Voltage difference from 2 cells of group 4 in Lithium battery combined for 20s

Battery Management
1

My DIY power supply for my motorbike
20 Lithium FePo4 33-1400 15Ah cells arranged in order of 20 and grouped into
Group 1 Group 2 Group 3 Group 4 Group 5
cell1→4 cell 5→8 cell9→12 cell13→16 cell l 16→20
After 4 months of use, I observed a difference of 1V between group 1 and 2. Same as 12V and 13V. The rest have the same voltage of 13V…
Is this normal or is there something wrong? From what?
I don’t use BMS module. Use only 1.2A active balancing module.

2

Hi,
This is normal to observe after the battery packs go through multiple charge/discharge cycles after some time. Is your active balancing connected between groups? or at Cell 20 and Cell 1? if the cell drop was higher, then you can anticipate a rogue cell in the group.