Would be worth testing the voltage of the cells, but lithium really doesnt like staying fully charged (which is also why it's bad practice to charge phones to 100%), so assuming the group charges the cells to full voltage, that voltage is going to drop a bit to a level the cell is happier with. So for simplicity i wouldn't read much into what happens between 100 and 95, if not 100 and 90. Now if the group drains itself from 90 down to 50 for no apparent reason, then either there's a strange ghost load, or the batteries are trash. Or both.