suggestions for solar charging battery-operated device
It's recommendable to use a charge controller between the solar panel and the battery. The charge controller is there to avoid damaging your battery by overcharging. When a lead-acid battery gets charged above about 98%, oxygen and hydrogen bubbles form around the internal plates in the battery.

In a "maintenance-type" battery, this vents out and can be replaced, in an "SLA" battery it can cause irreperable "gassing," but in "AGM" and "gel"/"deep cycle" batteries, the bubbles stay tralped against the battery plates. These bubbles reduce the available current &amp; capacity, while increasing the internal resistance of the battery.