I have built SEVERAL 3S7P 18650 battery packs, mostly for a pair of APC UPS's. I buy the battery boxes as a kit from Ali Express. The kits themselves are decent and the materials are pretty good. They can come with a 20A (or so) BMS. I put together around 30 of these about 12-18 months ago where 8 batteries fit into one of my UPS'. Roughly 2 months ago one of the UPS' master alarm went off indicating that there was an issue with the batteries. This tends to mean that the battery/s need replacing. My estimate on LiFePO4 batteries is that they should last far longer than about a year. In the last day or so the second one's alarm also went off.
I also built 2 battery packs for my 24v Dewalt power tools (6S2P, with a 6s BMS) and they work all right for about a month, but both packs are now not discharging at the 24v required for the power tool to operate correctly.
After some research on here it seems that a LiFePO4 battery pack should not just be charged using the internal mechanism of the UPS and rely on the BMS of the Battery Pack to maintain the battery pack. I do not understand why this is... The BMS is wired to each String of 7 cells and is supposed to balance the charge (and discharge) rate of each string. Obviously something is wrong so I would like someone to explain to me what I should actually be doing so that this does not fail (IMO) so quickly.
I also built 2 battery packs for my 24v Dewalt power tools (6S2P, with a 6s BMS) and they work all right for about a month, but both packs are now not discharging at the 24v required for the power tool to operate correctly.
After some research on here it seems that a LiFePO4 battery pack should not just be charged using the internal mechanism of the UPS and rely on the BMS of the Battery Pack to maintain the battery pack. I do not understand why this is... The BMS is wired to each String of 7 cells and is supposed to balance the charge (and discharge) rate of each string. Obviously something is wrong so I would like someone to explain to me what I should actually be doing so that this does not fail (IMO) so quickly.