Trickle chargers are used to maintain battery charge when batteries are not being used. This could be for a motorbike stored for the winter, or a car left in the garage while you are overseas.
They can be used over a long period of time, supplying a low amount of amperage.
Trickle charging means charging fully charged battery at a rate equal to its self-discharge rate. This enables the battery to remain at its fully charged level. Trickle charging will not keep a battery charged if current is being drawn by a load. A battery under continuous float charging voltage is said to be float-charging.