The technology for what used to be called trickle chargers and then float chargers has changed. The new device includes a sequence to de-sulfate the battery and automatically adjusts the rate of charge.
Here is one example that I use:
BatteryMINDer Model 1510
BatteryMINDer 1510 | 12 Volt Maintenance Charger with 10 Year Warranty
There are other models and other brands that do the same hi-tech battery care.