12V 375W phoenix inverter overload at 125 W

Seeing this crop up more than once now and it's worth discussing properly.

Had something similar with a Victron Phoenix 12V 375VA a while back — resistive loads should be the easiest thing an inverter handles, so when it trips on something like a soldering iron it's worth digging into why.

A few things worth checking:

• Battery voltage under load — if your 12V bank is sitting at 11.8V or below when the inverter kicks in, the unit has to draw significantly more current to compensate. A 125W load at low voltage can pull well over 15A from the battery side, and that's before accounting for inverter efficiency losses.
• Cable sizing and connections — undersized DC cabling or a dodgy terminal connection will cause voltage drop that compounds the above. Worth checking with a multimeter at the inverter terminals while under load, not at the battery.
• The load itself — some soldering irons have a brief inrush spike on startup even if they're nominally resistive. Not massive, but if the inverter's already marginal it might be enough.

What's the battery setup — flooded, AGM, lithium? And what gauge DC cable are you running? Fogstar Drift cells for instance can handle the current no problem, but an older AGM under load might sag badly.

The Phoenix is generally a solid bit of kit. If two have behaved the same way, I'd be looking hard at the DC side before blaming the inverter itself.

Anyone else seen this pattern?

@MarshChild worth checking what your battery voltage is actually doing under load when this trips. I had identical symptoms on my static caravan setup — inverter screaming overload at a fraction of rated load, turned out the battery voltage was sagging badly under draw and the inverter was interpreting that as an overload condition rather than a weak battery condition.

Victron's protection logic isn't always obvious. A 375VA unit pulling 125W is only around 10A at 12V — dead easy load — but if your battery internal resistance is high enough that voltage drops to say 11.2V the inverter starts compensating and the maths gets messier.

Stick a multimeter on the battery terminals while the load is running before assuming the inverter is faulty. Bet the answer's there.

@DODGuy is on the right track but I'd add — check your cable gauge and connection resistance too, not just battery voltage. I had this exact nonsense with my Phoenix 375 last winter. Battery showed 12.4V at rest but under load the voltage at the inverter terminals was dropping to 11.6V due to undersized cables. Victron's low voltage cutoff kicked in and it reported as overload rather than low voltage — misleading as hell.

Measure voltage directly at the inverter DC terminals under load, not at the battery. If you're seeing more than 0.3V drop across your cables, that's your problem. Proper 16mm² cable sorted mine completely.

Also worth checking your Fogstar or whatever cells you're running — if they're cold, internal resistance spikes and you'll see the same drop.

@SOC_Nerd and @DODGuy have covered voltage and cables, so I'll add the bit people miss — the low voltage disconnect threshold in the Phoenix settings. Default is 10.5V but if someone's nudged it up (or it's been configured for a lithium profile), it'll trip early and look like an overload when it isn't. Worth connecting VictronConnect and checking the actual fault history rather than guessing. The app tells you exactly why it shut down. For my garden office setup I had a similar phantom overload — turned out the LVD was set at 11.8V from a previous lithium config. Took me an embarrassingly long time to spot it.

@GlenDoug has nailed the LVD angle, so I'll come at this from a different direction — the inverter's own temperature compensation.

On my shepherd's hut setup, I had a Phoenix 375 doing exactly this on cold mornings. Turned out the unit had been tucked into a poorly ventilated locker; even though ambient temps were low, it was thermally cycling from the previous evening's use.

Victron's overload protection isn't purely current-based — internal temp plays into the derating curve. Worth pulling the casing away from any insulated surfaces and letting it breathe properly.

Also — have you run the VictronConnect app during an actual fault event? The history log timestamps the overload triggers and you can cross-reference battery voltage at that exact moment, rather than guessing afterwards. Changed everything for me diagnostically.