sunnuntai 21. heinäkuuta 2013

Call for Top balancers

I'd like to hear from any readers who are using or advocating "Top balancing". Specifically, I'd like to hear how you do it and what science do you think it's based on.

To me it more and more looks like "Top balancing" is something that you can't even do they way you think it can be done. Massaging as much energy in the cells by "boiling" them at a certain voltage doesn't even seem to result in evenly charged cells.

There's a couple of ways you could try to convince me otherwise. First you could "Top balance" your pack as well you can, discharge and recharge. If you have indeed managed to "Top balance" your cells they should all become full at exactly the same time, each and every time. In other words, if "Top balancing" works you'd need to do it only once, just like you only need to Bottom balancing once. From what I've gathered this is not so. And if it isn't, then it doesn't really work. You may think they're all full and balanced at the Top, but what you're really measuring is charge voltage. Nothing else.

Another test I'd like you to do is to "Top balance" your pack as well as you can and then disconnect the whole pack from everything, including BMS and any load. Completely isolated. Let it rest a day or two. Record voltages from each cell. Are they the same? If not, they cannot be at the same State of Charge. Open circuit voltage seems to be indicative or charge state, but only when left disconnected for a long time. This is because the Lithium ions need to settle before the reading is realiable and also the reason why "Top balancing" based on charge voltage doesn't seem to be based on anything.

If you are a proponent of Cell drift, which I haven't seen at all, I'd like to keep those cells disconnected and record the voltage of each cell once a day for as long as you can bother. If cell drift does exist and is measurable, you should see your cell voltages start to go down and differ from each other. Ideally this should be done on a new pack which hasn't been massaged to death by excessive periods of "Top balancing". This is because I'm pretty sure you can see the cells doing all kinds of weird things if you've been torturing them by cooking them at say four volts for days.

Disclaimer: All of my battery ramblings are based on my own experience and Jack Rickard's findings. They are applicable to CALB SE- and CA-series cells. Other cells and chemistries may at least require different voltages.

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