Charging Battery via Solar Panel

Circuit of Charging 12v Battery via Solar Panel, This will clear-up a lot of mysteries of the solar panel and There are 3 things you have to know before buying a panel or connecting a panel to a battery.

Charging Battery via Solar Panel


This will clear-up a lot of mysteries of the solar panel.
Many solar panels produce 16v - 18v when lightly loaded, while other 12v solar panels will not charge a 12v battery.
Some panels say "nominal voltage," some do not give any value other than 6v or 12v, and some specify the wrong voltage. You can't work with vague specifications. You need to know accurate details to charge a battery from a solar panel.
There are 3 things you have to know before buying a panel or connecting a panel to a battery.

2. The voltage of the panel when delivering the rated current. Called the RATED VOLTAGE

The UNLOADED VOLTAGE:  The Unloaded Voltage is the voltage produced by the panel when it is lightly loaded. This voltage is very important because a 12v battery will produce a "floating voltage" of about 15v when it is fully charged and it will gradually rise to this voltage during the charging period. This means the panel must be able to deliver more than 15v so it will charge a 12v battery.

Sometimes there is a diode and a charging circuit between the panel and battery and these devices will drop a small voltage, so the panel must produce a voltage high enough to allow for them.
The Unloaded Voltage can sometimes be determined by counting the number of cells on the panel as each cell will produce 0.6v.
If you cannot see the individual cells, use a multimeter to read the voltage under good illumination and watch the voltage rise. You can place a 100 ohm resistor across the panel to take readings.

RATED VOLTAGE: The RATED VOLTAGE is the guaranteed voltage the panel will deliver when full current is flowing. This can also be called the Nominal Voltage, however don't take anything for certain. Take readings of your own. The Rated Voltage (and current ) is produced when the panel receives bright sunlight. This may occur for only a very small portion of the day.
Solar Panel
You can clearly see the 11 cells of this panel and it produces 6.6v when lightly loaded. It will barely produce 6v when loaded and this is NOT ENOUGH to charge a 6v battery.

Solar Panel
This panel claims to be 18v, but it clearly only produces 14.4v. This is not suitable for charging a 12v battery. When you add a protection diode, the output voltage will be 13.8v. A flat battery being charged will reach 13.8v very quickly and it will not be charged any further. That's why the output voltage of a panel is so important.

Solar Panel
The panel needs to produce 17v to 18v so it will have a small "overhead" voltage when the battery reaches 14.4v and it will still be able to supply energy into the battery to complete the charging process.

RATED CURRENT:  The Rated Current is the maximum current the panel will produce when receiving full sunlight. The current of a panel can be worked out by knowing the wattage and dividing by the unloaded voltage.
A 20 watt 18v panel will deliver about 1 amp.


A solar panel can be used to directly charge a battery without any other components. Simply connect the panel to the battery and it will charge when the panel receives bright sunlight - providing the panel produces a voltage least 30% to 50% more than the battery you are charging.
Here's some amazing facts:
The voltage of the panel does not matter and the voltage of the battery does not matter. You can connect any panel to any battery - providing the panel produces a voltage least 30% to 50% more than the battery you are charging.

The output voltage of the panel will simply adapt to the voltage of the battery. Even though there is a voltage mismatch, there is NO "lost" or wasted energy. An 18v panel "drives into" a 12v battery with the maximum current it can produce when the intensity of the sun is a maximum.

To prevent too-much mismatch, it is suggested you keep the panel voltage to within 150% of the battery voltage. (6v battery - 9v max panel, 12v battery - 18v max panel, 24v battery - 36v max panel).But here's the important point: To prevent overcharging the battery, the wattage of the panel is important.  If the wattage of an 18v panel is 6watts, the current is 6/18 = 0.33 amps = 330mA.
To prevent overcharging a battery, the charging current should not be more than one-tenth its amp-hr capacity. 

For instance, a 2,000mAhr set of cells should not be charged at a rate higher than 200mA for 14 hours. This is called its 14-hour rate.

But this rating is a CONSTANT RATING and since a solar panel produces an output for about 8 hours per day, you can increase the charging current to 330mA for 8 hours. This will deliver the energy to fully charge the cells.
That's why a 6 watt panel can be directly connected to a set of (nearly fully discharged) 2,000mAhr cells.
For a 12v 1.2AHr battery, the charging current will be 100mA for 12 hours or 330mA for 4 hours and a regulator circuit will be needed to prevent overcharging.
For a 12v 4.5AHr battery, the charging current will be 375mA for 12 hours and a larger panel will be needed.


Some solar panels will discharge the battery (a small amount) when it is not receiving sunlight and a diode can be added to prevent discharge. This diode drops 0.6v when the panel is operating and will reduce the maximum current (slightly) when the panel is charging the battery. If the diode is Schottky, the voltage-drop is 0.35v.
Some panels include this diode - called a BYPASS DIODE.


There are two ways to prevent overcharging the battery.
1. Discharge the battery nearly fully each night and use a panel that will only deliver 120% of the amp-hour capacity of the battery the following day.

Here is the simplest and cheapest regulator to charge a 12v battery.
Full details of how the circuit works and setting up the circuit is HERE.
The solar panel must be able to produce at least 16v on NO LOAD. (25-28 cells). The diagram only shows a 24 cell panel - it should be 28 cells.
The only other thing you have to consider is the wattage of the panel. This will depend on how fast you want to charge the battery and/or how much energy you remove from the battery each day and/or the amp-Hr capacity of the battery.
For instance, a 12v 1.2A-Hr battery contains 14watt-hours of energy. An 6watt panel (16v to 18v) will deliver 18watt-hours (in bright sunlight) in 3 hours. The battery will be fully charged in 3 hours.

Circuit of Charging 12v Battery via Solar Panel
Circuit of Charging 12v Battery via Solar Panel


  • The pot is adjusted so the relay drops-out at 13.7v
  • The charger will turn ON when the voltage drops to about 12.5v.
  • The 100R Dummy LOAD will absorb 3.25 watts and that is the
  • maximum wattage the panel will produce with 100R load.


Here is a very clever circuit to find the charging current, if you don't have a multimeter.

Charging 12v Battery via Solar Panel circuit
Circuit of Charging 12v Battery via Solar Panel



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Electronic Bubble . Electronic engineering: Charging Battery via Solar Panel
Charging Battery via Solar Panel
Circuit of Charging 12v Battery via Solar Panel, This will clear-up a lot of mysteries of the solar panel and There are 3 things you have to know before buying a panel or connecting a panel to a battery.
Electronic Bubble . Electronic engineering
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