# Solar Panels Charging Process

## Solar Panels Charging Process

------by EverExceed Corporation

Believe that a lot of people know solar panels, most have seen V - I curve. But many people confuse for charging principle and process, let me answer that question for you, with 18 v panels to charge a 12 v battery, for instance:

First, we see 18 V panels V - I characteristic curve:

We can see clearly the two parameters: the short circuit current and open circuit voltage

1. Short circuit current: as the name implies, is the positive and negative side current of solar panels when the short circuit. And in general, the current is determined by the light intensity. The stronger the light intensity is, the more solar panels absorb photons, thus the stronger the photoelectric effect is, the greater the current is.http://www.everexceed.com

2. The open circuit voltage: as the name implies, is the voltage when solar panels do not connect any of the load, usually the voltage is related to temperature.

When use 18 v solar panels to charge a battery through the solar controller, people will find solar panels on both ends of the voltage will be clamping in the storage battery voltage. Then he may ask:”I know that the voltage is 18V when the solar panels are open-circuit. At that time, the voltage is higher than the battery voltage, and it can charge for the batteries. But when connected with PWM solar charge controller, the voltage of solar panels immediately becomes 12.5V, which is equal to the battery voltage. Why it still can charge for the batteries? Is that charge controller really so wonderful?” http://www.everexceed.com

Is it what leads us to have that misunderstanding? I think it is the different characteristics between daily batteries and solar panels. The main differences are the internal resistance Rs (This resistance is typically European level.).

Let’s look at the battery model. Here I want to explain that the solar panels are common lead acid batteries. They are all batteries and have internal resistance. In general, the resistance of solar batteries is larger than the lead acid batteries’. http://www.everexceed.comHere I choose the most commonly used linear models, which is very similar with Thevenin’s equivalent model.

Supposed that U∝ stands for the open circuit voltage of the solar panel; U1 stands for the terminal voltage of the solar panel; I stands for the charging current of batteries.

U1 = U∝ - R*i ①

From the equation we can draw the following conclusions: if the internal resistance of the battery plate is zero, then the V-I curve would be a bold line marked in the figure. That is to say, at that time, whatever kind of batteries are connected, the terminal voltage o f solar panels is forever equal to the open circuit voltage(18V). It is pity that this kind of solar panel is not existed. So many factors make the internal resistance to be some small or some big. But, this conclusion applies to any battery.

Return to that question: “when the solar panel voltage is equal to the battery voltage, cant it charge for batteries?” The answer is: Yes.

Q: Why? For the solar charger controller? What kind of role does the controller play? http://www.everexceed.com

A: It is not because of solar charger controller. Even if the solar panel connects with both ends of batteries, it also can charge for the batteries. And the terminal voltage of batteries will soon be clamped into the battery voltage.

Look at the specific charging principles directly connected to the solar panels:

According to Kirchhoff’s voltage law:

U∝ - R*i = Ui+ i*Rb ②

Ub = Ui + i*Rb ③

Ub stands for the terminal voltage of batteries when charging; Rb stands for the internal resistance of batteries; Ui stands for the open circuit voltage of batteris, U∝ stands for the open circuit voltage of solar panels.

Because the Rb is very small, the fact is that the charging terminal voltage of batteries is almost similar to the open circuit voltage.

Conclusion: Because the battery resistance is very small, it leads to the similar terminal voltage of batteries when charging and not charging. The internal resistance of solar panels is relatively large, leading to the measured terminal voltage of solar panel immediately "fell "battery voltage value when charging the batteries.

Figure (3) is the simplified solar controller scheme. When Ub is high, the Q switch is open. The solar panel then turned to recharge the battery. http://www.everexceed.comWhen the switching frequency is very fast, we will feel the solar panel has been charging the battery all the way. While the proportion of opening and closing the switches decides the charging voltage of the batteries.

Simply to say, the controller can prevent the overcharging damage in the process of charging batteries, including the over-current charging and the overvoltage charging.

Charging process:

Fast charging stage: When the output short-circuit current of solar panel is almost equal to the current at the maximum output power, if the short-circuit current is less than the maximum allowable battery charging current, the switch can be turned directly to the battery fast charge mode. When the output voltage reaches Eb1, it needs to be switched to constant voltage charging phase.

Constant voltage charging stage: through the constant pressure control mode the microcontroller make the output voltage constant in Eb2, then the charging circuit switch stays in pwm control mode. With the internal voltage of battery increases, when the charge current is reduced to Ib2, it shall be switched to the charge stage.

Overcharge stage: At this stage, the charging current is stably Ib2, and the charging circuit will make the charging voltage rises to Eb2. At that time, the battery is full and switches to the floating charging stage.

Floating charging stage: also known as trickle charging stage. http://www.everexceed.com It can prevent battery self-discharge and power loss. The charging circuit will provide a precise control of battery float voltage. And the controller with temperature compensation will keep the battery full capacity status.

Another function of the controller is to control the battery discharge and prevent battery over discharge. Because the operation process is simple, here we will not discuss it specifically.