What size is a Optima battery for a B-7800

[Speedybon]

I using an AC 51RPS 85 500cca I think it a group 51 series.

 

[Henro]

I would buy the largest that would fit in the space physically...

 

[Speedybon]

I just got my Optima battery today, and it fits perfectly, so if you have a B-7800 or something equal, a Yellowtop D51R works.
They are a bit pricey, but they will last a long time. I also use their charger to maintain it. And that is the biggest battery that will fit.