ī¼Current Gold in-line with a battery and compared the measured current drawn to the theoretical load of the circuit.

Measuring current

Measuring current

It’s a bit difficult to see but the estimate dummy load is 1003mA whilst the actual current drawn is 979mA. As the measurement continues the actual current falls to about 973mA in this case which, overall, isn’t too bad. I’ll test at other loads but we’re talking about less than 3% here so it’s no big deal in the scheme of things. If a battery test result shows a capacity of 2200mAh then it might actually be about 70mAh less. I would hope that the errors scale with current (i.e. stick at around 3%) but it’s also possible that there’s a fixed error or fixed + some component that scales. Either way, I’d like some estimate of what that error is, just so I know…

I’ve put my dummyLoad Arduino Uno code on Github. At the moment it relies on the user having some kind of serial connection to the Arduino to collect the data. You then have to copy that to a file and try to name things in such a way that you can figure out what you’ve done later. That’s harder than it seems, for me anyway.

What I would like is to be able to uniquely identify each battery and to be able to look at it’s test results however I want. Seeing how battery performance evolves over time might give some useful info and being able to compare different batteries is something I’ll definitely want to do.

Battery comparison

Battery comparison

I’m thinking a stand-alone GUI based application (written in Python and PyQt with a simple sqlite3 database) would fit the bill. This is the kind of stuff I’m working with a lot in the day job at the moment so I know it wouldn’t take long to do something fairly decent. It should interact with the Arduino and allow cutoff voltage to be overridden and the battery under test should be recorded in the database. It’s relatively easy then to add real-time graphs and graphing of historical results. All properly identified so I know exactly what has been done and, therefore, what the results mean!

Right, let’s see how much time I can kick loose over the next few weeks to get this done…

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