Susan,

Unlike your mobile devices, your boat battery will gradually fade.  A
voltmeter will let you see how the battery is doing.  When the battery can
no longer hold 12v after charging from your motor, an auto tiller won't
work.

The best way to long battery life is to trickle charge it when you aren't
using the boat.  A solar charger is very convenient for this purpose.

Regards,

Rick