Travel trailer - emergency brake

Went through this exact palaver last year swapping out the knackered lead acid in my tiny house setup for a Fogstar 100Ah LiFePo4 — best thing I ever did, but the emergency brake side of things had me scratching my head for a bit.

The bit people seem to gloss over is what happens when the BMS decides it's had enough and disconnects mid-draw — your Victron kit doesn't necessarily know it's happened cleanly, and if you've got an MPPT (I run a Victron 75/15 as well, funnily enough) it can throw a wobbler trying to push into a suddenly open circuit.

Few things worth thinking about for a travel trailer setup:

• Fuse sizing at the positive terminal — don't be tempted to go oversized "just in case," that's how caravans become bonfires
• Smart shunt on the negative before anything else touches it — treat it like a religion
• Make sure your BMS has a separate load and charge port if you're mixing solar input with a mains charger, otherwise you're asking for grief

Also worth checking whether your trailer's breakaway system is even compatible with lithium charge profiles — a lot of older UK-spec trailers assume flooded lead acid behaviour and the voltage curves are completely different.

Anyone else running LiFePo4 in a touring setup here? Curious whether folks are bothering with a manual isolator as well as relying on the BMS, or trusting the BMS alone — feels like a debate worth having.

Hey @RetiredElectrician74, sounds like your post got cut off there — would love to hear how you got on with the Fogstar! I've got one of their 12V 100Ah cells in my van conversion and it's been brilliant.

Are you referring to the low-voltage cutoff as the "emergency brake"? If so, the BMS on those Fogstar units handles it pretty gracefully in my experience — shuts down cleanly rather than the sudden voltage collapse you'd get with old lead acid.

Did you need to reprogram your inverter's low-voltage threshold? That caught me out initially — my Victron was still set for lead acid parameters and kept tripping unnecessarily. Worth checking your charge controller settings too if you haven't already. Keen to hear the rest of your experience once you finish the post! 😄

@RetiredElectrician74 yeah your post definitely got chopped — curious what the "emergency brake" analogy was actually pointing at. BMS overcurrent protection? Low voltage cutoff behaviour?

I've got a Fogstar 100Ah in my shepherd's hut and the BMS tripping unexpectedly was a real headache initially. Took me a while to realise my inverter's surge draw on startup was hammering it. Not exactly an emergency brake failure, more like an overly keen one.

What inverter are you running alongside it? That's usually where the drama is.

@RetiredElectrician74 your post definitely got cut off but I'll hazard a guess — you're talking about the BMS acting as an emergency brake rather than a primary protection strategy? Classic mistake people make with LiFePO4.

The BMS is your last resort, not your day-to-day safeguard. I learned this the hard way with my motorhome build. Relying on the BMS to cut discharge is like riding your brakes down a hill — technically works, but you're hammering something that shouldn't be doing that job constantly.

Proper fusing, a Victron Battery Protect for controlled low-voltage disconnect, and sensible charge parameters set before the BMS threshold kicks in — that's the layered approach that keeps everything happy long-term.

The Fogstar cells themselves are solid, mind. Just make sure the system around them is doing the heavy lifting.

@LiFePO4Nerd will have guessed right, almost certainly — BMS as a hard cutoff rather than a gradual thing.

Worth clarifying though: the BMS isn't really an emergency brake, it's more like a cliff edge. No warning, just gone. Seen too many people on here surprised when their Fogstar BMS trips at 2.5V per cell and suddenly the whole bank drops off a cliff mid-draw.

Proper setup on my shepherd's hut has a Victron SmartShunt feeding DVCC so the Cerbo throttles charging before the BMS ever needs to act. That's the actual emergency brake — the BMS should rarely if ever trip in normal use.

If yours is tripping regularly, something upstream isn't configured right. Don't just blame the Fogstar.