The battery is probably 80% (or more) the capacity of when new, which for most things is fine...
There many phones that, when video recording and displaying on the screen, would run the battery to zero even when on charge, i.e. using energy faster than you can replenish it! Same for a lot of games running on phones....
I gave up on SatNavs, so I have no recommendations! I got sick of taking off/putting on the windscreen, as they are what is known as "smack-head magnets".... Might get one again, now I live in a very quiet Close in very quiet village though....