Well, lithium is expensive, yes. But to do a realistic total cost-of-ownership comparison with AGM batteries...
Lithium costs - in round figures today - $1K per 100Ah at 12.8V. Assuming lithium provides 80% usable capacity without damage, that's $1K for 80Ah usable. $12.50 per usable 12V Ah
AGM costs - (again round figures) $750 for 220Ah at 12V. Assuming AGM is 50% usable capacity without damage, that gives us $750 for 110Ah usable. $6.82 per usable 12V Ah
So - for usable capacity, that makes lithium about twice as expensive as AGM.
Lithium has (depending on whose data you believe) 2x-10x the lifespan of AGMs (number of charge/discharge cycles, etc) So - that makes total cost of ownership roughly the same for lithium (if you take 2x) or better for lithium if you take a higher multiplier.
Caveats:
- If you don't keep/use the boat long enough to wear out a set of AGMs, the longer life of lithiums is irrelevant.
- Charging lithiums is easier on your generator (lets you charge at higher charge rates, so running the generator shorter times at higher loads) so that might also save some money long term.
- Lithiums are smaller, lighter, deliver higher max current, charge faster, have a much flatter voltage curve (easier on all the DC motors on your boat), and make you better looking. At least one of those may not be true.
- Lithiums require reprogramming your charging equipment, and some equipment may not work correctly with them.
- Lithiums are still new technology in the boating world, so we may find issues down the road. (Down the channel?)
Kevin