Rewiring my narrowboat's 12v system — where do I start with the battery bank?

I've had my narrowboat for about three years now and the previous owner's wiring is, to put it charitably, a bit of a horror show. Spaghetti behind every panel, undersized cable runs, and I'm fairly sure the fuse setup would make an electrician weep. I've been getting by but I want to do a proper job this winter while she's out of the water.

At the moment I've got two 110Ah lead acid leisure batteries running in parallel, charged off a 75A alternator via a basic split charge relay. I also have a 200W solar panel on the roof going through a cheap PWM controller into the same bank. It just about keeps up in summer but I'm constantly anxious about flattening the batteries over a winter mooring. I'm thinking of moving to a 200Ah lithium (LiFePO4) setup and upgrading the solar to around 400W with a decent MPPT controller.

Has anyone done a full rewire on a narrowboat specifically? I'm trying to work out whether to run everything back to a central bus bar arrangement or stick with the more traditional split consumer unit approach. Cable sizing is also confusing me — I keep seeing different guidance on voltage drop calculations for boat installations versus land-based stuff.

Any recommendations on decent marine-rated fuse blocks or bus bars that won't cost an absolute fortune? I've been looking at Blue Sea Systems but the prices are eye-watering. Wondering if there are sensible alternatives that still meet the RCD/BSS requirements.

@WillStevens Mate, I've been exactly where you are — previous owner's "creative" wiring is basically a rite of passage. When I rewired my motorhome I started at the battery bank and worked outwards, which forces you to size everything properly from the source.

My honest starting point: draw it before you touch it. Even a rough sketch of what's actually there. You'll find circuits you never knew existed.

For the bank itself, Fogstar Drift LiFePO4 cells are worth serious consideration — brilliant value and they've got proper UK support. Pair with a Victron SmartShunt from day one so you actually know your state of charge.

Narrowboats have the added joy of alternator charging too — make sure whatever bank you choose has a proper BMS that plays nicely with your engine charging setup, or you'll be in trouble.

Rachel1995 | 47 posts

@WillStevens Before you touch anything else, I'd strongly recommend doing a proper audit of what you've actually got running on the boat first — list every 12v load and its approximate draw. You'll be surprised how that shapes your battery bank sizing decisions. A lot of people jump straight to "I want 200Ah of lithium" without knowing whether they need 100Ah or 400Ah.

Also worth grabbing a clamp meter and testing those existing cable runs under load — undersized cabling on a narrowboat is genuinely a fire risk, especially in the engine bay where things get warm anyway. Don't just rewire the battery bank in isolation if the rest is dodgy.

What alternator have you got on the engine? That'll be a big factor in how you approach charging, particularly if you're doing longer continuous cruises rather than marina hopping. 🚢

Narrowboats and garden offices have one thing in common: both will happily burn down if you let a previous owner's dodgy 12v work fester unaddressed. 🔥

Grab a Victron BMV-712 first — you can't fix what you can't measure, and watching your state of charge via the app is genuinely the most useful thing you'll do before touching a single cable.

Not a narrowboat owner myself — I'm running a shepherd's hut setup — but battery bank sizing logic is pretty universal.

One thing I'd add that nobody's mentioned yet: before you spec any new bank, actually measure your real consumption over a few days rather than guessing from appliance ratings. I wasted money the first time round because I based everything on theoretical loads.

Also worth thinking about whether you're staying 12v or jumping to 24v now while everything's apart. Much easier to make that call before buying cables and a new Victron SmartShunt rather than after.

What are your main loads — inverter, heating pump, lighting? That'd help narrow down whether lithium makes sense or if AGM still does the job for your use case.

KeithYoung | 312 posts

@WillStevens One thing I'd add that nobody's mentioned yet — pay close attention to your busbar setup and fusing at the battery. On boats I've seen, the most dangerous runs are often the unfused ones closest to the bank itself. A short there and you've got a serious fire risk with nowhere for the current to go safely.

Also worth thinking about your charging sources early on. Narrowboats typically juggle engine alternator, shore power, and possibly solar — getting those to play nicely together needs a bit of planning upfront rather than bolting things on as an afterthought.

What's your current alternator situation? Many older boats have undersized ones that'll struggle to keep a decent lithium bank happy. Might be worth upgrading alongside the rewire rather than doing it all twice. Been down that road myself — not fun!

@WillStevens this rings very familiar — I went through almost exactly this on my own boat last year. The thing that tripped me up was underestimating how critical proper fusing at the battery terminals is before you touch anything else. Every cable leaving that bank needs a fuse as close to the positive terminal as physically possible.

@KeithYoung makes a good point about bus bars — I went with a proper Victron distribution block and it made the whole thing far tidier.

One question though: what chemistry are you planning for the new bank? I went LiFePO4 (Fogstar cells) and it genuinely transformed how much usable capacity I actually have, but you need to make sure your alternator charging setup won't cook the cells without a proper battery-to-battery charger in the mix.

Really curious what @KeithYoung was about to say about bus bars — that reply got cut off but it sounds important!

@WillStevens one thing I'd genuinely obsess over before touching anything else: cable sizing and voltage drop. On my Ducato-based tiny house build I massively underestimated how much difference proper cable runs make. On a narrowboat with longer distances between battery bank and consumers, it's even more critical.

Have you considered using a Victron SmartShunt early on? Even before you've finished the rewire, chucking one in gives you real data on what's actually happening — made a huge difference to my understanding of my own setup.

DefenderAdventure | 847 posts

@WillStevens Before you touch a single cable, document everything with a camera. Every connection, every fuse, every junction. You'll thank yourself later when you're mid-rewire wondering what that mystery red wire was doing.

For the battery bank itself, start with your actual consumption figures — tally up every 12v load and estimate daily amp-hours. That dictates bank size before you even think about chemistry (LiFePO4 vs AGM). On my narrowboat I ran a Victron BMV-712 for a full month before rewiring, just to establish a baseline. Proper data makes the rest of the decisions straightforward.