Narrowboat 12v system keeps tripping after adding a second leisure battery

Really common issue this, and almost always comes down to one of three culprits.

1. Mismatched batteries

If your second leisure battery isn't the same age, chemistry, and ideally same manufacturer as the first, they'll fight each other during charge cycles. One will always lag behind, causing the BMS or fuse to see irregular current draw and trip. Fogstar Drift lithium cells are brilliant for this reason — buy two identical ones from the same batch if you can.

2. Undersized fuse or breaker

Two batteries in parallel effectively doubles your potential discharge current. If your fuse was sized for a single battery system, it's now marginal. Worth checking what rating you're running — a lot of narrowboat installations I've seen use a 100A blade fuse that was already borderline.

3. Wiring imbalance

Are both batteries connected with equal cable lengths to a common busbar? Unequal runs create resistance differences, meaning one battery does the heavy lifting. Classic symptom is exactly the tripping behaviour you're describing.

From my own Vivaro-based setup — which I've since adapted experience from to help friends with boat installs — the fix that solved 90% of these issues was adding a proper Victron Battery Protect and moving to a busbar arrangement rather than daisy-chaining the batteries together.

Worth grabbing a cheap clamp meter and checking whether the current is splitting evenly between both batteries under load. That'll tell you immediately whether it's a wiring or a battery chemistry problem.

What batteries are you running, and how are they currently wired together? That'll help narrow it down considerably — others here will have seen the exact same setup I'd imagine.

@VivaroWanderer just going through something similar on my narrowboat right now — added a second Fogstar 100Ah AGM alongside my existing one (different age, which I now realise was a mistake after reading this).

Quick question though: my Victron BMV-712 is showing wildly inconsistent state of charge readings since the second battery went in. Is that a symptom of the mismatch causing uneven charging, or could it be a separate issue with how I've wired the sense wires?

Also, the tripping on mine seems to happen specifically under EV charging load — I've got a small portable EVSE running off a 230v inverter for topping up the car when moored. Would mismatched batteries behave worse under that kind of sustained high draw compared to normal domestic loads?

@SimonJohnson67 worth checking your BMS or fuse rating too — people often forget that adding a second battery effectively doubles your potential discharge current. If your blade fuse or ANL is sized for a single battery setup it'll nuisance trip under load even if the batteries themselves are perfectly matched.

On my static caravan setup I had exactly this after adding a second 100Ah alongside my original. Turned out my ANL fuse was only rated 80A — fine for one battery, marginal for two with the inverter pulling hard. Swapped to a 150A Victron ANL and sorted it immediately.

Also double-check your interconnect cable between the batteries is the same gauge as your main run. Undersized links are a surprisingly common cause of voltage drop that can look like a fault.

@DailyBuild also worth noting your alternator might now be seeing a deeply discharged second battery as basically a short circuit — engine alternators on narrowboats weren't exactly designed with "I'll just bung another 100Ah in" in mind, and they'll thermal-trip faster than my patience at a DIY superstore checkout.

@GrumpySpanner nails it on the alternator point — this caught me out badly on my own boat a few years back. Worth adding: check your cabling between the batteries as well as the fuse. If you've paralleled them with undersized cable (I used 6mm² initially, should've been 16mm²+) you get resistance heating under load which can trip thermal fuses even when everything looks fine on paper.

Also, @SimonJohnson67 — if your second Fogstar AGM sat on a shelf for a while before fitting, the resting voltage difference between old and new battery could be significant enough to cause a huge inrush current the moment you connect them. Always bring both batteries to the same state of charge before paralleling. A decent Victron IP65 charger will let you do each one independently first.

Bit of a faff but saves a lot of head-scratching later!