Besides cost, can someone tell me why people don't use batteries for an inverter that is maintenance-free? (no topping off of water required)
Absorbed Glass Mat (AGM)
AGMs are built to better stand up to repeated draining and recharging cycles than standard batteries. They are becoming standard equipment in more cars because modern features such as fuel-saving stop-start systems, electronic safety and convenience features, and power outlets for mobile electronics all increase the demand for power.
But AGMs can cost 40 to 100 percent more than highly rated conventional batteries. Consider buying one if you sometimes don’t use your vehicle for long periods and the battery loses its charge. An AGM battery can better tolerate a deep discharge, and it is more likely to fully recover if it is accidentally drained. "