Say you are a heavy AH user and going out for four days and you will need to do a "50-90" on the second day
You have 460AH in the four sixes and by the second day you are at 50%. But now you can only recharge to 90%, so for the second set of two days you are short 10% going in, because on the first set you started from full, not from 90.
After doing the 50-90, you decide you don't want to drop to 40% the next set, so now add the spare 12 with the jumper cables. This is like having that 10% back. 10% of 460AH is 46AH and 50% of the 100AH 12v is 50AH.
