PowerSwitch

I've bought a cheap inverter off ebay (24v, 600w nominal , 1200w peak)
by a company called A.M.P.S.

These seem to be a US small marine power supply company with a site in Worcester, UK.

Anyway, to test the inverter, I connected it to two 12V 110Ah batteries in series.

When I connected the inverter, it gave a large spark as I added the second terminal (negative, I think). Dropped the connector. On the second contact, no spark. However, the inverter works.

I then disconnected the inverter, and as I was putting it away, the connectors touched and gave another, medium sized spark.

So my question - is there a large capacitor inside the inverter, and is there a connection sequence which woud avoid the sparking?

I have a charge controller with PV panels I still haven't set up yet, would I be better advised to link this in between the batteries and the inverter?

Yes, most, perhaps all inverters do have a large capacitor connected accross the input.
This is likely to lead to sparking when connected to a battery, and also after use if the battery leads are shorted.

Sparking in close proximatly to batteries can be potentialy dangerous, since a mixture of hydrogen and oxygen may be present, which is explosive, and could be ignited by a spark.
Vented batteries that have just been charged are the greatest risk, sealed batteries are very low risk since they only emit explosive gases under fault, failure, or misuse conditions, not normally.

For temporary or emergency use, ensure that the area is very well ventilated to reduce the risk.

For regular use, it would be better to connect the inverter to the battery via a suitable switch.
Ensure that the switch is off before making or removeing the battery connections.
When desireing to use the inverter, turn the switch on. Sparking at the switch contacts will be much less than at the battery terminals, and should be at least 1M away from the batteries.

I would normally advise against connecting an inverter to the battery via a charge controller.
The load circuit of most charge controllers is of limited capacity, and not suitable for most inverters.
A 24 volt 600 watt inverter will use about 30 amps at full load, and perhaps twice that very briefly.
Some charge controllers can handle a load of 30/60 amps, but most are 20 amps or less.

Thanks that great.

Yes, my inverter does that. It's normal