Infrequently used batteries in storage will self-discharge over time, potentially leaving you with damaged cells amongst a myriad of other issues. Instead of plugging in the wall long-term, harness the power of the sun with an SCA 12V Solar Battery Charger. The perfect option for camping batteries, or anything being left long term - just connect the clamps and set the panel up in a sunny spot inside and you're ready to go. The amorphous solar technology is still effective in lower light conditions, so the power will be flowing even on cloudy days. This unit is not waterproof, so it is most effective when placed next to a window with lots of natural light.
This 1.5W solar charger panel is capable of maintaining 12V batteries up to 550CCA/40AH capacity - great for small cars and garden equipment like ride-on mower batteries. Don't waste money replacing infrequently used batteries, use the power of sunlight to keep your cells in tip-top condition.