Narrowboat battery bank keeps dropping overnight – 200Ah not enough?

Been through something similar getting my tiny house system dialled in, so a few thoughts that might translate across.

200Ah sounds reasonable but the real question is what chemistry are you running? If that's AGM or wet lead-acid, you're only realistically working with 100Ah usable before you risk damaging the bank. Lithium (LiFePO4) gives you the full 200Ah. Night and day difference.

Also worth logging what's actually pulling power overnight. A 12v compressor fridge alone can chew through 40-60Ah in winter when it's working harder. Add any heating controls, bilge pump cycles, phone charging, anchor lights — it adds up faster than people expect.

A few things I'd check:

• State of health on the existing batteries — older lead-acid loses capacity silently
• Resting voltage in the morning — if you're hitting below 12v on lead-acid you've been going deep for a while
• Parasitic loads — Victron BMV or similar will show you exactly what's happening overnight

What's your charging setup? Solar, alternator, shoreline? On a narrowboat I'd imagine alternator is doing heavy lifting when cruising but you're obviously not running the engine overnight.

Honestly if the budget allows, swapping to a decent LiFePO4 bank (Fogstar Drift cells are popular and reasonably priced for UK folk) with a proper BMS would likely solve it without needing to add capacity at all.

What size inverter are you running, if any? That's another common culprit — even in standby some draw a fair bit.

What are others on here running for overnight consumption on narrowboats? Curious whether liveaboards have cracked this one.

@Thommo is right to raise chemistry first — it's the single most important variable here.

Lead-acid at 200Ah? You've got roughly 100Ah usable before you're into damaging territory. LiFePO4 at 200Ah? Closer to 180-190Ah usable. That's nearly double the effective capacity from an identical nameplate figure.

On narrowboats specifically, you've also got parasitic loads that don't exist on land setups — bilge pump float switches, live well pumps, navigation lights left on standby, inverters idling. I've seen people lose 30-40Ah overnight just to background draw they haven't even measured.

Get yourself a proper battery monitor — Victron BMV-712 minimum. Don't guess. The BMV will show you exactly what your overnight draw actually is before you throw more batteries at the problem.

What chemistry are you running and have you actually logged the overnight consumption figures?

@Thommo @SolarNotSure both bang on about chemistry, but here's what nobody's mentioned yet — overnight loads on a narrowboat are brutal in ways a tiny house isn't.

I ran 200Ah AGM on my 57-footer for two miserable winters before finally caving and going Fogstar Drift lithium. The problem wasn't just usable capacity. It was the fridge compressor kicking on every 20 minutes through the night, the 12v pump, the Webasto pulling startup current — it all adds up to something savage.

Check your actual overnight draw with a Victron BMV-712 before throwing money at more batteries. Mine showed I was pulling nearly 80Ah between midnight and 7am — on a system I thought was modest.

Know your numbers first. Otherwise you're just guessing in the dark. Literally.

Great points from @Thommo, @SolarNotSure and @AngliaCamper — chemistry and overnight loads are the obvious starting points, but I'd also ask: how old is that battery bank?

Lead-acid in particular degrades significantly within 2-3 years on a boat, especially if it's ever been left partially discharged. A "200Ah" bank that's seen a few hard winters might realistically be delivering 120Ah or less. Worth doing a proper capacity test rather than trusting the label.

Also — are you monitoring actual consumption overnight? A cheap battery monitor (Victron BMV series is hard to beat for the money) will tell you exactly what's going out, which completely changes the troubleshooting conversation. Guessing loads rarely ends well!

What's powering overnight — heating, bilge pump, fridge? That'll matter enormously.

@T5Life raises a good shout about the less obvious stuff, but I'd add one more layer nobody's touched on yet — battery temperature.

Narrowboats sat on the cut in winter can see bilge temps drop to near freezing overnight. Lead-acid loses meaningful capacity below 10°C, and even LiFePO4 will throttle its BMS discharge at low temps (Fogstar Drift cells, for instance, cut discharge protection around 0°C).

Had the same creeping issue in my cabin setup before I moved my bank indoors. A 200Ah bank performing at 60% capacity due to cold is effectively 120Ah — suddenly your overnight drain makes a lot more sense.

Worth @Thommo checking where the batteries are physically located on the boat and whether there's any insulation around them. A cheap temperature logger overnight would confirm this quickly before spending money on more capacity.