Best way to size a battery bank for a narrowboat liveaboard?

Living aboard a 57ft narrowboat and trying to work out how big a battery bank I actually need. Currently running a 200Ah lead-acid bank (two 100Ah AGMs in parallel) and it's clearly not cutting it — I'm regularly running the engine just to top things up, even in summer.

Typical daily loads are roughly: 12V fridge running constantly, a few LED lights, phone/laptop charging, a small inverter for the odd 240V thing. I've seen estimates of around 80–100Ah per day for a setup like mine, but I'm honestly not sure how accurate that is without a proper monitor in place (Victron BMV-712 is on my list).

I've been looking at 200Ah lithium — something like a Fogstar Drift 200Ah — which would give me usable capacity closer to what the AGMs should have been providing in theory. But I keep second-guessing whether I need to go bigger, say 300Ah, given that I'm not always cruising (so alternator charging isn't guaranteed every day).

Has anyone sized up a similar liveaboard setup? How did you land on your final bank size, and did you regret going too small first time round?

HazelChild | 847 posts

@MuddyTinker The rule of thumb I'd suggest is to only use 50% of your AGM capacity regularly (to protect longevity), so your 200Ah is effectively only 100Ah usable. First step before sizing anything new — spend a week logging your actual consumption with a decent battery monitor like a Victron BMV. You might be surprised where the amps are going. Heating, inverters on standby, and bilge pumps are common culprits on narrowboats.

Once you know your daily Ah draw, multiply by 1.5 to give yourself a comfortable buffer, then pick your chemistry. Many liveaboards are switching to LiFePO4 precisely because you can use 80-90% usable capacity — effectively halving the physical bank size you'd need compared with lead-acid.

What's your current charging setup — alternator only, or solar/shore power too?

SmartSolarNerd | 203 posts

@MuddyTinker What's your actual daily consumption though? That's what I'd start with before throwing money at a bigger bank.

With my static caravan setup I made the mistake of guessing and ended up massively oversizing — wasted cash.

Grab a Victron BMV-712 or similar battery monitor and log your real usage for a week. Then you'll know exactly what you need rather than just upgrading blind.

Also — are you staying AGM or considering lithium? Fogstar do decent LiFePO4 at reasonable prices. You'd effectively double your usable capacity overnight without adding a single extra cell.

What's your charging setup currently — solar, alternator, shoreside hookup?

Robbo | 412 posts

@MuddyTinker Obvious question nobody's asked yet — are you running the engine regularly or relying purely on solar/shore power? Because that changes everything on a liveaboard situation.

Also, what's actually dying on you — are you genuinely running flat overnight or is the AGM just knackered from years of abuse? Two 100Ah AGMs that've been discharged below 50% repeatedly are probably giving you maybe 60-70Ah of actual usable capacity now rather than the theoretical 100Ah.

Did something similar sizing for my garden office — started with the loads first, worked backwards. What's your biggest consumer — inverter? Heating pump? That'll tell you whether you need 400Ah of lithium or 800Ah. Fogstar do decent value drop-in LiFePO4 if budget's tight once you've got your numbers sorted.

HMK_Sparks | 67 posts

@MuddyTinker Worth thinking about LiFePO4 if you're replacing anyway — I made the switch on my static caravan setup and the usable capacity difference is night and day. With lithium you're realistically pulling 80-90% vs that 50% @HazelChild mentions for AGM, so effectively doubling your usable bank without doubling physical size.

For a liveaboard narrowboat I'd be looking at minimum 400Ah LiFePO4. Fogstar Drift cells are popular in the UK boat community and reasonably priced. Pair it with a Victron BMV battery monitor so you actually know your state of charge rather than guessing — that alone changed how I managed my setup.

What's your inverter situation? That'll affect sizing too.

MoorRuss | 156 posts

With lead-acid you're only using ~50% of that 200Ah before you're damaging it, so you effectively have a 100Ah bank — which on a liveaboard is basically a polite suggestion rather than an actual power supply.

Mike1980 | 89 posts

@MuddyTinker One thing worth doing before you buy anything — actually log your daily consumption properly. I spent ages guessing before I stuck a Victron BMV-712 on my bank and realised my actual usage was completely different to what I thought.

Once you've got real numbers, the rule of thumb I've seen is size your bank so your average daily draw is no more than 20-25% of total capacity (for LiFePO4). That gives you headroom for bad weather days when solar isn't doing much.

What's your charging setup like? On a narrowboat you've presumably got the engine alternator — do you know what output that is? A weak alternator into a big LiFePO4 bank without a proper DC-DC charger like the Victron Orion can cause its own headaches.