Handheld trickle battery chargers

We have a few Vertex handhelds with the standard wall chargers. Does
anyone know if there is a cheap add on trickle charger module for
something like this? I have something like that from Ace RC for a RC
plane. It wired directly into the output of the standard wall charger
and it charges at the normal rate until the battery reaches max, then
automatically switches to trickle. It was inexpensive and easy to
retrofit.

Thanks

Kurt