Nur Überschuss für den Boiler nutzen

Been mulling this one over for a while now, so thought I'd finally put it out there.

My static caravan setup runs a pair of 400W panels feeding into a Victron SmartSolar MPPT 100/30, with a DIY 24V battery bank I built from some second-hand LiFePO4 cells sourced through Fogstar. The Cerbo GX ties everything together nicely and gives me a decent view of what's happening across the system.

Here's my dilemma — on a decent summer's day, the batteries are fully charged by mid-morning and the MPPT is throttling back hard. All that potential solar generation is just... sitting there doing nothing. Meanwhile I've got a 2kW immersion heater in the cabin that I'm currently running off LPG-heated water like some kind of caveman.

What I want to do is use only genuine surplus — once the batteries are at, say, 95% or above — to heat the water. Not drain the batteries, not complicate the system, just mop up what would otherwise be wasted.

I've had a look at the Victron ESS assistant and played around with some relay logic in VenusOS, but I'm not fully convinced I've landed on the cleanest solution yet.

Has anyone here set something similar up? I've seen the iBoost mentioned in a few threads but that's really designed for grid-tied systems. Wondering if there's a relay or smart switch approach that plays nicely with the Cerbo's logic.

Curious what others are doing — especially if you're working with a modest panel array and a self-build battery setup like mine.

@SparkySparky mate the thread title is in German but your setup is very much English — much like my Fogstar cells, works brilliantly but the manual's a nightmare 🔋

@DaleLover thread title's in German but at least the solar still works in any language, unlike my Renogy display which speaks exclusively in error codes and broken dreams 🔧

@SparkySparky classic diversion relay setup is what you want here. The Victron SmartSolar has an internal relay you can configure to trigger only when absorption/float is reached — essentially telling your boiler immersion "right, batteries are stuffed full, have the rest."

Set the relay output on the MPPT to activate at float voltage, wire it through a solid state relay (SSR) rated for your immersion load, and the boiler only ever sees genuine surplus. No microcontroller faff required.

Few caveats from my own motorhome build though — size your SSR properly, those cheap blue ones off eBay get hot handling 3kW immersions. Also worth logging through VRM to verify the relay behaviour before trusting it fully. Victron's documentation is decent on this but the community wiki fills the gaps.

What's your immersion element rated at? That changes things considerably.

@LiFePO4Nerd is on the right track with the relay, but worth flagging that the internal relay on the SmartSolar 100/30 is quite limited — it's a virtual relay really, better suited to triggering an external contactor than directly switching a boiler element.

What's worked well for me is pairing the MPPT relay signal with an iBoost or a basic Victron Energy Meter setup to properly track surplus. Alternatively, if you want something simpler and cheap, an Immersun second-hand off eBay can be had for buttons and handles the diversion logic itself.

The key thing with a static caravan install is that your battery hits absorption/float before the diversion kicks in — otherwise you're robbing your bank to heat water, which rather defeats the point.