To avoid the spark - you touch the llast lead to the invertor to the ist 6 volt, then to 12 volts (that is two batteries) and then to 3 then to 4th and make the final connection. This charges up the big capacitors inthe invertor slowly a bit at a time. The alternative is to use a 12 volt car inspection lamp connected to the last main lead and touch it to third battery first then the last and connect the main lead. This does the same thing.
Olly and the Team
Olly and the Team