What happens to a Victron 250-70 charge controller

Had this exact scenario play out on my static caravan setup last winter — LiFePO4 bank (non-smart, no comms) hit low temp cutoff on a particularly grim February morning, BMS disconnected the load side, and I sat there wondering if my Victron SmartSolar 250/70 was about to have a meltdown.

Short answer: it didn't. Carried on like nothing happened.

Longer answer: when the BMS opens that circuit, the MPPT essentially loses its output path. Victron's protection circuitry is decent enough that it just sees a high-voltage open circuit and throttles back — it's not like dumping current into a wall. The panel voltage climbs a bit, controller detects no absorption happening, sits in a kind of confused float limbo until the battery warms up and the BMS reconnects.

What I would worry about though is repeated thermal cycling causing the controller to log fault states, and whether your battery actually warms up fast enough in a UK winter to stop you losing a full day of marginal December solar. Spoiler: it doesn't.

Without a comms cable you're flying blind — the Victron has no idea why the battery disconnected, just that it did. A Cerbo GX with temperature sensor on the battery would at least give you visibility, even if it can't do anything about it.

Anyone else running non-smart cells in cold climates found a workaround that doesn't involve buying Pylontech and starting again? Asking for my increasingly-expensive-to-heat caravan. 🙄

@Rob1963 This is such a classic "February surprise" moment and I've lived it in my motorhome too.

What caught me off guard was the Victron 250/70 just kept doing its thing — merrily pushing current toward a battery that was essentially a brick wall. The controller itself was fine afterwards, but those moments where it's absorbing into nowhere always make me nervous.

The real villain in your scenario is exactly what you've identified — a dumb BMS with no low-temp comms. My fix was eventually adding a Victron temperature sensor and setting a conservative charge cutoff, so the MPPT throttles before the BMS throws the emergency stop.

The 250/70 is honestly overbuilt enough to survive the drama, but that disconnection event creates a voltage spike situation that I'd rather not repeat regularly. Worth checking your cabling terminations after it happens.

@Rob1963 this is exactly the failure mode that caught me out too on my static caravan before I properly understood BMS disconnect behaviour.

Worth clarifying the specific Victron 250/70 response here: when the battery-side suddenly goes open-circuit mid-charge, the controller sees a dramatic voltage spike — the panel array voltage essentially collapses onto the output with no load to absorb it. The 250/70 should handle this within its protection circuitry, but repeated events can stress the MOSFETs over time.

Key things worth checking after an event like this:

• Absorption/float logs via VRM — did it throw any fault codes?
• Inspect the controller casing for any thermal discolouration
• Consider adding a Victron Smart BMS or at least a temperature sensor to give the MPPT advance warning before hard disconnect occurs

The low-temp cutoff triggering without any charge controller awareness is the real design gap here.

@Rob1963 had this exact thing happen on my cabin setup — Victron MPPT kept pushing current into essentially a dead end once the BMS disconnected. What saved me was adding a Victron Smart Battery Sense with the low temp relay output wired to a remote on/off on the controller. Cuts the MPPT before the BMS even trips.

Worth noting the 250/70 does have a built-in protection that'll eventually back off if output voltage climbs too high with nowhere to go, but I wouldn't rely on that as your primary safeguard. Proper temp sensing is the real fix.

If your LiFePO4 bank is non-smart, a cheap external temp sensor relay is a decent workaround until you can upgrade.

When my boat's BMS pulls the plug mid-charge, the Victron just sits there looking confused like a Golden Retriever that's lost its ball — absorption voltage with nowhere to go.

@Cerbo_Master that Golden Retriever comparison is painfully accurate 😄

Worth adding — on my shepherd's hut setup I actually wired a small voltage divider across the battery terminals feeding the MPPT's battery sense input. When the BMS disconnects, the sense voltage doesn't suddenly crater, it bleeds down slowly through the resistors. Gives the controller a slightly softer "landing" rather than that harsh disconnect confusion.

Not a proper fix obviously — the real answer is a smart BMS with Victron comms, but that wasn't in budget at the time.

Also noticed the 250/70 specifically seems to recover more gracefully than some older Victron units once voltage normalises — mine just resumes the absorption cycle without throwing a fault that needs manual reset. Different firmware versions behave differently though, so YMMV.