Hi Jim,
There are batteries becoming widely available these days with the lithium/iron-phosphate chemistry (different than the lithium/cobalt chemistry in laptop batteries). These lithium iron phosphate (LiFePO4, or LFP) cells were first used in Dewalt brand lithium power tools. The main advantages are a vastly improved cycle life (2,000 – 7,000 cycles versus 500-800 for lead acid and 500-1000 for NiMH), high discharge current closer to lead acid, better deep-cycling performance, they won’t explode like laptop batteries and need no maintenance, venting or caustic chemical refills and they operate at almost 100% efficiency. If you topped these batteries off once per year they should remain good for a decade or longer.
The only downsides are the price, which is higher than lead-acid (in Dollars per Amp-Hour) but less than other lithium or metal-hydride batteries. There is also the need for a programmable charger, which can be expensive. The cut-off voltage is unique, unlike lead-acid, NiCd and NiMH batteries, and the batteries will be damaged from overcharging. I’ve been using a disassembled Dewalt battery pack