Multiplus Efficiency Loss on Heat Up

Been noticing something a bit odd with my Multiplus 12/3000 in the motorhome lately and wondered if anyone else has seen this.

When it first kicks in from cold, efficiency seems fine — but after it's been running a while and the unit warms up, I'm seeing noticeably higher draw from the batteries for the same load. Like, kettle on first thing in the morning versus kettle on after it's been running the heating circuit for a couple of hours. Same load, different consumption.

Checked the Victron Connect app and nothing's flagging as a fault. Temps aren't anywhere near the thermal throttle threshold either.

My gut says it might just be normal behaviour as the internal components reach operating temp, but I can't find anything in the Victron docs that properly explains the efficiency curve across temperature ranges. Could also be something specific to running a 12v system — the higher currents involved leave less margin for error I suppose.

A few things I'm wondering:

• Is this a known quirk of the Multiplus specifically, or inverters generally?
• Anyone running a Multiplus-II seen the same thing?
• Could poor battery connections exaggerate this effect as everything heats up?

My 280Ah Fogstar Drift cells are fairly new so I don't think it's the batteries sagging under load, but I haven't ruled it out completely.

Would be curious whether this is just physics doing its thing or something worth investigating properly. Anyone got data from their VRM dashboards showing efficiency over time?

CliveCrane | 847 posts | Midlands

@WildTinker — you've cut off mid-sentence there, but I think I know where you're going! Classic thermal throttling situation, which I've seen on my own 24/3000.

Worth checking a few things once it's warmed up: are the fans actually spinning freely and shifting air properly? Dust and fluff can really clog those vents on motorhomes — all the road muck gets drawn straight through.

Also have a look at the temperature sensor readings in VictronConnect if you haven't already. If the unit's genuinely running hot rather than just feeling hot, that's a different conversation entirely about ventilation spacing and ambient temps in your install bay.

What's your battery bank — lithium or lead? Makes a difference to how the Multiplus behaves under sustained load.

BatteryTim | 1,203 posts | Array

@WildTinker finish your sentence mate, we're all guessing here.

@CliveCrane probably jumping to conclusions as usual.

That said — Multiplus 12/3000 thermal throttling is a known thing. Once the internals get above a certain temp, it backs off to protect itself. Not a fault, it's working as designed. Check your VE.Bus monitor if you've got it set up, you'll see the derating happen in real time.

More likely culprits though:

• Poor ventilation around the unit
• Mounting position (vertical, always vertical)
• Ambient temp in the motorhome engine bay or wherever you've bodged it in

My 12/3000 in the shepherd's hut has run faultlessly for two years because I actually read the installation manual before bolting it in.

SunnyFisher | 312 posts | Array

@WildTinker the suspense is killing me more than a bad connection on a January night on the cut 😅

While we wait for the rest of that sentence — are you monitoring this through VictronConnect or just by feel? The Multiplus does have thermal derating built in, so if your ventilation isn't great it'll throttle back efficiency as temps rise inside the unit.

On the narrowboat I found mine was essentially cooking itself because someone (me) had mounted it in a cupboard with approximately the ventilation of a sealed tin of beans. Moved it somewhere with actual airflow and the problem largely disappeared.

What does your VRM data show for internal temperature during these episodes? That'd at least tell us whether it's thermal protection kicking in or something else entirely.

NorfolkSolar | 2,156 posts | Norfolk/Suffolk border

@WildTinker looks like the forum ate the rest of your post! Finish your thought when you get a chance — but if I'm reading between the lines, are you seeing the fan ramping up and efficiency dropping after 20-30 minutes of sustained load?

Worth checking what ambient temp it's sitting in. Motorhome installations can get surprisingly warm in the battery compartment, especially in summer. The Multiplus will throttle if it's heat-stressed.

Also worth a look in VictronConnect — check the history for any high temperature warnings. They don't always show up obviously but they'll be logged.

What's your battery chemistry and state of charge when this happens? Voltage sag under load could look like efficiency loss but actually be the batteries struggling rather than the inverter itself.