When a solar panel charges a battery, the PWM (Pulse Width Modulation) controller pulls down the panel voltage to match that of the battery.
What happens if a solar panel battery is discharged?
Fortunately, when the battery discharged, the output voltage is lower so the solar panel voltage will also be lower. When fully charged, the battery voltage will be high, but the current is very low—at this point, the drop-out voltage reduces to about 2V and the open circuit solar panel voltage also comes into play.
For example, if the open circuit voltage of your solar panel is 20V and the battery to be charged is rated at 12V, and if you connect the two directly would cause the panel voltage to drop to the battery voltage, which would make things too inefficient.
How does a solar panel work?
When a solar panel charges a battery, the PWM (Pulse Width Modulation) controller pulls down the panel voltage to match that of the battery. So an 18V solar panel gets pulled to 12V to match the 12V battery it is charging. As the battery continues to charge, the battery voltage goes up. The controller increases the solar panel voltage too.
What happens if you disconnect a battery from a solar panel?
This disconnects the battery from the solar panel. When the battery's voltage drops below 13 volts, the output of the 555 will go HIGH and deactivate the relay. This resets the system for another charging cycle. In the case of cordless power tool batteries, this will happen when you disconnect the battery.
So the worry is that if solar panels are running at the same time will they overcharge the battery because it now has two sources charging it. Well assuming there is a charge controller fitted on the solar installation then both charge sources, the battery charger and the solar system, are regulated.
If the solar panel is connected to the battery and there is no blocking diode or charge controller, the current could move backwards. With the solar panel not transmitting current to the battery, the current might flow out into the panel, draining the system. The chances of this happening are low but it is possible.