Hello,
I changed the battery on my Furuno GP 90. It is a lithium 2450. price 2 euro.  But it is difficult to do because the battery is soldered inside the GPS.  Removing the old one with a soldering iron is easy, but soldering a new battery without specific tools is very difficult.  I just fixed it without soldering it.  It's been 3 years and it's holding up.
Dominique
Irko
A54-16