I'm most interested in the application of replacing lead-acid packs with lithium-ion packs for UPS's and/or whole house/solar power banks. I've seen it stated Tesla's automotive packs are 6s with varying number parallel cells ~40-80p. I imagine the automotive application is on the extreme side of needing the most balancing.
I've not found good documentation on the ideal number of cells in series for balancing. Some concerns include:
- What is a high count of series cells
- Drawbacks and limitations of high count series.
- Do drawbacks consist of slight reduction in performance(less capacity/lifetime), or are the consequences complete failure in short order.
EDIT: I've edited the first sentence for clarity(in bold). My original version was not clear. Also, I only mention lead-acid for setting the context of the use case.
Edit2:
Tesla battery module with 444 cells, 6s74p 21v 7 wire bms
Sources:
https://www.youtube.com/watch?v=g_cC0NKvCDY
https://www.youtube.com/watch?v=44uovhbMPxw
Best Answer
Lithium-ion cells cannot tolerate overcharging at all. So the BMS balance channel is just one cell. Each cell or group of cells in parallel has to be managed independently of the others.
The cells may be arranged into groups or modules for convenience but each cell, or group of paralleled cells, will be managed separately. They are typically arranged in modules of 6-12 cells in series. The number of cells in parallel is not constrained and may be as done as a single cell up to many dozens as in the case of Tesla whose designs use small cylindrical cells. Since they are all in parallel they all have the same voltage so do not need to be treated individually.
In electric vehicles typically the total number of series cells are arranged to give about 400V (or 800v in a few eg Porsche Taycan and Kia). Usually each module of a few cells in series has its own circuitry that communicates with the central manager using opto-couplers or inductive coupling to isolate the hundreds of volts difference.
Balancing is usually only done while charging and diverts a small amount of the charging current from cells that reach full charge first. The balance may be fairly small, only tens of milliamps.
All implementations I know of use this passive balancing, although there are active balancing techniques where the excess charge is used to charge other cells by means of a DC-DC converter or capacitative charge pump they are in general too expensive,
Ni-Mh cells are much more tolerant about overcharge and can be managed as a series group of 6-12 cells. Toyota for example manages the NiMh cells in their hybrid vehicles in groups of 6 series cells with a nominal 7.2v.
Lead-acid may not need managing at the cell or cell group level at all as they have a convenient characteristic that their leakage current increases as they become fully charged. That leakage diverts current from the cell when it becomes fully charged, while other cells in series are still charging.