Second charging source on generator power

Had a similar head-scratcher with my van build recently, so this feels timely.

Running a Victron Multiplus-II 12/3000 and a bank of Fogstar Drift LiFePO4s. The Multiplus charger does a solid job at shore power or off a genny, but I've been wondering about adding a secondary DC-DC charger into the mix — specifically for when I'm parked up and running my little Honda EU22i.

The thing is, the Honda can realistically push maybe 1800W sustained without grumbling, and the Multiplus alone can technically pull more than that when it decides to go full beans on charging. I've had it trip the genny's overload protection more than once, which is deeply annoying at 11pm on a campsite.

So the question I'm actually chewing on: has anyone here run a second charger — say a Sterling or Victron IP22 — alongside their Multiplus from the same generator? Presumably you'd need to derate both units so the combined draw stays sensible, but I'm not sure if there's a smarter way to orchestrate it through VictronConnect or ESS settings rather than just manually throttling things.

For lead carbon specifically I imagine hitting the right charge profile is more critical than with LFP, so I get why you'd want maximum current available from the genny without cooking anything.

Anyone actually done this successfully? Particularly curious whether the Multiplus plays nicely with a second charger on the same AC input without getting weird about voltage sensing.

@CamperWayne — classic Victron setup, and I've been down exactly this road with my own emergency backup system.

The bit you'll want to watch: if you're thinking of adding a second charger (DC-DC or otherwise) whilst the Multiplus is pulling generator power, the BMS on those Fogstar Drifts can get confused by competing charge sources hitting it simultaneously. I ran a Victron Orion-Tr Smart alongside my Multiplus for a while and had to explicitly configure the Orion's absorption voltage below the Multiplus's setpoint, otherwise they'd effectively fight each other over who controlled the tail current.

VE.Bus or VE.Smart networking sorts this elegantly if both devices speak Victron — the Orion backs off when it sees the Multiplus is active. Without that link, you're managing it manually through voltage offsets, which is fiddly but workable.

What's the second source you're considering?

Worth flagging — the Multiplus-II already has a second AC input specifically for generator use, so depending on your setup you might not need a separate charger at all.

That said, if you're looking to add a DC-DC charger as a second source (say, from an alternator or solar controller running simultaneously), just make sure whatever you add is LiFePO4 compatible and ideally has its own BMS comms or at minimum proper charge profiles.

With my cabin setup I ran a Victron Orion-Tr Smart alongside the Multiplus for a while — worked fine, but you need to think about total charge current hitting the Fogstars at once. Check your BMS max charge rate and don't exceed it combining both sources.

What's the actual goal here — faster charging, redundancy, or something else? Makes a difference to the recommendation.

@EmmaEdwards82 is on the right track — but the real question is whether your genny can actually handle the Multiplus's inrush without the voltage sag making it throw a wobbler and disconnect anyway.

Set your AC input current limit in VictronConnect to something sensible — I run mine at 10A on a 2kW genny, stops it brown-out choking mid-charge. Also worth checking your ESS or PowerControl settings if you've got anything else pulling load simultaneously.

If you genuinely need a second DC charging source on top, a Victron Orion-TR Smart isolated DC-DC charger plumbed into your alternator circuit solves a completely different problem — don't conflate the two or you'll end up with a rats' nest and a headache.

@Rob1963 raises something I've had to deal with hands-on with my narrowboat setup — generator quality really does matter here.

Worth adding: if you're running a cheap inverter-style genny (the sort of thing you'd pick up from Machine Mart for a few hundred quid), the Multiplus can be very fussy about the AC waveform. I had endless nuisance tripping before I set the ac-input-current-limit lower in VE.Configure and enabled the weak AC input setting.

Also, on the "second charging source" angle — if you mean adding something like a DC-DC charger in parallel whilst the Multiplus is running, double-check your BMS can handle the combined charge current. Fogstar Drifts are solid but they're not unlimited.