Cooler connexion

Been thinking about this after a mate asked me something similar last week — running a 12v compressor coolbox directly off the load output on a Victron SmartSolar MPPT.

Short answer from what I've found: probably fine in practice, but not ideal.

The load output on something like a 75/15 is rated to 15A continuous, which most 12v coolboxes will sit well under during normal running. The startup surge is the bit that can catch you out though — compressor units can spike pretty hard for a fraction of a second on startup, and depending on how aggressive your low voltage disconnect settings are, you might get nuisance tripping.

What I'd actually recommend instead:

• Run the coolbox direct from the battery with an inline blade fuse
• Use the load output to trigger a relay if you want automatic disconnect at low SOC
• That way you're not stressing the controller's load terminals with compressor cycling

I've got a Brass Monkey coolbox running in my garden office setup and went the relay route for exactly this reason. Much happier with it and the Victron VE.Direct gives me proper visibility on what's actually pulling current.

Anyone running a compressor fridge direct off load output long-term? Curious whether people have had issues or if I'm being overly cautious here. Also — does this differ much between Victron and something like a Renogy Wanderer controller? Suspect the load output quality varies quite a bit between brands.

@SolarTrevor the load output on a SmartSolar is rated at 15A continuous — that's fine for most 12V compressor boxes at cruise, but the startup surge is what kills people. Compressor motors can pull 3-5x running current on startup, even briefly. Victron's own documentation is frustratingly vague on how the load output handles surge — it's not a proper relay, it's a MOSFET switch, so it will current-limit or shut off if you spike it hard enough.

My actual recommendation: don't use the load output for this at all. Wire the coolbox directly to your battery with a proper fuse, then use the load output to trigger a decent relay instead. Keeps the MPPT's load circuit out of harm's way and gives you proper current handling.

What's the specific compressor unit? Make/model matters here.

Has anyone actually tested what the inrush current looks like on startup for these compressor boxes? I've got a Mobicool MCF32 in my static caravan setup and I've always wired it direct to the battery rather than through the load output precisely because I wasn't sure about those startup spikes.

The SmartSolar load output — is there any soft-start protection built in, or does it just see whatever the compressor motor throws at it on first kick? I'd imagine a locked-rotor situation on a hot day could be fairly brutal.

Also worth considering: does running via the load output affect the MPPT's ability to prioritise charging? I'm thinking about scenarios where solar input is marginal and the compressor is cycling frequently.

@WattKaren good question on the inrush — I measured mine (Alpicool C15) with a clamp meter and the startup spike was around 6-8A for maybe half a second before settling to ~2.5A running. Not massive but worth knowing.

The thing I'd flag that nobody's mentioned yet: the SmartSolar load output has a low voltage disconnect built in, which is actually useful here. You can set it to cut the fridge before it drags your LiFePO4 too low overnight. Mine's set to disconnect at 12.8V and reconnect at 13.2V.

Works a treat in the tiny house setup. Running a Fogstar 200Ah battery so recovery's quick once the sun's up.

@WattKaren @LiFePO4Fan worth noting that the SmartSolar's load output has its own overcurrent protection, and a sharp inrush spike — even if brief — can trip it depending on the controller firmware and settings.

I ran a Dometic CFX3 off the load terminal of my SmartSolar 100/30 at the cabin last summer. Worked fine most of the time, but on cold mornings when the compressor was starting against a warm load it occasionally threw a fault. Moved it directly to the battery bus with a proper inline fuse and the problem vanished entirely.

Honestly, for anything with a compressor motor I'd treat the load output as a monitoring/lighting circuit and wire cooling loads straight to the battery. Less elegant maybe, but far more reliable. Victron's BatteryProtect is a tidy way to keep LVD functionality if you need it.