Basically there are two types of chargers. The old style just put out a voltage greater than the battery full charge voltage. When the battery absorbs all the charge it can the high charger voltage keeps dumping current into the battery. The battery dissipates the current as heat. In doing so it splits water into hydrogen and oxygen. If the battery produces hydrogen and oxygen faster than it can recombine it vents and the battery looses water. This is why there are warnings to not overcharge with the consumer old style chargers. Full charge is typically when the charger is down to 3 or 4 amps. With some of the old high current chargers it was possible to dump more current than that into a charged battery. That could produce enough heat in the battery to warp the plates and destroy it.
The new style chargers are usually called automatic chargers. Automatic chargers monitor the battery's state of charge by measuring the voltage across the battery. When it sees a full charge voltage across the battery it drops into a current limiting mode. It will not send more current to the battery than it can handle without loosing water. This would've been too expensive to do in the era of old chargers, moderrn electronics makes it possible.
Now this doesn't happen all at once. If your charger has "charging" and "full" lights you'll see them start to blink back and fourth at some point. The battery shows full voltage, the charger drops to current limited mode. The battery absorbs the charge, the voltage drops, the charger switches back to the quicker charging voltage limiting mode. And back and forth. When the battery is about full the charger will show "full" more than "charging."
As a battery ages the full charge voltage becomes slightly lower so you'll see more of this back and forth even when the battery has absorbed all the charge it can. Also starting and deep cycle/gel cell fully charged voltages are slightly different. You want an automatic charger with a switch to select between deep cycle and starting batteries.
I still have an old style charger. The only time it gets used is when a battery is so far discharged that the automatic charger can't sense that it's connected to the battery. Once the voltage is up enough it goes on the automatic charger. That's about the only good use for the old style charger today.
Plus with an automatic charger you can turn it on and forget about it. Can't hurt the battery. For longest life batteries should be recharged right away. When I get back from a fishing trip I throw the charger on and don't think about it until some convenient time the next day.
