Narrowboat 12v system keeps tripping after adding a second battery

Narrowboat wiring is basically just someone's motorhome fever dream but with added damp, so welcome to the club 🎉

Had almost identical grief in my shepherd's hut when I paralleled a second Fogstar Drift 100Ah alongside my existing one — turned out the two batteries were at wildly different states of charge when I connected them, and the resulting equalisation current was enough to upset everything downstream. Rookie error on my part, could've fried something expensive.

Few things worth checking before you pull your hair out:

• Are both batteries the same chemistry? Mixing lithium and AGM on the same bank is a special kind of chaos
• What's actually tripping — is it the BMS on one of the batteries, a fuse, or a breaker on the boat's consumer circuit?
• Cable sizing — adding a second battery doesn't mean your existing cabling is suddenly happy about double the potential load

If you've got a Victron SmartShunt in the mix, pull up the history data and see if you're getting a current spike at any particular moment. That'll tell you a lot about when it's tripping rather than just that it's tripping.

Also — and I say this gently — narrowboat 12v installs done by previous owners should be approached like a crime scene. Assume nothing is labelled correctly and everything is slightly undersized.

What BMS and fuse rating are you running? Might help narrow it down.

@CharlieCampbell66 ha, that description of narrowboat wiring is painfully accurate 😄

Can I ask — when you say it's tripping, is it the main fuse/breaker on the battery bank itself, or something further downstream? That distinction really helped me diagnose my shepherd's hut setup.

When I paralleled a second Fogstar Drift, the problem turned out to be mismatched State of Charge between the two batteries. The existing one was at 80% and the new one was fresh off the charger at 100% — the equalisation current between them was enormous and kept nuking my breaker.

Did you charge both batteries to exactly the same voltage before connecting them in parallel? Even a small difference across a low-impedance lithium bank can cause a massive inrush current. Worth checking your Victron Cerbo or SmartShunt logs if you've got one fitted — the data usually tells the whole story.

@CharlieCampbell66 curious whether your BMS ratings are actually matched between the two Fogstar packs — I made that mistake paralleling batteries in my cabin setup. Even if the batteries are identical spec on paper, if one BMS trips at a lower threshold it'll dump load onto the other and cascade into exactly the grief you're describing.

Worth checking with a clamp meter what each battery is actually pulling under load rather than trusting the spec sheet. Also — are you fusing each battery individually before the busbar? On my solar setup I learned the hard way that's non-negotiable when paralleling lithiums.

@OffGridFreak raises something really important — mismatched BMS ratings nearly had me tearing my hair out when I first paralleled batteries on my narrowboat.

One thing nobody mentioned yet: check whether both batteries are at identical state of charge before you parallel them. If one's sitting at 13.1v and the other at 12.7v, you're essentially short-circuiting them through each other the moment you connect. That current surge can absolutely trip a fuse or breaker before you've even switched anything on.

I had this exact scenario last winter on the cut. Volt meter, patience, and a good balance charge on both packs separately first sorted it completely.

A cheap battery combiner/isolator between them is also worth considering if you want to avoid this dance every time.

Hey @CharlieCampbell66, great shout from @OffGridFreak on the BMS matching — but also worth checking your interconnect cable sizing between the two batteries. Loads of people parallel up fine on paper but then use undersized links between the terminals, and that's where you'll get voltage drop and potentially trigger a BMS protection event under load. On a narrowboat especially, with all that vibration and potential for loose connections, a marginal cable can behave perfectly at rest then trip the moment you fire up the inverter or the bow thruster. What gauge are your parallel links currently, and how long are they? Also, are both batteries sitting at the same state of charge before you connected them in parallel? Dumping current between mismatched packs will cause grief too.

@CharlieCampbell66 narrowboat + parallel batteries + damp = the Holy Trinity of electrical chaos 😅

One thing nobody's mentioned yet — check whether your negative busbar is actually rated for the combined load. Sounds obvious but I once spent three days chasing a gremlin only to find my busbar was rated 100A and I was cas