It also keeps the parallel strings isolated from each other so that a pack with a slightly higher voltage will not discharge and attempt to charge a weaker pack.
it is most noticable on SLA and NiMh packs where each parallel string has it's own self discharge speed. the pack the string that discharges the fastest will drag down the other string.
Same effect occurs with parallel strings where one sags more than the other under load.
or any other situation where one string has a higher voltrage than the other.
I thought that one string, or battery balancing the other was the whole purpose of putting the sub-packs in parallel. Admittedly, in the case of SLA's it is usually better to use "buddy pairs" than parallel strings.
And, how do you charge the pack with the diode in place?
Also, how does one BMS cutting out (opening the circuit) damage the other BMS?