Anyone else had their JK BMS trip out when charging from a split charge relay in cold weather?

Been having a right faff with my setup over the past few weeks. Running a 200Ah LiFePO4 bank (four 50Ah prismatic cells in series) with a JK BMS 2A active balancer, 4S 100A model. Every time the temperatures drop below about 5°C and I'm driving, the BMS cuts out maybe 20-30 minutes into the journey. Alternator charging via a standard 140A split charge relay, no DC-DC converter in the mix.

My suspicion is it's the low temperature charge protection kicking in, but here's the thing — I've got the low temp cutoff set to 0°C in the JK app, and the cells themselves are sitting at around 7-8°C when it trips. Doesn't seem to happen when I'm charging from solar or hook-up, only when the relay connects and dumps current straight in. Makes me wonder if there's a brief inrush spike that's either triggering an overcurrent fault or some other protection I'm not seeing clearly in the app logs.

Has anyone else seen this with a JK BMS and a dumb relay setup? I'm half-tempted to just fit a Victron Orion-Tr Smart DC-DC and be done with it — the current limiting and low temp lockout on those is supposed to be dead reliable — but I'd rather understand what's actually going wrong first before spending another £150-odd. Curious whether tweaking the overcurrent trip threshold or the delay settings in the JK app would even help, or if I'm barking up the wrong tree entirely.

Roger1983 | Posts: 847 | Location: Peak District

@StormyNomad Classic cold weather LiFePO4 issue this. Your JK BMS almost certainly has low temperature charge protection kicking in - it defaults to cutting charge at around 0°C on most models, which is exactly what you want it to do honestly, as charging LiFePO4 below freezing causes lithium plating and permanent cell damage.

Worth checking your BMS parameters in the JK app - look for "Cell Low Temperature Protection" settings. Some people bump the threshold down slightly, though I'd be cautious about that.

Longer term solution is either insulating your battery box properly (closed-cell foam makes a massive difference) or fitting a small self-heating element. A few members here have used seedling heat mats on a thermostat with good results. Where are your batteries located? Van/shed/boat makes a difference to what's practical.

BayLisa | Posts: 312 | Location: Afloat somewhere damp

Had exact same nightmare on the boat last January. JK was cutting out because the cells were reading below the low temp charging cutoff — mine was set to 5°C by default which is basically useless on the water.

Worth checking your BMS parameters in the JK app — there's a dedicated low temp protection setting that most people never touch out of the box.

I ended up fitting a self-heating LiFePO4 from Fogstar which sorted it properly, but if you're not replacing cells yet, a simple battery blanket or even lagging the bank buys you time.

Also — split charge relays dump current in fast, which hammers cold cells harder than a proper DC-DC charger would. A Victron Orion-Tr Smart would let you set temp-compensated charging properly. Pricey but worth it.

BayTim | Posts: 312 | Location: Array

Had exactly this on my boat last January. Split charge relay dumped current straight into the cells at about 2°C and the JK tripped low temp protection — which is exactly what it's supposed to do, frankly.

What's your low temp charge cutoff set to in the JK app? Default is usually 5°C. You can lower it slightly but I wouldn't go below 0°C on LiFePO4 — you'll plate the anodes and wreck the cells.

Proper fix is a DC-DC charger (B2B) instead of a dumb split charge relay — Victron Orion-Tr Smart being the obvious choice. It'll respect the BMS comms and won't throw current at cold cells.

What alternator are you running off? That changes which B2B size makes sense.

Had this exact problem last winter in my shepherd's hut. JK was cutting out because the cells were reading below the low temp charge cutoff — totally correct behaviour tbh, it's protecting your cells from plating.

Worth checking what your BMS low temp threshold is set to. Mine was factory default at 5°C which was triggering constantly.

Two options really:

1. Add a self-heating LiFePO4 setup (Fogstar Drift cells have this built in)
2. Fit a temperature sensor on the cells themselves, not the BMS board — sometimes the probe placement gives false readings

I ended up wrapping my bank in some cheap camping sleeping mat foam and that alone kept temps above cutoff overnight. Dead simple fix.

Split charge relay timing can also dump a big current hit on cold cells — worth adding a Victron Battery Protect to soften that.

Tommo55 | Posts: 847 | Location: North Yorkshire Dales

Worth checking your JK's low temperature charge cutoff settings if you haven't already. Mine was set to 5°C from the factory which caught me out something rotten last February. You can adjust it in the JK app under Protection Parameters - I dropped mine to 0°C once I'd confirmed my cells could handle it, though obviously you want to be careful there.

The other thing @StormyNomad - split charge relays can be a bit brutal with inrush current on cold cells. Have you considered adding a DC-DC B2B charger instead? Victron Orion-Tr Smart handles cold weather charging properly with built-in temperature compensation. Bit more outlay but mine's been faultless through two Yorkshire winters now. The relay is essentially uncontrolled charging which your BMS will always be fighting against.

What temperatures are you actually seeing at the cells when it trips?

MPPT_Wizard | Posts: 1,204 | Location: Array

This bit me badly last February with the shepherd's hut build. What nobody's mentioned yet is the rate of temperature change being the culprit as much as the actual low threshold.

Split charge relays are notoriously brutal — engine fires up, relay slams shut, and suddenly your BMS is seeing a chunky current surge hit cells that might be sitting at 4°C. The JK sees that voltage spike across cold internal resistance and panics.

What sorted mine was adding a Victron Cyrix-Li-Charge instead of a dumb relay — it's actually LiFePO4-aware and plays nicer with BMS protection events. Bit more outlay but I've not had a single nuisance trip since.

Worth ruling out whether it's genuinely a temperature cutoff or actually an overcurrent protection event — the JK logs both differently in the app.

SaltyTinker | Posts: 312 | Location: Scottish Borders

@StormyNomad the JK's low temp cutoff is definitely the culprit, but worth mentioning that the onboard temperature sensor placement matters a lot here. If yours is sitting near the middle of the pack rather than against an outer cell, it'll read warmer than reality during a cold soak. I solved mine by adding a cheap NTC thermistor taped directly to the coldest cell with self-amalgamating tape. Also double-check your split charge relay isn't sending voltage spikes on engagement - cold weather makes relays lazy and that initial surge can occasionally trigger overcurrent protection too.

DriftKing | Posts: 2,156 | Location: Peak District

@StormyNomad worth noting that split charge relays can also cause the BMS to see a slightly odd charge profile compared to a proper DC-DC charger. The voltage spike when the relay engages can sometimes trigger protection circuits independently of the temp cutoff others have mentioned. If you haven't already, try logging your cell voltages at the moment it trips - the JK app makes this dead easy. Might reveal whether it's genuinely thermal protection or something else entirely. A B2B charger would solve both issues in one go if you're open to upgrading.