Battery Charging Question...

  • Does the idea that batteries last longer when only charged to 80% still hold when the battery is managed by a BMS?

  • @RickN Correct. TLDR; the BMS protects the cells while in use and charging but not against a stagnating full charge. Realistically if you're using it often enough it's not that big of a concern. Leaving it for months without use and it's recommended to have it sit at a 40-60% charge then charge it before use.

    The 80% charge recommendation is primarily for prolonging the lifespan of the cells in the battery without. Most manufacturers will actually ship cells at 40% charge (depending on chemisry) as it's considered the most nominal state for the cell to be in. Li-ion (or pretty much any lithium battery) is considered unstable at its peak voltage. At this point the cell deteriorates (very) slowly which is why most will tell you to not leave batteries at 100% charge over a few weeks/months without usage. Keeping the charge lower means the cell deteriorates much less. The deterioration is usually just a loss in charge capacity and not a complete cell failure.

    On the flip side leaving a cell too low (like at 0%) over a long period can cause it to fall below it's recommended voltage and cause the cell to lose significant capacity and most BMS's will refuse to try charge it. This isn't as common since most products don't let you get low enough for this to occur under normal circumstances.

    The BMS in the Onewheel can't solve a stagnating full charge itself with how it's made. It will protect the cells from being overcharged, balance cells that have a different voltage and turn off when a short is detected, the voltage gets too low or temperature exceeds the min/max acceptable for operation. There are probably a few other protections in there I've glossed over bu those are the main ones and for the most part that's good enough.

    Some products have smart BMS's that if unused for X period of time will self discharge to a safer state to save life. I've got some DJI Mavic batteries that do this after 2 weeks of not being used which is really handy because I'm always charging a battery and not using it for months on end!

  • @Lia Thx, I was interested in general battery use. I see people throwing out the "If you want the max life out of your batteries never charge them past 80% or use the lower than 20%". As a OW user and owning multiple higher end cordless hand tools, just monitoring the batteries while charging could become a full time job.

  • @RickN Agreed, with battery health you usually end up with diminishing returns. At a certain point you spend more time trying to give your battery that extra mAh lifespan than using the device.

    A problem with never going above 80% is the BMS can't balance the cells since almost all BMS's I've seen do a balance at 100% over a long period depending on how out the cells are form each other. Without a balance charge cells can get so far out from each other that the margin of usable capacity shrinks quicker than any cell degradation ever could.

    It's a science and most after looking deeper into it accept it's not worth religiously maintaining. For the OW my rules are simply if I can ride the next day on what charge I have leftover don't charge the board, if I plan on riding far the next day do a balance charge overnight.

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