Inverter amperage

Pretty much - but allow for some power wastage in the inverter itself - so probably more than 100A max load.

They have a deal with the makers of car fuses :-)

(Or perhaps only expect low loads like battery chargers, and shavers etc)

Not much, I think you have spotted the limitation, they draw *loads* or current to compensate for the lower voltage. (ones that use series batteries have a much easier time, since they at least halve the current demand)

The inverter won’t be 100% efficient.

Assuming it is 90% efficient - a reasonable working number- on full load it will need 1333 W which is 111A at 12V.

I’d expect the fuse on your cigarette socket to be 10 or 15 A.

That said, it shouldn’t be drawing anything like 111A with no load or even just a shaver. I suspect a shaver is a few Watts at most. The inverter will have a ‘standing’ / resting current but I’d expect that to be an amp or so. Therefore, it should have been drawing a few amps.

My guess is, the inverter hadn’t been used for some time. When first connected the inrush current to charge the capacitors popped the fuse.

These larger inverters aren’t designed to be connected via those sockets. They should be wired to the battery via decent cable and a fuse of the recommended rating.

When it has a defined load, it is going to be pretty tame (long term). You should connect a load to your inverter, one that will glow when the inverter is working properly. You should not leave an inverter with no load on it.

It could have a transient behavior, and draw enough current to blow a fast blow fuse. But it is pretty hard to guess why it would do that.

You could set up a car battery on the lawn, put a 100 amp fuse in series with it, and connect the inverter and a 60W incandescent as a "proving load" that the thing works, and test it that way. Or, you could connect it with pieces of wire guaranteed to burn through, if the current is too high.

But as you can see in the picture, the 12V side only needs to deliver

The nice thing about batteries, is you can run all sorts of insulting circuits from them, and they don't even whimper. I had a bench supply once, and connected a flyback circuit to it, and the back-EMF into the bench supply, blew out the MOSFET used for sensing the output voltage. Whereas when I ran the flyback off lantern batteries, the circuit caused no problems at all. Batteries are really nice for this sort of circuit (inverters, flybacks, joy-buzzers, Tesla coils). That's the flyback I used to run my gas laser in uni (it's the kind of laser you select, when you want to get a credit for building a laser). Batteries are a great source of power, when the circuit is "dangerous".

Paul

Hmm…. Does if affect the calculations if some numpty has wired the input to the inverter with reversed polarity :(.

I suspect so.

I have now connected it using its crocodile clips leads, didn’t have a mains load on, but its LEDs lit which is progress.

Checked the cigar lighter cable. Its fuse hadn’t blown on account of it doesn’t have one.

Don’t think the crocodile clips leads do either.

Some of you may have seen my other thread. What I think has happened is an internal fuse (piece of wire) in the cigar lighter socket has blown :(.