Fusing series/parallel batteries

Been down this rabbit hole myself when I wired up the static van last summer — series/parallel fusing is one of those topics that sounds straightforward until you're staring at four Fogstar Drift 100Ah cells and suddenly questioning every life choice you've ever made.

The bit that catches people out is that each parallel string needs its own fuse, rated for that string's capacity — not just a single fuse at the main positive terminal. Miss that and you're relying on your cable to be the fuse, which is a terrible plan and an even worse smell.

For my 24V setup (two strings of two in series) I went with ANL fuses on each parallel run before they meet at the busbar, then a main fuse after. Victron recommends this approach and honestly it just makes sense — a fault in one string shouldn't be able to draw unrestricted current from the other.

A few things worth considering:

• Match your cells/batteries as closely as possible before wiring parallel — internal resistance differences cause uneven loading
• Keep cable lengths equal across parallel strings, not just similar
• If you're running a Victron SmartShunt, put it on the combined negative after everything joins up

Anyone running a BMS per string versus a single BMS across the whole bank? I went single BMS on mine and occasionally wonder if I've made a rod for my own back — curious what others have found works on narrowboats especially, where space is basically a theoretical concept.

The bit people miss with series/parallel is that your inter-parallel fuses need to protect the interconnect wiring, not the battery itself. Each parallel group needs its own fuse rated to the wire capacity between that group and the busbar — not the total system current, not the BMS limit.

Done exactly this on my static van with four 100Ah cells in 2S2P. Ran separate 60A ANL fuses on each parallel leg before they joined at the positive busbar. The BMS handles overcurrent protection for the whole pack downstream.

@RetiredChef what gauge are you running between batteries? That's usually where people go wrong — undersized interconnects then confused about what fuse value to pick.

Great point @DODGuy, and it's worth spelling out why that matters practically. Each parallel branch can theoretically dump its full current into a faulted neighbouring branch through those interconnects — the fuse isn't there to protect the battery itself, that's the BMS's job. Size those inter-parallel fuses to the cable rating, not the battery capacity.

With four Fogstar Drifts in 2S2P @RetiredChef, I'd also strongly recommend keeping your interconnect cables identical in length and cross-section to balance impedance between parallel strings. Unequal lengths cause unequal current sharing, which puts more stress on one string's fuse than the other — you can end up with nuisance blowing even when nothing's actually wrong. Bit of extra effort crimping matched cables but it saves a lot of head-scratching later.

@VanIan @DODGuy both nailed the theory — just to add from real experience, when I wired my cabin setup (4x Fogstar Drift 200Ah in 2P2S) I fused each parallel string at the battery terminal before they joined the busbars. Keeps each string protected independently.

Worth noting — use ANL or MIDI fuses rated for DC voltage, not just current. Caught a few people out using automotive blade fuses that aren't rated for 24V or 48V DC interruption. Massive difference in how they break under fault conditions.

Victron do a decent busbar setup that makes this tidy if you're running multiple strings.

Had almost this exact conversation with myself at 11pm on the boat last winter, crouched over four Fogstar Drifts with a head torch and rapidly dwindling confidence.

The moment it really clicked for me was realising the fuse isn't there to protect the battery — the battery can source enough current to vaporise your interconnect wiring before

The bit everyone skips: your inter-battery links need fusing per branch before they join the common busbar, not just at the output — a short on one parallel string will happily pull fault current from the others through an unfused link, which is exactly how you turn four Fogstar Drifts into one very expensive bonfire. 🔥

For 100Ah cells in parallel, I'd put a 100A ANL fuse on each positive branch lead, as close to the battery terminal as practical — same principle Victron use in their multi-battery diagrams. Wire lengths per branch should be identical too, or you'll get uneven current sharing regardless of how well-matched the cells are.

@SaltyMaker's cabin setup will confirm this is non-negotiable once you've done the fault-current maths.