MPPT controller behavior when BMS is lost

Been mulling this over since I noticed some odd behaviour on my static caravan setup last winter and wanted to get others' thoughts.

When the BMS loses communication with the MPPT — whether that's a CAN bus dropout, a wiring fault, or the BMS itself going into protection — what should the MPPT actually do? And more importantly, what does it do in practice?

On my setup I'm running a Victron SmartSolar with a lithium pack that has its own BMS. From what I've read in Victron's own documentation, the MPPT is supposed to fall back to a fixed absorption voltage if it loses the BMS signal, rather than just continuing to push whatever it was doing beforehand. The intention being that it defaults to something safe rather than potentially overcharging.

What concerns me is the gap between documented behaviour and real-world behaviour. I've seen references to a brief window where the controller carries on regardless before the timeout kicks in. For a caravan sitting unattended — which is exactly my situation — that window worries me.

A few things I'd genuinely like to understand better:

• Does the fallback voltage get set at configuration time, or is it a hard-coded default?
• Is there any difference in behaviour between MPPT units on VE.Direct versus those on VE.Can?
• Has anyone actually tested this by deliberately pulling the comms cable and monitoring what happens?

I suspect most people with Fogstar or similar third-party lithium packs are running without managed comms entirely, so this may be less relevant for them — but for anyone with a properly integrated BMS talking to their controller, it seems like an important edge case that doesn't get much attention.

@Compo seen this exact scenario on my boat setup. The behaviour really depends on whether your MPPT is configured to treat comms loss as a fault condition or just carry on blind.

On my Victron SmartSolar, I've got it set so that if the BMS communication drops (via VE.Direct to the Cerbo), it falls back to the fixed absorption/float voltages I've programmed in. Not ideal but it won't cook the cells.

The danger is MPPTs that just keep pushing at whatever voltage the BMS last told them was acceptable — if that was a high-charge command mid-cycle, you could be sitting there overcharging with no cutoff.

Worth checking your MPPT's "BMS assistant" or equivalent settings and explicitly defining the fallback behaviour rather than relying on defaults. Fogstar and others publish BMS comms timeout specs too — worth cross-referencing.

Great topic @Compo. Worth distinguishing between communication loss and load disconnect from the BMS, as MPPTs respond quite differently to each.

If your BMS hard-disconnects the battery (common during overvoltage or overtemperature protection), a decent MPPT should detect the voltage spike and throttle back or shut down entirely. The problem is cheaper units can keep pushing current with nowhere for it to go, which isn't ideal.

Communication loss alone (CANbus or RS485 dropping out) is trickier — most MPPTs will simply revert to their standalone voltage profile, which may not suit your battery chemistry at all. If you're running lithium, that could mean charging to a profile set up for lead-acid.

What MPPT make/model are you running? Some brands handle BMS comms timeout much more gracefully than others, and there are often firmware settings worth checking first before assuming it's a hardware issue.

@ExpertBuild makes a crucial point there. On my shepherds hut setup I actually tested this deliberately last autumn — pulled the VE.Direct cable mid-charge on my Victron SmartSolar 100/30 to see what'd happen.

It just carried on charging, completely blind, using whatever absorption/float voltages were baked into its local config. The Fogstar LiFePO4 BMS had no way to shout "stop!" via that route — it had to physically disconnect the load to protect itself.

That's the scenario that keeps me up at night, honestly. If your MPPT's standalone voltage settings are even slightly mismatched to what your BMS expects, communication loss isn't just an inconvenience — it could be a slow-motion overcharge event before the BMS finally pulls the plug.

The moral I took from it: treat your local MPPT voltage profiles as your last line of defence, not an afterthought.

@ExpertBuild's comms-vs-disconnect distinction is the one most people miss — but there's a third wrinkle nobody's mentioned: what voltage the MPPT defaults to when it goes blind.

Victron kit typically falls back to absorption voltage and just sits there, which on a lithium bank with a dead BMS is a slow-motion disaster waiting to happen. Tested this myself on my motorhome rig — Victron SmartSolar kept pushing 14.4V into a bank the BMS had already isolated. Panels were still harvesting, controller was "happy," battery was screaming silently.

Set a sensible low temp cutoff and absorption timeout in VictronConnect before you need them — not after. Fogstar cells especially hate prolonged absorption with no BMS babysitting.