rogue cell

[pctree]

So I strung 16 LiPo batteries together after I had them sit in parallel for a couple of days and wanted to charge them mostly full so I could then top charge them with a small bench charger to 3,65V. I hooked up a Daly 16s 48V 150a BMS with all the wires conected and checked the voltage across the pins which all read almost identical at about 3.345 V . These are 230 Ah cells. It was real cloudy today and I hooked it to my Growatt SPF 12000 inverter and was getting about 500 Watts of charge and when hooked up there was 0V across the BMS terminals. I checked about 30 mins later and to my horror the inverter said the battery voltage was 57 volts. I checked the actual battery voltage and it was 53.6 and the BMS had something like 5 volts across it. After I disconected everything I found the second cell from the negative was at 3.66 volts and most were in the 3.4 range.

Any ideas what is going on here ?

 

[BentleyJ]

Are your referring to LiFePO4 (LFP for short) cells or Li-Polymer otherwise known as Li-ion. Their respective voltage is much different. Seems like your cells are Li Iron Phosphate? If this is so then 3.66V is not a problem and 3.4V would be normal just after charging but that much of a difference indicates that Top balancing was not completed before the cells were connected in series.

 

[Craig]

As the prevoius poster said this is not necessarily a problem. However definitely keep an eye on that cell it could be that it is bad and has a lower capacity which would lead to its voltage increasing so fast. If it is a bad cell it will probably drop faster than the others as well. This is why I test each cell independently before assembling a pack.

 

[pctree]

But my main question is how do I proceed? These cells have never been top balanced which is why I was trying to get them to a higher state of charge so that I could balance them but seems like I cant. Shouldnt the BMS prevent this kind of thing ?

 

[pctree]

pluss why was there a 5v difference across the BMS

 

[rhino]

If you are saying one cell was at 3.66V and the others were much lower then you need to discharge power separately from that one cell since it is charged more than the others. You can use something like a automotive halogen light bulb The BMS voltage you are reading if I understand what you are saying is because the BMS turned off charge/discharge to protect that one cell. This is also why some use a separate active balancer to automatically do this for you or a BMS with active balancing built-in like a JK BMS.

 

[TacomaJoe]

I think your BMS did its job and stopped the charging when one of the cells hit 3.66V. When it did that, it is effectively an open circuit. You have the battery voltage on one side and the charging voltage on the other. As noted, you need to drain the 3.66V cell with some sort of load then fire up the charging again. Watch the cell voltage and when one is getting close to 3.66V drain again.

The trick with charging with the BMS (cells in series) is that you can charge the 16 cells quicker than using a 5V supply in parallel.

 

[Skypower]

Had to see this thread. Based on the title, I thought it was a story of intrigue.