Evening all,
I have a few 40Ah 12v batteries, they are half the size of regular batteries and in my research into getting a second battery for my 90 I realise I want something like an 80Ah 12v leisure for my second battery. These are deep discharge high cyclic sealed lead acid batteries, and I was wondering if they would be any good wired in series and then wired with a split charge with my main battery.

Am not too knowledgable with car batteries, so I'm sorry if this is a rubbish suggestion.

depending on the make most sealed lead acid batteries like low current and constant voltage charge.
best thing to do is look on the makers website.

If they are mat impregnated lead acid batteries- then this is what motorcycles use and no problems. also you want to wire in parrallel not series or you will have 24v

"Am not too knowledgable with car batteries"

If your not that knowledgable, nothing wrong with trying, personally, I'd just get one of these;

Numax CXV-X 1000 Amp - Leisure Batteries - Numax Leisure Batteries