Hi forum, asking for a bit of advice regarding my Pylontech batteries please.
I noticed an abrupt drop in SOC during the early morning one day following 3 days of cloudy weather where the batteries had not reached 100% SOC. On that day, one battery out of the 3 showed only 1 charge SOC light while the others showed 3 SOC lights (from memory). By the afternoon 2pm 2 batteries were showing almost full SOC lights by the one that had started out at 1 SOC LED had only reached 2 SOC LEDs with the 3rd LED flashing, indicating it had not charged anywhere near as much as the others, but the battery was not charging fast, there was almost no solar production even on a bright sunny day, e.g. the battery BMS was somehow not requesting high charging power. I shut everything down and took that battery out of the parallel stack. The remaining 2 batteries continued to work well together after that.
I have 2xUS2000B+v2 and 1xUP5000 running in parallel from my Sunsynk 5kW inverter. the 2xUS2000 are my oldest batteries nearly 4 years old now (2021) and I only added the UP5000 to the system in December 2025. Before adding the additional UP5000 I confirmed with the supplier that it was OK to add an additional new battery to the slightly older existing batteries. Batteries are linked together via the link port and the comms is from the UP5000 master battery to the inverter as recommended. Battery connection to the inverter is from the top battery in the stack for + and from the bottom battery in the stack for - as recommended. There are no errors reported on the inverter or error lights showing on the batteries. The batteries were individually charged to 100% SOC before connecting them together in parallel for the first time.
A while after this event, I had time to physically pull the batteries out of the stack and what I noticed is that there is very slight physical damage/bulging of the case on the battery that I identified as showing the low SOC and not charging full in the same way as the others. It’s one of my older US2000 batteries. When inspecting the case of the other US2000 there is also some very slight bulging in the case. It’s very slight to where just taking a photo of it, it isn’t evident in the photo, maybe I must try to rest a straight edge against the case and take a photo again. So it’s a very slight bulging (I’ve seen much much worse in pics on this forum).
I haven’t connected to battery view software (I don’t have the cable yet) so I have no idea if that would show me some low/high cells. I was planning to get the cable and check, but now that I see the slight bulging I’m sort of thinking maybe I should rather just contact the supplier I bought them from and ask them to check it and return it for warranty if necessary.
My Sunsynk was set to 53.5V charge voltages, which the installer set per the Pylontech user manual (still have the manual, have double checked it). I have read that some have suggested lower voltages so I have now set the inverter to 52.4V in the hopes of protecting my UP5000 which has no bulging (yet?). There was always comms between battery and inverter though.
I guess my question is, is there any way that what I saw could be considered ‘normal’:
- Abrupt SOC drop and then not charging to matching SOC after.
- Very slight bulging on the case.
I’ve sort of tried to rationalize it a little bit in that maybe the SOC became unbalanced because there were so many days in a row of cloud, where the individuals batteries couldn’t do cell balancing internally. And from RC hobby some slight swelling of LiPo batteries (I know it’s a slightly different chemistry) is pretty normal and expected and only if it’s severe would it be a big concern.
Or are my batteries toast and need to be returned ASAP?
