Upgrading from AGM to lithium: What else needs to be replaced

Why simply swapping batteries is rarely enough

“Lead out, lithium in” works insofar as the battery will subsequently deliver power and be recharged. It is not optimal, and in some configurations, it is dangerous. The reason lies not in the battery, but in the rest of the system: a battery bank of LiFePO4 cells behaves fundamentally differently from lead during charging and in the event of a fault, and the existing charging infrastructure is designed for lead.

NoteIf you choose a BullTron battery with P-Bus compatibility, you can save the battery management shunt: the battery reports its values directly to the Control Unit PCU. Find out what this means in the article Integrating a BullTron battery directly into the P-Bus.

Three differences are crucial.

First, the end of charging. A lead-acid battery needs to be held at a float voltage permanently after being fully charged, otherwise it sulfates. A LiFePO4 cell wants exactly the opposite: the charging current must be terminated when it reaches a full state. A lead-acid charger that continues to hold it at 13.6 to 13.8 volts will permanently operate the lithium battery bank at its upper limit.

Second, the internal resistance. LiFePO4 delivers and accepts significantly higher currents than a lead-acid battery bank of the same size. Cable cross-sections and fuses that were unremarkable for years with AGM may then be undersized — not during normal operation, but in the event of a short circuit.

Third, the BMS. A lead-acid battery cannot disconnect itself from the onboard power supply. A lithium battery can, and it does so if a cell leaves its safe operating range. Everything connected to the network at that moment must be able to handle this — especially the alternator.

Typical values. The specifications of your cell manufacturer take precedence. DIN EN IEC 62619 serves as the safety standard for lithium cells in industrial and maritime applications — a good point of reference for evaluating a supplier's safety documentation.

Calculation: How much lithium replaces your AGM battery bank

This is where the most expensive planning mistake occurs, and it almost always goes in the same direction: buying too much. Calculations should be based on usable capacity, not the nominal capacity on the label.

FormulaNew nominal capacity = (old nominal capacity × old depth of discharge) ÷ new depth of discharge

An example: You have 400 Ah of AGM. At a 50 percent permissible depth of discharge, 200 Ah of this was usable. A battery bank of LiFePO4 cells providing the same 200 Ah of usable capacity requires 200 ÷ 0.8 = 250 Ah nominal capacity. So, 400 Ah of lead-acid becomes 250 Ah of lithium, not 400, and certainly not 800.

Two reasons favor going slightly larger anyway. First, the old battery bank may have already been too small — if you regularly discharged deeper than 50 percent, that was not comfort, but wear and tear. Second, a larger battery bank extends service life for the same consumption because each cycle becomes shallower. However, the surcharge should be a conscious decision, not just doubling out of habit.

The three conversion variants

Variant 1: Replace battery, adjust charging sources

The AGM battery bank is replaced, and all existing charging sources are switched to a lithium charging curve or replaced. The prerequisite is that each of them can be adjusted at all: shore power charger to 14.2 to 14.6 volts without continuous floating, solar controller to a LiFePO4 profile or a suitable charging end voltage.

This variant is the cheapest and suitable for systems that primarily charge via shore power and have little engine running time. Its weak point is the alternator: a standard alternator with an internal regulator will charge the lithium battery bank, but without protection against load dumping when the BMS disconnects.

Variant 2: Charging converter between alternator and auxiliary battery

A battery-to-battery charger takes power from the starter circuit and generates its own regulated lithium charging for the auxiliary battery. This means the alternator only sees the starter battery — it always works against a consumer, even if the auxiliary battery's BMS disconnects. At the same time, the current with which the alternator is loaded can be limited.

For most conversions with a standard alternator and mixed battery onboard system — AGM starter, LiFePO4 auxiliary — this is the right choice. It can also be easily retrofitted in existing systems because nothing needs to be changed on the engine.

Variant 3: High-output alternator with external regulator

The standard alternator is replaced by a high-output model, controlled by an external regulator with temperature sensors on the alternator and battery. The regulator runs a true lithium curve, limits the excitation if the alternator temperature is too high, and receives the charge enable signal from the BMS before it has to perform a hard disconnect.

