Unbalanced Charging/Discharging

[pacard17]

hello! need some help from the brain trust!

built a 100ahr lifepo w a jbd bms. using xiaoxiang bms v3.1 1026. thoroughly top balanced the cells to 3.65v. when i put it in use, i noticed that cell 3 was charging faster and discharging faster. meanwhile cell 1 wasnt fully charging. i think this was due to cell 3 hitting charge limits and disabling charging.

so i disassembled it and top balanced the battery again. then i swapped the position of cell 1 & 3. now, cell 1 is overcharging and cell 3 isnt getting to full charge. is this symptomatic of a bad bms? anyway to fix it? or should i just buy another?

TIA,
pc

 

[Bob B]

It seems like the problem moved with the cells .... that would not indicate a BMS problem.

Give us a little more information about your cells and where you purchased them .... but, it sure doesn't seem like the top balance worked for some reason.

 

[pacard17]

no, it did just the opposite. when i moved cell 1 to cell 3, it began charging more quickly. whereas, in it's previously location in the battery pack, it was charging more slowly or not at all if #3 reached it's protection cutoff level.

all batteries were top balanced via desktop power supply w no bms attached. i wanted to make sure they werent adulterated by the bms.

i bought the cells (4s) from batteryhookup.com

 

[Steve_S]

This typically indicates battery cells with different internal resistance values which will charge/discharge at different rates and a BMS cannot correct for that.

Other possible causes:
- poor connections, loose busbars, or dirty (oil/grease) contact surfaces on busbar or batt terminal face.
* Note that even Burrs & Ridges on a busbar can create some issues. Clean everything off with 99% Pure Rubbing alcohol, check for any ridges/burrs around the busbar holes, file smooth and rewipe to ensure everything is clean. Also note that many (most) companies have a light Wax Coating over the aluminium to prevent corrosion during storage/shipping etc.

IF the cell IR is different (unmatched cells) there is not much that can be done as that is within the cell. You can mitigate some issues by being more conservative such as NOT charging above 3.450V per cell 13.8V (Bulk/Absorb) for a 12V Pack. Float at 13.7V (3.425V per cell). General Rule for ABSORB for LFP is 10 Minutes per 100AH of capacity, your EndAmps/TailCurrent is 5A if you have a setting for that with your charging equipment.

 

[pacard17]

This typically indicates battery cells with different internal resistance values which will charge/discharge at different rates and a BMS cannot correct for that.

Other possible causes:
- poor connections, loose busbars, or dirty (oil/grease) contact surfaces on busbar or batt terminal face.
* Note that even Burrs & Ridges on a busbar can create some issues. Clean everything off with 99% Pure Rubbing alcohol, check for any ridges/burrs around the busbar holes, file smooth and rewipe to ensure everything is clean. Also note that many (most) companies have a light Wax Coating over the aluminium to prevent corrosion during storage/shipping etc.

IF the cell IR is different (unmatched cells) there is not much that can be done as that is within the cell. You can mitigate some issues by being more conservative such as NOT charging above 3.450V per cell 13.8V (Bulk/Absorb) for a 12V Pack. Float at 13.7V (3.425V per cell). General Rule for ABSORB for LFP is 10 Minutes per 100AH of capacity, your EndAmps/TailCurrent is 5A if you have a setting for that with your charging equipment.

this is actually what i did to deal w the issue until i could set aside the time to top balance again. glad my hunch was correct. this is my first time building a battery. learned soooo much in doing it.

 

[pacard17]

sorry if i missed someone's post. oddly, the posts visible in this thread depends on how i access them. emailed notification vs website notification vs just refreshing a web page.