Sailing Catamaran: converting factory AGM house bank to 400Ah LiFePO4 — what equipment do I need?
I’m planning to replace the AGM house bank on our sailing catamaran with twin diesels, 12V system with 400Ah of LiFePO4. Capacity is not the goal — I only need about the same usable storage as today. The goal is longer battery life under daily cycling in Caribbean heat.
Existing system (all factory, already Victron):
5 × 150Ah AGM house bank (750Ah)
MultiPlus Compact 12/2000/80-50, 120V
SmartShunt 500A
Cerbo GX
3 × 400W solar, each on its own SmartSolar MPPT
Starboard engine: 115A alternator wired directly to the house bank; house bank also starts this engine
Port engine: own start battery and 115A alternator, joined to the house bank through a charge relay
Generator (manual start), windlass 1500W and 12v watermaker both on the house bank
No shore power most of the time
My rough equipment list:
400Ah LiFePO4 (undecided between drop-in batteries with internal BMS and Smart Lithium with Lynx Smart BMS)
A separate lead-acid start battery for the starboard engine
Orion XS 12/12-50 from the start battery to the house bank for alternator protection
Class T fuse, disconnect, new cables and lugs
Reprogram MultiPlus, MPPTs, SmartShunt, and Cerbo (DVCC) for lithium
Questions:
Is that the complete list, or am I missing something?
For a 400Ah bank, is Smart Lithium + Lynx Smart BMS worth it over drop-ins, or is a drop-in with CAN to the Cerbo enough?
One Orion XS or two (one per engine)? Or can the port relay stay as it is?
Any concerns about BMS high-temperature charge cutoff in a warm engine-room compartment?
Important question: have the five domestic-bank batteries been factored into the vessel’s stability calculations in any way?
If they were taken into account in the ballast plan, for example in terms of weight and mounting location, you’ll need to proceed accordingly when replacing them.
I would equip both engines with one starter battery each, and add an XS on each side to connect to the domestic bank (both XS units controllable via the Cerbo). This protects the alternators in the event of an emergency shutdown. Future XS software will allow back-charging.
How are the essential vessel systems currently connected to the batteries? Are photos of the connections available?
With the Smart Lithium batteries and the Lynx BMS, you have optimum control over the batteries, as they can be connected to the Cerbo. In addition, you can still connect important systems directly to the batteries so that they are always supplied with power. Emergency shut-offs must be external and, where necessary, implemented via a BatteryProtect; on a boat, this solution is ideal.
Drop-in batteries usually have an internal shut-off. If you’re unlucky, everything suddenly goes offline. With several batteries connected in parallel, you may not notice that some of them have already shut down, causing the remainder to be overloaded. The drop-in batteries must also be able to communicate with one another, and their BMSs must support parallel connection with mutual communication.
Stability: No, the batteries were not part of a ballast plan. Areté weighs about 13,600 kg (30,000 lb) dry. The five AGM batteries (FCD 12-150) weigh about 215 kg together. Two Victron 200Ah Smart Lithium batteries weigh about 41 kg. The change is about 175 kg, or 1.3% of displacement, and the new batteries will go in the same compartment as the old ones. The boat is a catamaran, so this is not a stability concern. The engines, generator, and tanks dominate weight balance.
How the essential systems are connected today (from the Fountaine Pajot 12V drawing):
The house bank is 5 × 150Ah AGM, 12V.
The starboard engine has no separate starter battery. It starts from the house bank, and its 115A alternator charges the house bank directly.
The port engine has its own starter battery and its own 115A alternator. A relay connects the port battery to the house bank when the engine is running.
The windlass (1500W, 12V) runs from the house bank. It is interlocked so it only works when the starboard engine is running.
Charging: MultiPlus Compact 12/2000/80, three 400W solar panels each on its own MPPT, and a generator (manual start only). A SmartShunt 500A and Cerbo GX are already installed.
The watermaker is 12V.
Your proposed layout: I agree with it. Adding a starter battery to the starboard engine, and putting an Orion XS on each side between the starter battery and the house bank, solves the alternator problem cleanly. It also means the lithium bank can shut down without stopping either engine. Two questions:
Port side already has a relay that couples the starter battery to the house bank. I assume we remove that relay and let the Orion XS be the only path. Is that correct?
The windlass draws up to about 125A, and the interlock means the starboard alternator is always running when it operates. My plan is to move the windlass to the new starboard starter battery, so the alternator carries most of the load and the lithium bank never sees the inrush or depends on the BMS contactor during anchoring. Does that match what you would do? If yes, what size starter battery would you choose for that side, since at idle the alternator will not supply the full 125A and the battery must cover the difference?
Lynx BMS vs. drop-in: Your point about parallel drop-in batteries is the deciding one for me. In charter use, nobody is watching the system, and a silent shutdown of one battery followed by overload of the others is exactly the failure I want to avoid. I plan to go with Smart Lithium + Lynx Smart BMS so the Cerbo has full visibility and DVCC controls the MultiPlus and the three MPPTs. For a 400Ah bank with two alternators at 115A each, is the Lynx Smart BMS 500 sufficient, or would you go to the 1000?
I would choose both starter batteries of the same type. It makes sourcing replacements easier.
I would connect the anchor windlass directly to the house bank, with appropriate protection. A warning light controlled by the system would indicate that it should not be used, although this could be overridden if necessary.