I had one flat module so I stripped it and charged / tested all cells and couldn't find any obvious fault - reassembled it but it flattend itself again ?
I've admired your setup - nice , It looks like you're running without BMS - How often do you check and how often do you find they need balancing ?
When you charged up the module, did you take note of how much energy was used in charging them up ?, something simple like x amps for y time at z volts ? So you could work out the approx capacity of them ?, i.e. to work out if the cells are normal or end of life low capacity.
Did the module flatten itself whilst running in the original the Growatt box with its BMS ?, i.e. could it be that a faulty BMS is causing this ?
I ran my system as a 24v system without BMS last year for nearly a year and most cells stayed within 0.05v and the worst one was only 0.1v different. This year, the system reconfigured to 48v the cells are similar in variance. I find if you limit the charge to 80% to 90% they stay in balance and you don't need to bleed off a high voltage cell with a BMS because you don't charge the cells high enough to get a high cell.
I use a remote voltage monitor so I can keep a close eye on the bank voltage so I instantly know what the approx charge level is.
I also have a remote voltage test point so I can easily monitor the bank down to two or three cells at once.
So the above checks will alert me to anything going wrong and require more investigation and approx once a month I pick a random bank or two and check every cell. Not found any problems yet.In fact the only cells I have ever had to re-balance were the original ones which had been running in a Growatt box under their BMS before I re-purposed them.
