How to Convert a Boat From AGM to Lithium

A conversion is a rework of the boat's DC system, not a battery swap. The batteries are usually the least difficult part; the charge sources, the conductors and the protection are where the work and the cost actually sit.

How do I plan an AGM to lithium conversion?

Plan it in six stages and do them in order: audit the loads, decide system voltage, specify the bank, specify and reconfigure every charge source, recalculate conductors and protection, then commission and document. Deciding the bank before the charge sources is the usual mistake, because a charging strategy that cannot keep up makes the bank size irrelevant. AGM to lithium conversion walks through the full project.

How do I decide what has to be replaced and what can stay?

Assess each component against two questions: can it be configured for a LiFePO4 charge profile, and is it rated for the currents the new bank will allow? Most DC appliances, lights, pumps and instruments stay untouched. Chargers that cannot do a lithium profile, alternator arrangements with no current limiting, and conductors sized for a sagging lead-acid bank are the usual replacements. AGM to lithium conversion lists the typical scope.

How do I handle the alternator during a conversion?

Handle the alternator before the batteries arrive, by deciding between a DC-DC charger sized below its safe continuous output and an external temperature-controlled regulator. An alternator that was perfectly happy charging AGM can overheat charging lithium, because lithium keeps accepting current instead of tapering. There is no configuration of a standard internally regulated alternator that makes this safe on its own. Lithium alternator charging covers both approaches.

How do I reconfigure my shore charger for lithium?

Set the shore charger to a LiFePO4 profile with the battery manufacturer's absorption and float voltages, and disable equalisation and any desulphation stage. If the charger has no lithium or user-definable profile, replace it — running a lead-acid profile into a lithium bank leaves the BMS arbitrating charge termination, which is not what it is for. Shore power lithium charging covers the settings.

How do I size cables for the new bank?

Recalculate every major conductor at the currents the lithium bank allows, not at what the AGM bank delivered. Lead-acid limits current through its own internal resistance and voltage sag; lithium holds voltage and supplies what the load asks for. Circuits that were adequate can become marginal, particularly between the bank and the inverter. Run each circuit through the cable size calculator at the new figures.

How do I upgrade overcurrent protection for a lithium bank?

Replace any device whose interrupting rating is below the fault current the lithium bank can deliver at that point, and resize every device to protect the conductor it now feeds. Interrupting capacity, not the trip rating, is the specification people miss — a device can be correctly sized to trip and still be unable to safely interrupt a lithium bank's short-circuit current. Marine battery fuses and breakers covers the selection.

How do I reuse my existing battery compartment?

Assess the compartment against four requirements: it can restrain the new batteries against movement in all directions, it stays within the battery's operating temperature range in an Australian summer, it is dry and accessible, and it allows short conductor runs to the main loads. Lithium banks are lighter and often smaller than the AGM banks they replace, which can free space but can also leave batteries loose in an oversized cradle. Marine lithium battery compartment covers the rebuild.

How do I keep the boat usable during the conversion?

Sequence the work so the boat keeps a starting battery, bilge pumps and navigation lights throughout, and do the distribution rework before the old bank comes out where that is possible. Conversions overrun; a boat that cannot be left safely at its berth mid-project is a problem. Agree the sequence with the installer before work starts rather than discovering it.

How do I commission the new system?

Commission in a fixed order: confirm every charge source's configured voltages against the battery datasheet, energise with a small load, verify the shunt reads the right sign and magnitude, bring the bank to a full charge so the monitor synchronises and cells balance, then test each charge source individually under load. Record every setting as you go. Installation checklist is a usable commissioning list.

How do I test the system before relying on it?

Test it by deliberately running the cases you are worried about while still alongside: the largest combination of loads you expect to run together, a full discharge-and-recharge cycle, and a simulated BMS disconnect if the design allows it safely. Watch terminal temperatures during the heavy-load test. Finding the limit at the dock is far better than finding it on passage.

How do I avoid the mistakes that make conversions expensive?

Avoid the four that account for most overruns: sizing the bank before the charging, leaving the alternator unprotected, reusing conductors and protection that no longer suit, and not budgeting for certification and documentation. Each is cheap to address in planning and expensive to address after commissioning. Hidden costs of a lithium conversion covers the budget side.

How do I convert on a limited budget?

Convert in stages that each leave a working system: do the distribution, protection and monitoring properly first, then add battery capacity as budget allows, sizing the cable and busbars now for the bank you intend to end with. Staging the copper is what makes a conversion expensive; staging the batteries does not. Cheapest way to lithium a boat covers the approach, and future-proofing a marine lithium installation covers doing the permanent parts once.

How do I know the conversion worked?

Verify against the numbers you started with: a month of shunt data showing daily consumption and generation close to the audit, a bank that reaches full charge as often as the design assumed, no BMS protection events in the history, and terminal temperatures unremarkable under the heaviest load. If any of those disagree with the plan, the disagreement is the thing to investigate. How to monitor and maintain a bank covers the ongoing checks.