NiCads and Ni-MHs can benefit from a maintenance, or trickle, charge. However, there are very few chargers that do it properly. Too much current can damage these cells.

It is my understanding that SLAs do well with a maintenance charge and this can extend their life. You, the end user, have to decide how much you want to invest in equipment.

Looks to me you have a good charger that you can leave on the battery indefinitely without worry that the battery would be damaged. Monthly charges over the summer would work well too. Will you remember to keep your schedule? If you think you would forget then you might just leave it on the charger all the time.

Or you could just use the light through the summer.