Little battery maintainers

[Kneale Brownson]

What usually fails inside one of the little battery charger/maintainers?

I have a couple of three-year-old Sears 2 Amp (12 volt)/4 amp (6 volt) 
DieHards that say they feature float-mode monitoring.  One still works 
fine--raised an old battery's measured output from 11.8 to a sustained 
13 volts--the other's front lights light up but it doesn't do diddly 
when connected to a battery for multiple days.

So what could be wrong inside?  Easy fix for a fumbling old solderer or 
just trash it?