It's mostly electronics. The more sophisticated ones have voltage monitoring circuits that adjust the charge rate according to the terminal voltage of the battery. A trickle charger simply supplies a constant low current and doesn't monitor terminal voltage. The terms "float charger" and "battery maintainer" are usually used interchangeably to describe an adjustable-rate charger that keeps a battery topped off without overcharging it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!