One of the notorious details of lithium batteries is the need to keep all of the cells in balance. With a string in series, the total voltage can stay within safe limits but individual cell-level voltages can be dangerously high if other cells are correspondingly low. The trick is to keep all of the cells “balanced”, and that is what LiPo chargers have to do actively. Inside power tools and laptops the balancing is handled behind the scenes; for phones there is no need since they use single cells.
The “cell-level” voltages that matter in my homebrewbattery are the packs that contain 10 (soon to be 20) cells in parallel. I divided the individual cells up into packs to try and achieve similar capacities for all seven, but it was not possible to achieve perfection. Even then, the capacities I measured were at a 1 A discharge current – and actual performance of each pack will also depend on other parameters such as internal resistance. I had no way to predict how well the packs would stay balanced after a few charge-discharge cycles.
In the long term I want to get a nice battery monitoring system that takes care of balancing, but my design goals are to start cheap and bootstrap the battery. To keep with the data-focused approach that has proved interesting so far, I decided to simply log the pack voltages occasionally to “manually monitor” the state of balance. My target was to have the packs stay reasonably balanced for a week at a time, and if that was possible then I’d use a hobby charger to do an active balance every weekend. Here’s the data so far:
After nearly a month the packs are still almost indistinguishable from each other in voltage – the variation is within bounds of +/- half a percent! During October 10-12 the packs became a bit more unbalanced, but that corresponded to the deepest level of discharge during the experiment so far (a week of cloudy days and storms made it hard for the solar charger to keep up with battery use). A more prominent imbalance at deeper discharge is not surprising. Imagine a set of buckets of slightly different size that start full of water, so that they are all at “100%”. If exactly the same volume of water is removed from each bucket, then there will be differing amounts of water left in each. When most of the water is gone, there may be a bucket which is essentially empty and one which is still 20% full – the imbalance will appear enormous. Filling the same quantity of water back into each bucket will return them all to 100% full and exactly balanced. This shows up for my battery, since the packs returned to closer balance when the battery charged after those darker days.
It is interesting to ponder what the “safety threshold” for imbalance might be. The PCM 60x charge controller is configured to reach a maximum battery voltage of 28.6 V, which is about 4.085 V per pack. I’m deliberately stopping short of the 4.2 V upper limit for LiPo cells, because this provides a safety buffer and increases the longevity of the cells. If a pack was undervoltage and another was overvoltage by compensating amounts, the dangerous point would be when the high pack went past 4.2 V. This extreme situation corresponds to the packs pushing past an envelope of +/- 2.8%.
This chart is live – and I’m adding it to my statistics page. Follow along with me as I wait for imbalance to creep in – but at this rate I’ll be fine until Christmas!