question

@conceptura

Is it the fuse tripping or the inverter?

If it is the inverter tripping, then a high resistance connection has been added. So either something is not tight enough lugs not crimped right, or the fuse has added issues.

If i recall correctly 12 3000 would need 2x50mm² for both positive and negative with a 400A fuse (so 2x 200A for each cable).

Is it a proper fuse?

Similar issues are common if you use crappy automatic breakers.

Thanks for the replies and insights. It"s a jtron fuse that I added. https://www.amazon.fr/dp/B09XHV4HR8?psc=1&ref=ppx_yo2ov_dt_b_product_details , the 300A verion. It trips the inverter and not the fuse.

On the lugs I soldered them on with pre-fluxed tin solder, inserting the wire into the lug and then filling the lug with molten solder. I didn't crimp as I don't have a tool for that and the wire was held rock solid. I wonder now whether the solder has enshrouded the wire and created a high resistance barrier between the lug and the wire. What do you think? Thanks.

It's possible that you melted the plastic housing into the internals of the breaker. I've never seen that type of breaker before but I know from looking at it that it was not designed to be soldered.

Hat exactly happens when the Inverter trips?

It may have something to do with the resistances which are supposed to be lower now since the cabling is improved.

Another point could be the Restwelligkeit of which I dont know the english word, sorry.

If it Was the breaker that tripped, I remember having Reader about breakers that trip way ealier than their designated amperage due to warming-up components. This is why I avoid the Electromechanic breakers.

A 300A breaker for 20 euro... can't be good. I think the Multiplus trips because of low voltage. This is another type of thermal breakers which usually do more harm than any good.

With higher currents they will have a huge voltage drop. If it weren't for the Multiplus to trip, I'm quite sure it would start smoking/melting if you really would run 200A through it.
If you have a MK3 you will probably see a huge voltage drop (or just use a multimeter to check).

Just get a good MEGA fuse + holder (10-30 euro or so).
Or if you insist on a breaker, expect to pay 200+ for a proper quality one.
And just a tip: Don't solder. Always crimp. Solder is not suitable for any installation where vibrations are involved.

It's your wire size. I had this problem also using 2/0 cable. Coffee maker would run for about 30 seconds then inverter would shut down. No faults in the history. I used 4/0 cable for everything and have had no issues since.

Thanks to all of you for your input and sharing experience. Absolutely fantastic learning for me. Actually things seem to have settled down now suggesting something else was going on but now the toaster runs fine without tripping. That said to advance my/our learning I would point out/ask:

(i) no cables melted. When I added the fuse I upgraded my battery--->inverter cable from 35mm2 to 50mm2. This should be more than enough unless I am mistaken. I am not sure whether "kkesp" you are referring to awg references but if you know the equivalent in europe I'd be glad to know what gage you are referring to.

(ii) I soldered without knowing much about crimping but there is no vibration involved in my system so I assume I'm ok to not worry about that.

(iii) Fuse quality..ok €17 is not much but the fuse feels good quality. It's robust, the fixing points are good, the screws to tightened onto the cables are sturdy etc and it works. However I'd be interested in researching the other technologies/sources mentions ( Mega or NH fuses mentioned by DJsmiley and Alexandra)

(iv) Voltage drop - the voltage drops only half a volt when the load is applied (i.e. when I toast some bread :-)) so today I went from 12.7 to 12.2V..doesn't seem too dramatic.

All in all everything seems ok for now but I would like to be ready with solutions (ie better fuses etc) of the problem crops up again.

Thanks all and wish you a great day!!