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Solar Battery Charging

Choosing Solar panels for Battery Charging

12 volt solar panels for 12V power
The most common question is what type of solar panel to use for camping power. Don't use solar panels like those seen on suburban home roofs designed for supplementing AC power, their voltage is different.
Use solar panels that are designed for charging batteries. These are nominally rated for charging 12V and 24V Batteries

OFFGRID v GRID-TIED SOLAR POWER Typical 12V solar panel Typical grid-tied solar panel
Type of solar cell
Number of solar cells per module
Open-circuit voltage (Voc)
Optimum operating voltage (Vmp)
Typical application

 
Monocrystalline or Multicrystalline
36
22V
18V
Connects to a solar regulator for charging 12V batteries
Monocrystalline or Multicrystalline
60
37V
30V
Connects to a power inverter to supplement AC power usage

 

Using several solar panels to increase power
Use several solar panels connected in series to multiply the output volts or connected in parallel to multiply the output amps.
For instance, two 12 volt / 5 amp solar panels connected in series will have an output of 24V/5A and the same panels connected in parallel will have an output of 12V/10A.

 

Match solar panel size to battery usage
Another common question is what wattage of solar panels is required to power camping lights and appliances. The answer depends on what size battery is used to store the solar power and how long the battery power lasts. If you can answer that then you will be able to easily estimate the required solar panel size.
The solar panels are battery chargers. Deep cycle batteries, the type used for power storage, are sized according to their amp-hour capacity. If you have a 100 Ah battery that requires recharging from a 50% depth of discharge (DOD) then you have used 50 Ah that need to be replaced. Battery size depends on your daily power usage, but a simplified rule is that you need 1Ah of battery capacity for every watt of solar power, so a 100Ah battery will usually be used with a 100 watt solar panel.

Solar battery charging cycle

Ignore the watts but check the Amps
Although everybody chooses solar panels according to their wattage, when used to charge batteries it is more relevant to check the output amps rather that their rated watts because we can directly relate the amps provided by the solar panels to battery Amp-hours.
For instance if we select an 80 watt solar panel we can check its nameplate data on the back of the solar panel and it will state the peak output watts, voltage and amps.
If the peak current flow is about 5 amps then realistically it will produce about 4 amps of current flow in regular use during the day, so that in one hour the solar panel will produce 4Ah and in 6 hours it will produce 24 Ah.
So to recharge a partially depleted battery with 50Ah in one day with 6 hours of maximum sunlight, a simple calculation shows that at least two 80 watt solar panels will be required. They will be connected in parallel to double up the amps output to about 8 amps.

Use a solar regulator for proper battery charging
A solar regulator or controller should connected between the solar panels and the battery to ensure the battery is not overcharged.
It must be able to control the maximum voltage and amperage produced by the solar panels. It will be rated accordingly, so for the two 80 watt solar panels a solar regulator rated at 12V/10A regulator will be required.