Ironbender, yes they do work. A slow, 2.5 amp trickle may not be much, but it heats the batteries INSIDE, which increases cold crank amps.
Heres one memorable example where a trickle charger saved my a s s, from the stupidity of diesels in below zero weather:
I had just finished 14 consecutive 12 hr days on the pipeline and my shift was over. It was 57 below zero ambient in the morning. My coolant looked like a green slushy before start up. I thought that a piece of cardboard in front of the radiator was sufficient enough for driving in that type of cold.
Somewhere between salcha and north pole, i lost lights and even dash lights went dim. What happened was i didnt put a real bib on the grill, so the intercooler was getting blasted by 50 below. It tripped an air temp sensor, so the cummins air intake grid heater was permanentley on. The grid heater pulled more amperage than the alternator could replace and drained the dual batteries.
Emegency pit stop in north pole, i plugged in the truck and overnighted at a rental cabin. I had a napa trickle charger, oil pan heater and block heater.
I made a temporary bib out of some insulated carharts and attached it to the grill with self tapping sheet metal screws. Duck tape wouldnt stick.
Truck fired right up. Off to willow i proceeded, to go run sled dogs! Batteries had plenty of charge to crank over that big cummins 6bt
