DFord wrote:
I second the advice to buy true deep cycle golf cart batteries. There are no 12 volt batteries that will perform nearly as well in this application.
Other than that, keeping the batteries connected to a BatteryMinder whenever possible will help prolong their life. These chargers pulse the battery after their fully charged to prevent the sulfation that kills batteries. The cheapest place to buy them I've found is Northern Tools - https://www.northerntool.com/shop/tools/product_200332201_200332201
You don't have to keep the charger going continuously, just plugin for a day or two every month.