Marine battery chemistries: flooded, AGM, gel, lithium

Four chemistries dominate marine service, each with a different sweet spot. FLOODED lead-acid (Trojan T-105, Rolls, Deka): cheapest per Ah, tolerates neglect if you check water monthly, 300-800 cycle life to 50 percent depth of discharge. Must be vented (H2 gas), must be upright. AGM (Absorbed Glass Mat, Odyssey, Lifeline, Optima Blue): sealed, no maintenance, faster charge acceptance than flooded (up to 40 percent of Ah), can mount at any angle except upside down, 500-1000 cycles at 50 percent DoD. About 2x cost of flooded. GEL (Sonnenschein, Trojan Gel): sealed, most tolerant of deep discharges (60-70 percent DoD without damage), but charge slow (max 15 percent of Ah acceptance) and hate high temperature. Niche today. LITHIUM (LiFePO4 chemistry, Battle Born, Lithionics, Victron): 3-5x cost of AGM, 3000-5000 cycles to 80 percent DoD, half the weight for same usable Ah, accepts nearly full charge current continuously. Requires alternator regulation change (see AGM-to-lithium conversion), a BMS on every bank, and low-voltage disconnect on loads. Rule of thumb for house bank sizing: total daily Ah consumption times 3 for lead-acid (never below 50 percent), times 1.3 for lithium (can use 80 percent). For a 200 Ah daily load: 600 Ah AGM or 260 Ah lithium.