Remote Start Genset on AC Load

Been wrestling with something similar on my backup setup and thought it'd be worth opening up the discussion here.

Running a Victron Multiplus-II 5000 paired with a Fogstar Drift 200Ah lithium bank, and I've been trying to get my generator (a Honda EU22i) to kick in automatically based on AC load demand rather than just SOC thresholds. The use case is almost identical — I've got an immersion heater on a timer that pulls about 2.5kW, and when that fires up alongside the fridge and a few other loads, the battery bank gets hammered harder than I'd like for an emergency backup system.

The challenge I keep hitting is that the VE.Configure assistant for generator start has fairly blunt instruments — you can trigger on SOC, voltage, or even AC load, but getting the hysteresis and time delays dialled in properly to avoid the genny cycling on and off every few minutes takes some proper experimentation.

A few questions for the group:

• Has anyone used the AC load trigger specifically (not just SOC) with a Multiplus or Multi RS setup? What thresholds did you settle on?
• Is there a meaningful difference in behaviour between the Multi RS and the Multiplus-II when it comes to generator assistant logic? The RS seems to have slightly different firmware behaviour from what I've read.
• Anyone running a Kohler or Pramac unit with a proper digital remote start interface rather than the two-wire start Victron typically assumes?

Would be particularly interested in setups where the battery is acting primarily as a buffer/backup rather than deep cycling daily, since the charge logic feels like it's optimised for solar self-consumption rather than my scenario.

@SolarJake the Multiplus-II has a built-in AC generator start/stop via its aux relay — worth digging into VEConfig if you haven't already. You can set SOC thresholds to trigger it automatically.

With the Fogstar Drift you'll want to make sure your BMS comms are sorted though. If the BMS trips before the genset kicks in properly you'll get a nasty disconnect mid-charge.

Few things I'd check:

• Generator start SOC vs low battery disconnect — leave enough headroom between them
• Victron's ignore AC input setting can cause headaches if ticked accidentally
• Warm-up delay before accepting load

I ran into this exact mess on my garden office setup last winter. Ended up adding a small delay relay to give the genset time to stabilise before the Multiplus accepted it. Pain but sorted it.

@QuietTrekker is right about the aux relay, but worth noting the Multiplus-II's generator start logic works off SOC thresholds or AC load — not just one or the other. You can stack conditions in VEConfigure so it only fires up the genny when both SOC is low and load demand is high. Stops unnecessary run cycles.

One thing I'd double-check with the Fogstar Drift — make sure your BMS comms are feeding SOC accurately into the Victron via the VE.Can or VE.Bus otherwise the start trigger can be unreliable. Had this exact issue on my cabin setup before I sorted the DVCC settings.

Also consider a time delay on start to avoid the genny kicking in during brief load spikes. Configurable in VEConfigure under the generator tab.

@EmmaEdwards82 makes a fair point about SOC thresholds, but one thing neither of you have mentioned is the relay hysteresis — if you set your start and stop SOC too close together, you'll get the genny short-cycling, which is murder on the brushes and AVR over time.

On my static caravan setup I run a Victron Cerbo GX alongside the Multiplus-II, which gives you far more nuanced generator control via the GX generator start/stop assistant — you can stack conditions (SOC and AC load and time window), so it won't fire up at 2am just because a cloud passed over.

Worth also checking whether your Fogstar Drift's BMS is correctly feeding SOC data over to the Cerbo via VE.Smart or a shunt — inaccurate SOC readings will completely undermine any threshold-based start logic.

@MandyRoss good shout on the hysteresis — that's something I got caught out on with my motorhome setup. Related question though: does the Multiplus-II's generator start logic play nicely with AC load-based triggering as well as SOC?

I've seen mentions of it starting the genset when load exceeds a certain wattage threshold, but couldn't find clear guidance in the VEConfig docs on whether both conditions run simultaneously or if it's one or the other.

On my shepherd's hut build I'm planning to trigger the genset before the battery hits a damaging SOC, but I'd also want it to kick in during a sudden heavy load spike regardless of SOC. Are both trigger conditions active at the same time, or do you have to pick one?

@MandyRoss @SaltyGrafter hysteresis caught me out too — my genset was basically doing the hokey cokey for three days before I figured out what was happening. On, off, on, off. Neighbours loved it.

One thing worth adding: if you're using the generator start assistant in VEConfig, double-check