Solar panel holding batteries are connected in parallel with one another, usually wired from a charge controller placed in the circuit between the solar panels and the holding batteries. A charge controller regulates voltage and current going to the batteries during charging.
A parallel rather than serial connection keeps the charging voltage and the battery output voltage near 12 volts for all batteries in the system, while increasing the current available from the batteries. A charge controller conditions the voltage and current received from solar panels and outputs voltage and current that efficiently charge the batteries without damaging them.
Most so-called 12-volt solar panels provide 16 to 20 volts under optimal temperature and sunlight conditions. This level of voltage would overcharge and damage most 12-volt batteries if the panels were connected directly to the batteries. The charge controller converts higher voltage into a stable 12 volts for safe charging. Under less-than-optimal temperature and sunlight conditions, solar panels provide lower voltage, usually at or above 12 volts for most of the daylight period of the day.
Solar panel holding batteries may also be connected in parallel to an inverter to convert the DC voltage of the batteries into AC voltage that can be used with most household appliances or to supplement a standard 60-Hertz AC household system.