Each time a battery is run to a low point, it loses life, it will not last as long. Do not allow a battery to fall under a voltage of say 11 volts. FYI, a low voltage can kill the fuel pump, and some other sensitive electronics.
You should have a voltmeter of some kind. If the voltage is under say 10 volts, then the engine will be harder to start than if you disconnect it. Once a battery gets that low it needs more charging than just a casual drive. If it gets that low than just to start the engine, one battery cable should be removed.
We are trying to help, it is very hard when you don't know what any of the voltages are. Regards,