Yet another Lifepo4 charging question

Question

I have 16 * 3.2v 12AH Lifepo4 cells. I would like to use them as a 8*2 configuration, so to get 24AH 24V. I need 24V but could do with 12AH, 24AH would be better do. Now my question is, would it be ok to use 1 24V (8 cells) BMS and connect this BMS to the 2*8 packs in parallel, that is, charge 2 cells at the same time. Or would it be better to use a separate BMS for each 8 cells pack but then connect the 2 packs in parallel.

Answer 1

Would it be ok to use 1 24V pack (8 cells) BMS and connect this BMS to the 2*8 packs in parallel - that is, charge 2 cells at the same time. Or would it be better to use a separate BMS for each 8 cells pack but then connect the 2 packs in parallel.

Probably the latter, because:

• Most BMS's are expecting/designed for the load of one cell.
• You are already using high-power (12Ah) cells.
• If one cell fails, it is easier to spot and fix, and doesn't take out it's neighbor.
• The more protection that can be thrown at lithium batteries in general, the better.

Note that not all BMS's charge the cells, or are very clear about how they charge them. You may want (or have to) find or build a charging solution specifically for LiFePO4 chemistry and this particular pack configuration. There are many such charge management IC's for LiFePO4 if you are interested in rolling your own. It's a lot of work, but then there is no guesswork as to whether it will charge correctly or not.

LiFePO4 cells require very specific charging and management characteristics which are more stringent than other rechargeable types. A lot of BMS's I see out there SAY they are for multiple cell types, but don't trust this. Look at the datasheets for the cells and BMS and make sure they agree. The better ones could have a selector jumper for the type of cell used, so the charging characteristics can be fine-tuned. (Accidentally allowing a LiFePO4 cell to reach lithium-ion's 4.0v or 4.2v because the title stated LiFePO4 was supported but actually was not... could have disastrous consequences.)

For example, this one gives very vague specifications and no official datasheet. "Single overcharge protection voltage: 3.75V" - this exceeds the charge cut-off of the LFP-26650-3300 cell.

That said, LiFePO4 seem to be among the safer of the various lithium battery types available currently, so wise choice. Remember to consider thermal effects with such a large pack.