By Boat Battery editorial team · 2 minute read · Editorial policy
Lithium gives you 80% of the label in usable energy against AGM's 50%, at about a third of the weight, and it recharges far faster. AGM costs less on the day and does not care about cold charging.
Compare usable energy and ownership costs
The comparison people make is price per amp-hour, and on that measure AGM wins comfortably. The comparison that decides the boat is price per usable amp-hour over the life of the bank, and on that measure lithium wins by a wide margin, provided the boat cycles enough to get there.
Usable capacity and weight
Usable capacity is the first difference. You may take a LiFePO4 bank to 80% depth of discharge routinely. Take an AGM bank past 50% regularly and its life collapses. So 400 Ah of lithium delivers more usable energy than 600 Ah of AGM, in less than half the weight and a smaller box.
Charge acceptance and recharge time
Charge acceptance is the second, and on a cruising boat it matters more. AGM's acceptance falls away sharply above 80% state of charge, which is why the last 20% takes hours of engine time. Lithium accepts near-full current until it is nearly full, so the same solar array or the same hour of motoring puts far more back in.
When AGM remains a practical choice
Where AGM still wins: initial cost, tolerance of cold charging, familiarity for any electrician anywhere, and the fact that a failing AGM bank fades rather than disconnecting. On a boat that spends most of the year on a marina berth on shore power, an AGM bank is not a mistake.
Estimate payback from your own use
The break-even on a coastal cruiser that spends thirty or more nights a year at anchor is typically four to six years. On a boat used a dozen weekends a year, the lithium bank will likely be replaced before it pays back.
Related installation planning resources
Use these notes to prepare your own measurements and questions. Check the exact equipment manuals and have the proposed system assessed for your vessel before buying or changing electrical components.
Calculate your boat battery capacity
The house bank calculator takes your loads, your nights on the hook, your solar and your alternator, and returns the bank, the DC-DC charger, the AGM equivalent and the weight you take off the boat.
Open the calculator