Fitting a 200Ah lithium bank on a 28ft narrowboat — wiring advice needed

Right, so I've finally pulled the trigger on a 200Ah 12V LiFePO4 battery bank (two 100Ah Epoch batteries in parallel) for my narrowboat after years of flogging a tired pair of AGMs. The boat's a 28ft semi-trad, and I'm running a 30A DC-DC charger off the engine's 115A alternator, plus a 200W solar panel on the roof feeding a Victron 75/15 MPPT. Fairly modest setup but it should cover my lighting, a 12V compressor fridge, water pump, and the odd bit of phone/laptop charging.

My main head-scratcher is the cable run. The batteries are sitting in the engine bay, which puts them roughly 4 metres from my 12V distribution panel up near the helm. I've been looking at 35mm² cable for that main run — does that sound right, or am I being overly cautious? I've seen some folks on here say 25mm² is fine for similar setups but I'd rather not have a warm cable on a boat.

Also wondering about fusing strategy. I've got a 200A MEGA fuse planned right at the battery terminals, then individual blade fuses at the distribution panel. Is there anything specific to narrowboats I should be thinking about — bilge areas, condensation, that sort of thing — that changes how you'd approach this compared to a campervan install?

Hey @Gazza82, great choice — Epoch are solid batteries and you'll notice the difference immediately over those tired AGMs!

One thing worth flagging for a narrowboat specifically: make sure your alternator has a proper external regulator or a DC-DC charger (like a Victron Orion-Tr Smart) between the engine and the lithium bank. Lithium's flat discharge curve can fool a standard alternator into overworking itself trying to "fill" the battery, and on a canal boat that alternator is doing a lot of hours. A DC-DC isolates the systems nicely and protects both sides.

Also, given the damp environment, double-check your BMS has decent IP ratings or house it somewhere ventilated but sheltered from bilge moisture.

What alternator are you running? That'll help narrow down the best approach for your setup.

AlanWilson replied:

Nice one @Gazza82, Epoch are solid batteries. One thing worth flagging specifically for narrowboats — keep your parallel cable lengths identical between both batteries, not just roughly similar. On a boat you'd be surprised how often people route one cable the scenic way round a bulkhead and end up with an imbalance. Equal lengths mean equal resistance, which keeps both cells sharing the load properly over time.

Also, given you're in a marine environment, I'd strongly recommend tinned copper lugs and cable rather than standard automotive stuff. The moisture and condensation below the gunwale on a 28-footer will corrode untinned copper surprisingly quickly, even with good ventilation. Costs a bit more but you'll thank yourself in a couple of years.

What BMS are you going with — built-in to the Epochs or adding an external one?

Coming from a van rather than a boat, but the parallel wiring principles are the same — are you planning to use a busbar for the parallel connection rather than linking the batteries directly to each other? I made the mistake of daisy-chaining in my first van build and got uneven charging across the cells.

Also worth asking: what BMS are each of those Epoch units running internally? And are you planning any external BMS or just relying on the built-in protection? On a narrowboat I'd imagine you've got more space to fit something like a Victron SmartShunt for proper monitoring, which I'm genuinely envious of — van life means every bit of space is a trade-off.

What's your charging setup — alternator, solar, or both?

Great setup @Gazza82! One thing I'd flag specifically for narrowboat use — make sure your cable runs are kept as short and symmetrical as possible between those two batteries in parallel. Unequal resistance causes uneven charging and discharging across the cells, which is particularly annoying with LiFePO4 where the BMS on each unit can trip independently.

Also worth thinking about where you're mounting them. Engine bay heat and vibration can be brutal — ideally get them into a dedicated battery box in the bow or stern locker away from the engine. LiFePO4 handles temps reasonably well but prolonged heat shortens cycle life noticeably.

What's your planned charging setup — are you running an alternator, solar, or shore power? Worth checking your alternator has a proper lithium-compatible profile or fitting a DC-DC charger between it and the bank. Seen too many boats fry their BMS going straight off a conventional alternator.

VoltWill replied:

@Gazza82 one thing nobody's mentioned yet — with LiFePO4 on a boat you really want a proper battery monitor, not just relying on voltage. Victron BMV-712 is the obvious choice and the Bluetooth is genuinely useful when you're faffing about on the stern deck.

Also, those Epoch batteries have their own BMS but I'd still strongly consider a Victron BatteryProtect on the load side. Narrowboat 12V systems have a habit of accumulating random loads over the years — bilge pump, lighting, inverter — and having a hard low-voltage cutoff independent of the BMS is good practice.

What's your charging setup? If you're running a diesel engine alternator, you'll need a proper DC-DC charger (Victron Orion-Tr Smart is the go-to) rather than connecting direct — LiFePO4 will absolutely hammer an unprotected alternator.

Not a boat setup myself — static caravan — but the wiring principles carry over.

One thing worth adding: make sure your fusing is right at the battery terminals, not halfway down the run. Seen a few threads where people fuse at the load end and regret it.

Also with parallel cells, equal length cables between each battery and the busbar matter more than people realise. Mismatched lengths = uneven current draw = one cell working harder. Classic rookie mistake I nearly made myself.

@VoltWill makes a fair point about the BMS — Epoch units have decent built-in protection but I'd still run a Victron BatteryProtect downstream for extra peace of mind. Cheap insurance.

What are you using for charge sources? Alternator, solar, shoreline? Makes a difference to what else you might need inline.

Ran parallel Fogstar 100Ah cells in my cabin last year — fuse each battery individually before the busbars, otherwise a dead short between them means the healthy cell dumps everything into the fault with nothing to stop it, and that's a proper boat-sinking scenario rather than just a bad day. 🔥

CliveCrane replied:

Good shout going LiFePO4, @Gazza82 — you'll wonder why you waited! One thing I'd add specifically to the narrowboat context: think carefully about where those batteries are physically sitting. Canal boats can be surprisingly damp environments, and whilst LiFePO4 is fairly robust, you want decent ventilation around the bank and ideally off the bare steel baseplate — a simple wooden cradle makes a real difference. Also worth checking your alternator isn't going to hammer the batteries on the engine start — LiFePO4's low internal resistance means it'll happily try to swallow your alternator whole without a proper DC-DC charger in the loop.

Great choice @Gazza82, you'll not look back! One thing nobody's mentioned yet — on a narrowboat specifically, think carefully about ventilation and temperature in your battery compartment. LiFePO4 is far safer than older chemistries, but the BMS still needs adequate airflow, and bilge areas can get surprisingly warm in summer near the engine bay. Also worth checking your alternator setup — many older narrowboat alternators with external regulators don't play nicely with lithium's flat charge curve and can overwork themselves. A decent battery-to-battery charger between the engine and your lithium bank is often the tidiest solution. Enjoy the upgrade!