If you ever drain both batteries down and you only have 12V vehicles around you have a few options, use boosters to charge one battery for 15 or 20 minutes from a running engine ( 13.8 to 15 V ) Then switch your booster cables to the other battery and you should be able to start her up. Always put your 2 cables to the same battery, never to the chassis ground as one batteries negative terminal is at +12V potential. Of course if you have two vehicles, each vehicle can boost one of the vehicles ( again don't let the two vehicles touch, there chassis will be at 12 V Difference potential, you will arc a nasty hole if there two bumpers were to touch) Lastly, these 24 Systems will give you far less problems if you replace you batteries as a matched pair. If you don't, you will be changing batteries for ever as one over charges, and the the other undercharges. ( I have been running 24 LC's for 20 years , and lived the pain )