i like the gel cells ..
They're lead-acid and can deliver plenty of current if it's demanded. The gel (instead of liquid acid-water) prevents sulfation so they will survive being left unattended or partly discharged for a pretty long time. It's a totally sealed battery .. O2 and H gases that are generated internally recombine into water. (there's an emergency vent)
reasonably priced, zero-maintainance, won't leak even if you bust one open and can they be found in various rectangular shapes and sizes (amp-hour ratings) to fit into wherever spot is available.
i've got one on a bike .. havent touched that bike in 9 months.. just walked over there, turned the key and the starter motor cranked it over.