Smart Battery Protect to cut off part of 12v circuit

Been mulling this one over for my motorhome build actually, so feels timely to share.

I've got a Victron Smart Battery Protect (the 65A version) sitting in my parts box waiting to be wired in, and I've been wrestling with exactly this dilemma — whether to run it across the whole 12v load circuit or just target the fridge specifically.

My thinking is to use the SBP purely as a fridge killer. When the battery drops to, say, 11.8v, the fridge is the biggest draw and the least critical thing to lose — I'd rather keep my lights and USB charging alive whilst the fridge sacrifices itself. Feels like the smart approach rather than a hard cutoff of everything.

The snag I keep coming back to is the inrush current when the compressor kicks in. My fridge is a decent-sized unit and I've read a few horror stories about SBPs not loving that spike, even on the 65A model. Has anyone had issues with that?

I'm also wondering whether it's cleaner to:

• Run the fridge on its own dedicated SBP, or
• Use a Cerbo GX relay to switch a separate relay/contactor for the fridge (more elegant but more faff)

My Fogstar lithium cells have their own BMS cutoff as a last resort, so the SBP is really just the early-warning layer — I want to protect the battery before the BMS throws its toys out.

Anyone else running their SBP on a partial circuit rather than the whole leisure load? Curious whether the wiring setup is as straightforward as it looks on paper.

@Stacey the Smart Battery Protect is brilliant for this exact job, but there's one gotcha that caught me out when I wired mine into the garden office — it's one-directional only. Current must flow from battery side to load side, no exceptions. Try to run it backwards and you'll get some very confusing behaviour.

Also worth knowing: you cannot use it on the output side of a charger circuit. Loads only.

The BT monitoring through the Victron Connect app is genuinely useful though — I had mine set to disconnect at 11.8V and reconnect at 13.2V, and watching the voltage curves over a few weeks taught me more about my actual consumption patterns than anything else had done.

What loads are you planning to protect against low-voltage cutoff? That'll determine whether the 65A is comfortably rated or whether you're pushing it.

@Glen1983 yeah go on then, finish your sentence 😄

@Stacey one thing people miss with the SBP — it's load disconnect only, no charging through it. Wired mine wrong first time round in the cabin, nearly fried the unit. Also can't run it bidirectionally so plan your circuit accordingly.

The BT app integration is decent though, you can set custom voltage thresholds which is what makes it actually useful rather than just a glorified fuse. I've got mine cutting non-essential loads at 12.1v to protect the Fogstar cells underneath.

What's your battery chemistry? Thresholds vary quite a bit between AGM and lithium — get them wrong and you're either babying the bank unnecessarily or wrecking it.

@GrumpyBuilder beat me to the "finish your sentence" energy, but the other thing worth knowing is the SBP cannot be used on the input side of a battery charger — load side only, else you'll fry it spectacularly.

Also worth noting: you can daisy-chain two SBPs if you want to protect different voltage thresholds for different circuits — lights stay on longer than the inverter, for example. Done exactly this on my narrowboat with a pair of them sitting in front of a Fogstar 200Ah lithium, works a treat.

One more gotcha: the remote on/off pin needs a proper low-voltage signal, not just floating — caught plenty of people out on here before.

@ExPostie86 curious what you were going to say there about the SBP not being used for something — you've done the same as @Glen1983 😄

Genuine question though: in my setup I'm wanting to protect just the 12v lighting circuit while keeping the fridge running regardless. Would the SBP handle that if I wire it in series with only the lighting loads? Or am I better off looking at a relay-based solution instead? I've got a Victron Cerbo GX already so I could potentially trigger something via the relay outputs on that rather than running a separate SBP just for one circuit.