I do maintenance of many off the grid solar systems. The optimum word here is emergency backup battery. I an assuming that this light is only used from time time. First of all you can’t over charge a battery, that is say put more current in to it than it can hold. In simple when a battery is fully charge, it is fully charged, And that’s it. Any more attempt to raise the charge is hopeless. It is just turned to heat and short life the battery. No matter the charger. So the term over charging just means heating up the battery. A simple trickle charger or low current charger will do the job. The PWM charger and other are make for multiple cell battery that are use a lot and that may have unexpected heavy discharge . These chargers can equalize the weaker cells and bring then up to full charge by doing what is called a bulk charge. But that is more that you need to know.