A quick point to note: FWIW> DO NOT charge a Sealed lead acid,(SLA), Gel-Cell with a normal car battery charger. They put out too much current & are not regulated enough for the Gel-Cell. You MUST USE a DEDICATED Gel-Cell charger which switches to trickle charge when it "senses" the peak voltage.
If you buy one of those jump starters, they come with the 240 volt plug packs & 12 volt chargers built-in/supplied.
Don't forget, more capacity is better.

L.