Many solar panels generate between 16V and 18V under light load conditions, while some 12V solar panels may fail to charge a 12V battery.

The terminology can be confusing: some panels indicate “nominal voltage,” while others provide no specific value beyond 6V or 12V, and some even list incorrect voltages.

To effectively charge a battery with a solar panel, precise specifications are crucial.

Before purchasing a solar panel or connecting it to a battery, you should understand three key aspects:

1. Unloaded Voltage
2. Rated Voltage
3. Current

1. Unloaded Voltage

Unloaded voltage refers to the output voltage of the panel when it is lightly loaded. This measurement is essential because a fully charged 12V battery reaches a “floating voltage” of about 15V during charging. Therefore, the solar panel must produce more than 15V to effectively charge a 12V battery.

Additionally, if a diode and charging circuit are present between the panel and the battery, they may cause a slight voltage drop, so the panel’s output should account for this.

You can estimate the unloaded voltage by counting the number of cells on the panel, as each cell typically produces around 0.6V. If the cells aren’t visible, use a multimeter to measure the voltage under good lighting conditions. A 100-ohm resistor can help you take readings.

2. Rated Voltage

Rated voltage indicates the voltage that the panel will deliver when it is supplying its maximum current. Sometimes referred to as nominal voltage, this value should be verified through your own measurements.

Rated voltage is achieved when the panel receives direct sunlight, which may only happen for brief periods throughout the day.

For instance, a panel with 11 visible cells may produce 6.6V when lightly loaded and will struggle to provide even 6V when fully loaded, making it unsuitable for charging a 6V battery.

Similarly, a panel labelled 18V might only output 14.4V, which is inadequate for charging a 12V battery. With the addition of a protection diode, the output might drop to 13.8V, which is insufficient for further charging once a flat battery reaches that voltage.

Conclusion

It’s crucial for a solar panel to produce a voltage between 17V and 18V to ensure there is enough “overhead” voltage to charge a battery effectively when it reaches 14.4V. Understanding these specifications will help you choose the right solar panel for your battery charging needs.