Some thoughts based on recent discussion:
There are manual battery chargers, automatic battery chargers, battery charger/maintainers, and battery maintainers.
Manual battery chargers, which are almost unavailable today, require constant monitoring because they will just keep throwing a charge at the battery until they boil away the electrolyte and ruin the battery.
Automatic battery chargers are better but not by much. They shut off when they sense the battery is (almost) fully charged. However, many (if not most) are just single stage chargers.
Battery charger/maintainers are best because almost all are multi-stage chargers, capable of fully recharging a (partially) drained battery and keeping the battery's charge topped off. (I'll leave it up to you to read up on what the charging/maintaining stages are.)
Note: Some multi-stage charger/maintainers have too many stages/functions/features. The extra stages/functions/features are only suitable for old, nearly worn out batteries that should be turned in for recycling rather than trying to "save" them.
Finally, there are battery maintainers. These are low-amperage devices that are typically permanently mounted in the vehicle and connected to the battery. They are good for maintaining a battery's full charge but are not powerful enough to recharge a drained battery.
I have received reports of some battery maintainers draining a battery when left unplugged, in an idle/stored vehicle, for an extended period of time. However, I have a strong basis for doubting these claims.
Newer vehicles have a small steady parasitic power drain, due to all the electronics, that can significantly drain a battery if the vehicle is idle/stored for an extended period. Personal computers have small batteries to maintain memory state when powered off ... provided they're not left off too long. Newer vehicles should also have the same type of small batteries but most do not. The implicit design assumption is the vehicle will be operated at least every week or ten days, therefore offsetting the battery drain by recharging via the alternator.
So, the supposed problem with battery maintainers may actually be due to a newer vehicle with lots of electronics.
