I've yet to get a gel battery thats a good fit in the 550's battery space, plenty will fit and leave room to spare, and on paper be same / higher speck. It's just easyer for me to stick a car battery in the sidecar but the gel battery in dads bmw has outlived it's previous lead acid equivalent and not failed despite being on a bike that lives outside without maintenence all winter.............. There are lots of suppliers out there online, I'd expect one thats a close fit for the cb550 for