Adding a 24-Volt System to a 30-Year-Old Catamaran
We bought a boat with no working electrics at all. Three years later, we’re about to more than triple our usable power — and change the entire voltage of the boat to do it.
When we got Paikea, “electrical system” was generous. The batteries held a charge for about two hours. The solar panels — old BEP units from Australia — didn’t work. No autopilot, no working instruments, nothing that told us depth or wind underway. We bought two AGM batteries in Italy early on, a real expense at the time, and that’s what we cruised and lived on for three years: 200 amp-hours of AGM capacity, which in practice meant about 100 usable amp-hours if the batteries were in good condition. Livable, but a constant budget to manage.
Why we’re jumping straight to 24 volts
Converting from 12 volt to 24 volt sounds like an odd move on a 30-year-old boat, and it raised eyebrows when we mentioned it. But a conversation with the electricians at Gunboat changed our minds — people who deal with this at a level well beyond ours. Their logic was simple: our old wiring is tired, with three decades of minor corrosion at every terminal and connection. Doubling the voltage halves the amperage running through those same wires for the same power delivered, which means less heat, less strain, and meaningfully more life out of wiring we’re not planning to rip out and replace.
It also solved a very real cable-sizing problem. At 12 volts, running the amperage this new battery pack needs would have meant cable as thick as a thumb — expensive, heavy, and a pain to route through a boat this size. At 24 volts, that drops to cable roughly finger-thick. That difference alone made the decision close to a no-brainer.
The tradeoff is that most of the boat’s 12-volt consumers — lights, instruments, autopilot, windlass — need to either be swapped for 24-volt equivalents or run through a converter. We’re doing both: a DC-to-DC converter steps 24 volts back down to 12 volts for the lighting and instrument circuits that aren’t worth the cost of upgrading yet, sized generously enough to handle known draw with headroom for an extra 50 euros — cheap insurance against undersizing it. The fridge runs on either voltage, so that was solved before we started. The watermaker already came fitted with a 24-volt pump. The only genuine casualty in the whole conversion is the anchor windlass motor, which has to be swapped from 12 volt to 24 volt to keep up.
The battery pack itself
The new lithium pack comes in at roughly the same weight as the old AGM bank, but with more than double the usable capacity — going from 200 amp-hours of AGM to 720 amp-hours of lithium, with a much higher percentage of that actually usable compared to AGM’s typical 50 percent depth of discharge. It’s a genuinely different amount of power available to the boat, not just a modest step up.
Managing it properly meant two separate battery management systems rather than one — a deliberate choice, not an oversight. The cells in this pack are second-hand, salvaged from a pack that had previously been discharged below a safe voltage with no BMS protecting it at the time. They’re usable, but not perfectly matched to each other, which means the balancing system has to work harder to keep every cell in line. Running two BMS units, one per pack, in parallel rather than one BMS managing both packs together, means every individual cell gets monitored rather than just the pack as a whole — worth the extra cost given the cells’ history. Each unit is rated for three amps of balancing current, which is genuinely high output in the battery world, and exactly the headroom needed for cells that aren’t a perfectly matched set.
The only real weak point in an otherwise well-priced, no-frills system is an exposed circuit board with no protective coating from the factory. That’s getting a conformal coating before it goes into service — silicone is the recommended choice for a marine environment over acrylic or urethane alternatives, since it holds up better against damp and salt air over time.
Solar to match the new capacity
None of the added battery capacity means much without the generation to back it up. We started with roughly 150 watts of solar total. Two 295-watt panels now sit over the dinghy davits, bringing us to just under 600 watts, and the plan is to add semi-rigid panels across the new hardtop roof to push total capacity toward 1.1 to 1.2 kilowatts, depending on budget and what actually fits.
Semi-rigid panels are the right call for a walkable roof surface specifically — rigid panels aren’t an option where people are standing on them regularly. But not all semi-rigids are equal, and this is one area worth spending real money rather than chasing the cheapest option available. Cheap semi-rigid panels have a documented history of overheating and, in worst cases, fire risk — a very different risk profile to a cheap rigid panel, which might just underperform. Rather than guess, the panels under consideration are ones already seen in service on other rigged boats during professional rigging work — a chance to see real-world reliability before committing rather than relying on marketing specs alone.
Untangling a genuine chicken-and-egg problem
None of this rebuild has been simple to sequence. At anchor, with no shore power and a broken orbital sander, sorting the battery bank became the prerequisite for running the inverter, which is the prerequisite for running any of the tools needed to prep and paint the space the battery pack lives in. Three days of work went into templating solar controller layouts and wiring plans before a single screw went into the bulkhead — the unglamorous, unfilmable part of a project like this that still has to happen before anything visible gets built.
Along the way, the teardown turned up its own small lesson in corrosion. Water tracking down behind the interior lining near a fuel tank bolt had turned an aluminium washer into a crumbling white crystalline mess — a clear, physical reminder of why dissimilar metal corrosion and proper anti-seize treatment matter on a boat that spends its life in salt air, and one more thing added to the growing list of jobs this rebuild keeps uncovering.