Matching the old bank's amp-hours case for case.
Size on usable capacity. The lithium bank is typically smaller.
Installing
How to Convert a Boat from AGM to Lithium Batteries
Size the new bank on usable capacity rather than matching the old label figure, fix the charging path before connecting anything, and replace the fusing. A straight case-for-case swap is the most common way to make this go wrong.
- Steps
- 7
- Last reviewed
- 4 September 2026
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Equipment and information required
- Your measured daily draw
- The alternator's rating
- The existing mains charger's model number
How to Convert a Boat from AGM to Lithium Batteries: steps
Size on usable capacity, not the old label
You only ever used about half your AGM bank. A 400 Ah AGM bank delivered roughly 200 Ah usable, which is about 250 Ah of lithium. Matching 400 Ah of lithium buys capacity you will never cycle.
Fix the alternator path first
An empty LiFePO4 bank will accept everything an alternator can produce and keep asking. Fit a DC-DC charger sized to the smaller of a quarter of the bank and half the alternator before the bank goes in, not after.
Check the mains charger has a lithium profile
A lead-acid charger's absorption and float voltages are wrong for LiFePO4, and indefinite float holds the bank where it ages fastest. If it has no lithium profile, it is replaced, not reprogrammed.
Check the solar controller the same way
A suitable charge profile follows the exact battery manual, including any permitted float setting. Disable equalisation unless explicitly allowed; a flooded lead-acid equalisation cycle can exceed lithium voltage limits.
Replace the fusing
Fit a Class-T fuse, or equivalent interrupt rating, at each pack's positive terminal. The existing ANL or MRBF fuse was chosen for a bank that could not deliver these fault currents.
Fit a shunt-based monitor
LiFePO4's voltage curve is flat across most of its range, so voltage tells you almost nothing about state of charge. A shunt counting amp-hours in and out is the only useful gauge.
Do not leave the AGM bank in parallel
The two chemistries have different charge profiles and different internal resistance. Paralleled, the lithium bank supplies most of the load and the lead bank rarely charges properly.
Common installation and calculation errors
Keeping the old bank in parallel to save it.
Mixing chemistries degrades both. Keep lead only as a separate, isolated cranking battery.
Related equipment and battery guides
More on installing
How to Plan Marine Lithium House Battery Bank WiringFuse each pack at its own positive terminal, bring the packs to a common bus with equal-length cables, put the shunt on the negative between the bank and everything else, and size every run for the current the new system can pass.How to Install a Boat Battery Monitor and ShuntPut the shunt on the bank's negative with nothing but the battery on its battery side, run every other negative in the boat to its load side, then calibrate it at a known full charge.How to Wire a Marine DC-DC Battery ChargerFeed it from the cranking battery or alternator output, land its output on the lithium bank, fuse both sides close to their sources, and give it an ignition trigger so it only runs with the engine.How to Connect Boat Solar Panels to Lithium BatteriesWire the panels to an MPPT controller with a true lithium profile, land the controller's output on the bank through its own fuse, and connect the battery side before the panel side when commissioning.