Why boat owners switch to lithium
On a boat the case for lithium is stronger than almost anywhere else. A lead-acid or AGM house bank gives you only about half its rated capacity before you should recharge, and it gives that back slowly. LiFePO4 fixes both.
- More usable energy from the same space. A 100Ah lead-acid delivers roughly 510 to 640 Wh usable at 50 percent depth of discharge. A 100Ah LiFePO4 gives around 1,024 to 1,150 Wh, so you can often halve the amp-hours and keep the same usable power.
- Weight off the boat. Lithium is about 70 percent lighter for the same usable energy, and being sealed it can mount low, which helps trim and stability.
- Fast recharge at anchor. Lithium accepts a high charge current almost to full, so a short engine or genset run puts far more back than it would into lead.
- Deep-cycle life. 2,000 to 5,000 cycles against a few hundred for typical lead, so the cost per usable cycle is lower despite the higher price.
Keep the start battery, switch the house bank
The usual marine setup is to leave a lead-acid or AGM engine start battery in place and convert the house bank to lithium. Cranking suits lead chemistry, and keeping the banks separate avoids the problem below.
Never wire lithium in parallel with lead-acid. The two sit at different voltages and will fight each other, and a lead fault can drag the lithium BMS into a shutdown. Charge sources can be shared through the right device, but the banks themselves stay separate.
What to change before you swap
A lithium battery that fits the tray is not automatically plug and play. On a boat, three things matter most.
- Protect the alternator. Lithium pulls as much current as the alternator can give, which can overheat one wired straight to the bank. Fit a DC-DC charger or an external regulator with a lithium profile and a temperature sensor.
- Fuse for high fault current. Lithium can deliver enormous short-circuit current. Fit a correctly rated main fuse close to the positive terminal; on larger banks a Class T fuse is the common marine choice, and check the cable gauge suits the new currents. See fusing and wiring.
- Set the charge profile. Set the shore or mains charger and any solar MPPT to the lithium profile: about 14.2 to 14.6V bulk and absorption, float at 13.6V or off. Lead-acid settings will not fully charge lithium and a high lead float holds it at the wrong voltage.
How big a lithium house bank do you need?
Two ways to size it. The quick way: lithium needs only about half the amp-hours of lead for the same usable energy, so a tired 2 x 110Ah lead bank (about 220Ah, roughly 1,300 Wh usable) is matched by a single 100 to 120Ah lithium with room to spare. The thorough way: add up your daily loads (fridge, instruments, autopilot, lights, pumps) in watt-hours and the finder tool will size the pack and check the fit.
| Typical boat | Lead-acid now | Lithium replacement |
|---|---|---|
| Weekender or day sailer | 1 x 80 to 100Ah | 1 x 50 to 100Ah LiFePO4 |
| Cruising yacht (fridge, pilot) | 2 x 110Ah | 100 to 200Ah LiFePO4 |
| Liveaboard or large bank | 4 x 110Ah or 2 x 6V | 200 to 300Ah LiFePO4 |
See the lithium battery that replaces your boat's bank
Enter your current battery by group size, amp-hours, or brand and model. Get the match, the weight saved, and whether it fits your tray.
Size my lithium battery