21700 45a

[Blackgolem]

Hello community real short I have a jkbms and I'm using the 21700 45a cells to get 72v but pack won't fully charge only gets to 79v why could someone tell me how if they have same setup. Oh pack is 20s 4p controller is 60a in the ap it only allows me to set SOC to 4.18v not 4.2? HELP.

 

[harryS]

Even at 4.18V, you should see more than 79 V if the battery is balanced. I suppose you can use your JK app to read out the cell voltages to see if they're balanced, Are all twenty banks at 3,95 volt? Or are a few at 4.18V and the charger has stopped with the rest below that value?

 

[Gionnirocket]

Even at 4.18V, you should see more than 79 V if the battery is balanced. I suppose you can use your JK app to read out the cell voltages to see if they're balanced, Are all twenty banks at 3,95 volt? Or are a few at 4.18V and the charger has stopped with the rest below that value?

Exactly what I was going to suggest... but I got lazy when I first read it
Getting the impression this is a new pack... Did you make sure your cells manufacturer lot and internal resistance were all matched?
Also just double check your chargers output voltage.

 

[Blackgolem]

Even at 4.18V, you should see more than 79 V if the battery is balanced. I suppose you can use your JK app to read out the cell voltages to see if they're balanced, Are all twenty banks at 3,95 volt? Or are a few at 4.18V and the charger has stopped with the rest below that value?

Exactly the case so does that mean(in my mind) I need to rebuild the pack and charge all cells individually to 4.2v ?

 

[Blackgolem]

Here's the app read out of voltages

 

[Blackgolem]

Exactly what I was going to suggest... but I got lazy when I first read it
Getting the impression this is a new pack... Did you make sure your cells manufacturer lot and internal resistance were all matched?
Also just double check your chargers output voltage.

Well I guess I'm not able to check the IR and I didn't get a whole batch at once so that could be the problem but I've built a few personal packs b4 without checking those parameters and they've worked fine for years. I would just hate that these 21700 45a cells are trashed. Thanks to you and gionnirocket for responding

 

[Gionnirocket]

I have no experience building packs...
But from what I understand you currently have a 0.23v voltage differential. If the BMS has a balance feature, which it seems to have, I'd try charging it at a very low amperage to see if the BMS can correct things. It may take a few attempts.
If the internal resistance of the cells varies too much it might not be possible or the fix will only be temporary.
Rule 1 in building a pack is to have the cells as closely matched as possible. That's why matching manufacturer lot is important.

edit:
Found this on the manufacturer website.
Look how close the numbers are here for both voltage and resistance.

 

[harryS]

If you've built packs, then you know it can take a long time to balance them, and it is unbalanced. Make sure your BMS is set to balance and let it charge for a day. I don't have a smart BMS so you have to RTFM to figure why it's maxing at 4.18,

If they are all cells of the same make and roughly the same age, they should be able to balance.