i had one and the battery kept loosing its charge so took it apart and its only a 17ah battery in the one i had,looked for new battery but almost as costly as getting a new unit.in the end i did not and have a spare 100a terry battery.
it fitted both 2.7 but the terminals are wrong for my 3l.
in the winter i did not use 3l for 3months and needed a jump,i put it on charge but lucky for me a neibour had called out a local garage to get there car started and i knew him from old and he got there car started and came over and jumped mine but it was a big power pack.i was happy as he did not take the fiver i offered as he said they paid lol.
i kept the compressor bit as was going to fit it into front of caraven with cioled hose,never did though.
so the big ons are ok for big ons and the smaller ones ok for cars in my view.