Treat your battery as a powerful electrical source that contains hazardous acid and which can explode like a bomb. Adding a Solar Panel If you wish, you can add a solar PV panel to convert sunlight into electricity and charge your battery. Always fuse the connection from the panel to battery and provide an easy power disconnect switch. For small capacity panels, you can leave the panel connected to the battery all the time. The charging current is small enough that it is mostly trickle charging the battery and will not cause harm if continuously charging your battery.
More solar panels means a greater current flow.
If your cable run length from the solar panels to the batteries is long, you absolutely must use a fat gauge wire. If you add additional solar panels, you will need to add a charge controller to your setup. If your charging current is too high, you will overcharge the batteries and evaporate the electrolyte. To avoid over charging your batteries, a charge controller measures the batteries voltage. When the voltage reaches a fully charged state, the charge controller disconnects the panels from the batteries.