I am considering one for my 182. What do you have to do to the battery
box and what is the difference in weight?

Javier Henderson wrote:
I got an RG35AXC in my Skylane earlier this year. Piece of cake to
install, there's a small mod needed to the battery box lid, requiring
a highly specialized tool (a utility knife). The battery is STC'd for
various airframes, the STC paperwork was available from Concorde free
of charge.

I needed the involvement of an IA because of the STC issue, but
further replacements can be done by myself.

They're maintenance free, and they're not venting corrosive
fumes. They're a little more expensive than flooded cells, but in
AMU's, the difference is a pittance.

Go for it.

-jav