One lithium battery. The work of two AGMs, for less per year.

In usable power, roughly yes. You can use nearly all of a lithium battery's 1,280 Wh, but Renogy advises not running its AGM below 50%, so a 100Ah AGM gives about 600 Wh. Two 100Ah AGMs (1200 Wh usable, 128 lb) match one 100Ah lithium (22.05 lb).

If you use the battery regularly, yes. One Redodo 12V 100Ah cost $242.89 and two Renogy 100Ah AGMs $417.98 when we checked. At 100 uses a year the lithium works out to about $24 a year and the AGMs $84, because the lithium is rated for 4,000 full cycles against 500 half cycles. If half of one AGM's capacity is all you ever need, a single AGM is about $34 cheaper up front.

Usually, for house, trolling and solar batteries. Check that your charger, converter or solar controller has a lithium (LiFePO4) setting, and turn off any equalize or desulfation mode. Don't use these deep-cycle lithium batteries to start an engine.

Brand, warranty and where they are built. Battle Born's 100Ah ($799) is assembled in the USA with a 10-year warranty; LiTime's ($335.99) and Redodo's ($242.89) carry 5-year warranties and the same 4,000-cycle rating where stated. Energy per battery is the same 1,280 Wh.

It discharges in the cold but must not be charged below 32°F, per Battery University. Buy a self-heating model if you charge on frosty mornings; AGM tolerates cold charging but loses capacity in the cold too.