Comment by awakeasleep

4 years ago

The “charge to 80%” advice isnt generally useful to PEV riders in 2021

It is true that a lithium _cell_ lasts longer when charged to 80%, but a _battery of cells_ must be “balanced” and PEV BMSes only do that balancing in the constant voltage stage of charging in the hours after the charger indicates the battery has been fully recharged.

For anyone just learning about this, an unbalanced cell is usually the cause of a battery going “dead” and can even be the cause of a fire. Very few batteries die because they have outlived their charge cycle count.

This is different than your cell phone which has at most two cells (but usually one) in the battery. In that case, the BMS can monitor and charge individual cells. In a PEV the BMS usually monitors several groups of cells, and can not charge individual cells.

This can change in the future! I believe electric car BMSes generally do it. But its an added expense and PEV manufacturers have not started spending the money. Ill be happy if another commenter can correct me and show that there is some manufacturer who is ahead of the curve.

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awakeasleep

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labawi  4 years ago

By charging and balancing individual cells, you mean group of cells that are connected in parallel (the 4P in 12S4P)? I don't see how or why would you balance individual parallel connected cells, unless you had a different battery configuration.

Simplest "balancers" drain cells that have too high a voltage as you are approaching full charge. You do need to occasionally top the battery off, to keep it balanced. Seeing they already have sense wires and discharge resistors, they could fully monitor state and balance even without top off, but that's additional code complexity.

There are nice BMSs that can shuffle charge between cell groups - they increase range by helping the weakest link, and should help longevity, by again, decreasing strain on the already worst cells. I don't know how common they are, just that they are not typical. There you can also have good and sub-optimal algorithms.

I have heard of individual cell monitoring and maybe balancing, but I have no idea what, how or why it does it.

  • jacquesm  4 years ago

    The pack I built has a balancer, that's what all the red wires on the photograph are for. It uses a charge pump to move some charge around so that the high cells get clipped and the low ones brought up. It keeps the pack within 2 mV. The unit it monitors is the cell group, all 10 of them have a wire to the balancer and a ground for reference. It is very accurate and it wills substantially contribute to the life-span of the pack.