This is the most powerful variant and the only one that delivers high charging currents even at low RPM. It is worth it for long-distance cruising and wherever engine hours are scarce. For alternator outputs over about 100 A, the belt drive must also be checked — a simple V-belt does not transmit this power reliably.

What to check before the conversion

Each charging source individually

Go through the list completely, including sources one rarely thinks of: shore power charger, solar controller, alternator, wind generator, fuel cell, generator. Each of them needs a lithium charging curve and the charge enable signal from the BMS. A single forgotten source is enough for the system to regularly go into emergency shutdown.

Cable cross-sections and connections

The current limitation of the BMS protects the cells, not the wires. Check the cross-sections between battery, distribution, charger, and charging converter against the currents that are now possible — not against those that flowed previously. Inspect all connection points for oxidation and contact resistance: what was unnoticeably warm at 50 A will become hot at 200 A.

Fusing

The main fuse belongs as close as possible to the positive terminal and needs a switching capacity that matches the short-circuit current of the new battery bank. For larger lithium battery banks, this regularly leads to Class T fuses. A fuse that cannot safely cut off the short-circuit current will not burn out in the event of a fault, but will weld or ignite an electric arc.

What happens when the BMS disconnects

Run through the fault scenario in your mind: the BMS shuts off while the engine is running and the anchor winch is working. Which devices lose power? What does the alternator do? Do the bilge pump, anchor light, and radio remain powered? Safety-relevant consumers do not belong behind the auxiliary BMS, but on the starter battery.

State-of-charge display

The discharge curve of LiFePO4 is almost flat — between 90 and 20 percent, there is often less than 0.3 volts difference. A voltage display is therefore useless as a state-of-charge indicator. An ampere-hour counter with a shunt in the common negative line is not a luxury with lithium, but part of the basic equipment.

Exceptions and limitations

Winter operation. If the vessel remains in operation over winter and cannot be safely kept above the minimum charging temperature, converting without heated cells is not a good idea. The BMS will then correctly block charging — only to leave the system without any replenishment.

Starter battery. It remains lead-acid. For short, very high currents and continuous charging from the alternator, this is the appropriate technology.

Chargers without switchable float charging. If floating cannot be deactivated or lowered to about 13.2 volts, no setting will help — the device must be replaced.

Systems without documentation. If no one knows which wire goes where, the conversion is the wrong time to find out. Record first, then convert.

Typical mistakes

• Capacity doubled instead of halved. Calculations must be based on usable capacity.
• Charger not adjusted. Float charging continues; every charge ends with an emergency BMS shutdown.
• Alternator unprotected. Without a charging converter or external regulator, load shedding during a BMS disconnect destroys the rectifier.
• Only the main charging source wired with charge enable. The retrofitted solar controller continues to charge unchecked.
• Fuse not adjusted. Switching capacity and short-circuit current no longer match.
• No shunt monitor. The state of charge cannot be read from the voltage.
• Safety consumers behind the BMS. They shut down exactly when they are needed most.

The order of conversion

1. Record inventory. List all charging sources, cross-sections, fuses, and consumers. Take photos of the current state.
2. Calculate capacity. Based on usable capacity, considering a consumer list and days of autonomy.
3. Determine variant. Based on engine running time and the existing alternator, not just budget.
4. Convert charging technology first. Charger, solar controller, charging converter, or external regulator — all before the battery swap, while the old battery bank still forgives mistakes.
5. Adjust fuses and cross-sections. Place main fuse at the positive terminal, check and replace wires.
6. Install monitoring. Shunt into the common negative line, connect monitor, and set capacity.
7. Replace battery. Only now.
8. Check charge enables. Test each source individually, consciously provoke an emergency shutdown, and observe what happens.
9. Full charge and synchronize. Charge completely once so that balancing and the ampere-hour counter reach a defined starting point.

Step 4 is the one most frequently skipped because the new battery has already been delivered and is in the way. Anyone who handles the charging technology only after that is operating the new battery bank in the meantime on an infrastructure that does not fit it — and the first charging cycles are exactly the ones you should not waste.