If having only one solar panel charging your battery isn't adequate, then wiring in a second panel will improve charging times and stronger battery charges. Having a higher power input than battery output is ideal for charging batteries. Quicker charges result in better performance of the system being run by the battery. Batteries charge better when you increase the amperage input by wiring panels in a parallel wiring scheme, as instructed below.
Splicing two panels together
Using wire strippers, strip 9 mm (3/8 inch) of wire insulation from the two wires coming off each solar panel.
Strip 9 mm (3/8 inch) of insulation from the wires of the battery terminal connector.
Crimp both positive (+) or red wires coming off the solar panels into one end of the in-line crimp splice.
Crimp both negative (-) or black wires coming off the solar panels into one end of the in-line crimp splice.
Crimp both red and black battery lead wires into the ends of their respective in-line crimp splices. Black connects to black; red connects to red.
Install blocking diode -- optional
Cut positive (+) or red lead wire, stripping 6 mm (1/4 inch) of insulation from each end of the cut piece.
Trim leads on diode to 1.2 cm (1/2 inch) and solder into place; connect the cut piece together again. The silver ring on the diode should be in the direction of the battery.
Clean with rubbing alcohol and tightly wrap in electrical tape.
Strip 9 mm (3/8 inch) insulation from both red and black lead wires.
Crimp battery terminal connectors onto lead wires.
Test power output from solar panels to ensure correct splicing connection with multimeter.
Use zip ties or cable lacing to secure wires together to prevent damage to splice connections.
Secure connectors to their respective battery terminal; Black wire to negative (-) terminal and red wire to positive (+) terminal.
Instructions are for a parallel wiring scheme. Do not twist wires together before crimping. The twists will weaken the conductor wires. Blocking diodes are optional on 12 volt systems. They should be used on 24 volt systems to prevent power from being discharged through the solar panels during the night. Clean the blocking diode solder connection with rubbing alcohol before wrapping with electrical tape. Brown coloured rosin flux from the solder is corrosive and will damage the diode and wiring. Make sure not to crimp any insulation in to the splices or terminal connectors. Crimping insulation will result in poor conduction and a less secure crimp.
Do not allow bare black and red wire conductors to touch each other. Shorted wiring may cause personal injury, damage to wiring, solar panels and the battery or even start a fire. Do no over-crimp or over-tighten zip ties; it will damage the wire, possibly causing a short.