Remember... there are two batteries..
2 batteries in series.... Again not gonna do it. Your best/safest bet is to remove the batteries and charge them in a real charger then replace them in opposite positions so you get even wear on the batteries. Label them A and B or 1 and 2 if it helps. So battery one is on the left two is on the right. Then after charging them put them back in with battery one on the right and two on the left. Switch them like that every time you remove them and charge them.