Multiplus C sudden loud buzz and Overload/Low Battery - No load

DIY
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1

Hi everyone,
I’d be really grateful for any pointers on this, as I am completely stumped. I installed the Multiplus C 12/2000-80 on my boat together with 2 new 300Ah Lifepo4 batteries. Wired everything up, fused for 300A between Multiplus and batteries and each battery fused again with 250A. 70mm² cabling and a no more than 1,50m between Multiplus and batteries. Voltage drop from battery to Multiplus DC input 0.01V.

Switched on for the first time yesterday. Everything worked fine. Had AC power, charged the batteries, fans lightly spinning, low humming sound. Turned everything off again, AC off and battery disconnect switch off and went home. Today before working on something else, I turned on again and it started charging at 60A, which is what I set. The batteries were at 13.2V when I started. While charging, they read 13.6V. About 2 hours later, I came back into the cabin and sat down to read a manual for something and suddenly the Multiplus makes a LOUD harsh buzz and the error LED comes on. I connected the USB-dongle and it showed:

Screenshot_20260425_154134
Screenshot_20260425_1541341005×780 66.8 KB

You can hear the buzz here: Imgur: The magic of the Internet
At the same time the residual current breaker in the boat hall tripped and the lights out there went off. Now I looked around this forum and the manuals etc. and this combination of errors seems to point to excessive voltage ripple on the DC side, mostly caused by undersized cabling or loose connections. So I checked every connection mechanically, measured resistance and the aforementioned voltage drop. There is nothing. Everything solid. But even if there was something, that would not explain the breaker tripping.
On the AC side I had a charger for my Bosch 12V batteries connected. It drew 40W at the start but by the time this happened it had finished charging and there was about 7W load on the AC output. I disconnected the output completely to make sure that this was not the problem but it still does not start again, AC input connected or not. And if I connect the AC input again, the breaker trips straight away.
Has anybody seen something like this before? What could be going on? I am confused because it worked fine for quite a time, charged and inverted.
Best regards,
Tom

2

Sounds like the inverter has failed. Remove from Boat, bench test and prepare for a warranty claim.

3

Hey Mike,
thanks for your answer. Bench testing would involve connecting AC and seeing if it also blows the breaker with nothing else connected? I connected it like that before I installed it to configure it for Lifepo4.
This seems really unusual to happen after a combined maybe 3 hours of operation, doesn’t it?
I forgot to mention that I also tried to measure the ripple on the DC terminals. But it turns on only for the duration of the buzz and my multimeter only registers it rising up to about 0.85V during that short time. Since this is with no load connected, I don’t know where even that would come from.

4

Maybe one more observation: I was (by pure chance) sitting right next to it when it first made the buzzing sound. No magic smoke came out and there was no weird smell or anything.