MPPT SOLAR CHARGER
In this tutorial, you will be introduced to a special type of charger that is used with solar panels. This type of charger, the MPPT solar charger, will use maximum power from the panel to minimize losses. What is MPPT, how it works and why it is used will be explained below and we will explain how to connect this charger.
HOW DOES IT WORK?
The solar panel consists of solar cells and each cell provides us with certain current and voltage. In order to achieve higher current, the cells are connected in parallel and for higher voltage, the cells are connected in series. This type of solar panel provides us with a certain current at a certain voltage, which can be seen on the I-V graph.
If we draw more current from the panel, the voltage decreases, and if we have a lower current, the voltage increases to the rated value of the panel voltage. The voltage and current, i.e. the power that a panel can provide depends on both solar radiation and the temperature at which the panel operates. If the radiation is higher and the temperature is lower, we will achieve higher voltage and current, or higher power, and if we have a higher temperature and lower radiation we will get less power from the panel, which means that the power is proportional to the radiation and inversely proportional to the temperature.
As we can see in the I-V graph, we have an operating point (which depends on temperature and radiation) at which, for a certain voltage (or current), we get maximum power from the panel. Since the voltage and current of the panel are not equal to the needs of the battery (load), the battery will determine the voltage and current and the panel will not always provide us with maximum power. To avoid this and always use the maximum power of the panel, we use the MPPT charger. The MPPT (Maximum Power Point Tracker) is, in fact, an algorithm that searches for the maximum power point. It finds the voltage at which maximum power is obtained from the panel for the current conditions (temperature and radiation), and then it converts the voltage using the converter to the value required by the battery to charge the battery with the maximum current. As the temperature and radiation constantly change, the point of maximum power also changes and we must constantly adjust the voltage of the panel to obtain maximum power.
Using such a charger at high temperatures and low radiation, we can draw more energy because the panel operates at the point in which we achieve the maximum power. They are also suitable for use when we have such conditions that require the highest possible energy in poor panel conditions.
In order for our charger to find the voltage at which we obtain maximum power from the panel, we can use several techniques for finding the voltage.
The techniques are divided into direct and indirect and we will explain the most important techniques used. The direct techniques measure the current and voltage of the panel, calculate the power and adjust the maximum power point accordingly. Indirect techniques have a constant voltage value that is previously adjusted and then they operate using these values, that is, they do not measure values in real-time.
•Perturb and observe technique
•Incremental Conduction technique
•Fixed voltage technique
•Fractional Open Circuit Voltage
Perturb and observe technique
This technique measures current for a given voltage and calculates power. If the power is greater than the previous power, it increases the voltage by a certain level and repeats the calculation process until the power obtained is lower than the previous power, which means that we have passed the point of maximum power and need to go back. If the radiation value changes faster than the time it takes for us to find the maximum power point, an error may occur because one value will be measured and processed and the actual values will be different. If the step in voltage is high, we will never be able to reach the maximum power point and if it is too low, it will take too many steps to reach the maximum power point so we need to find the optimum and choose an appropriate voltage change step.
In the image, we can see that for point A it is necessary to increase the voltage to get closer to the point of maximum power. While the calculations were done, the radiation changed and now we have a new curve, but the algorithm does not know that because it contains data from the last measurement when the radiation was higher. Because of this, we will pass the point of maximum power on the new curve, but if we compare points A and B, we see that we have more power in B point and the algorithm will calculate that we still need to increase the voltage, and in fact, we need to reduce it to get maximum power. In this case, we will have an error and it will take time to correct it and obtain maximum power from the panel.
Incremental Conduction technique
This technique uses voltage and current derivations (changes) to determine whether to increase the voltage or decrease it to reach maximum panel power. First, we measure the voltage and current and determine their change (derivation) by subtracting the previously measured value from the current value.
When we divide the change in current by the change in voltage, we get conductivity. The image shows when the operating point is left or right from the maximum power point. If the change in current divided by the change in voltage equals the negative value of the current divided by the voltage, then we are at the point of maximum power.
Fixed voltage technique
The fixed voltage technique uses a constantly adjusted voltage that is adjusted by the user for certain radiation. This method is not too efficient because the conditions (radiation and temperature) are constantly changing, thus changing the position of the maximum power point, but it is the simplest and therefore used.
With this method, the maximum power point voltage needs to be adjusted at least two times, for winter conditions and summer conditions, because this is where we have the largest change in conditions and therefore the change of maximum power point voltage.
Fractional Open Circuit Voltage
The maximum power point voltage can be obtained by multiplying the open circuit voltage by an approximation constant. As we use an approximation constant, we will not always achieve the point of maximum power; the power will be slightly lower, but very close to the maximum power. This technique is simple because we do not need to over-calculate and measure, but the problem is that we need to measure the open circuit voltage, i.e. the voltage when the panels are not connected to anything. This would mean that for each measurement we have to disconnect the panels, which is not possible, so we use an additional measuring cell for this technique, using which we measure the voltage and which is placed in the same place with the panels for us to achieve the same conditions.
HOW TO CONNECT IT?
This charger is easy to connect, we connect it between the panel and the battery, and we can also connect the load to it. The charger can be power supplied by 5 to 32 V panels and it can charge lithium (Li-ion, Li-poly, LiFePO4) batteries in combinations of 1S (one battery), 2S (2 batteries connected in series), 3S (3 batteries connected in series) and 4S (4 batteries connected in series) respectively up to 14.4 V. By changing the resistor on the board, we select the configuration (1S, 2S, 3S or 4S) and it comes configured for 1S.
It can charge the battery with up to 2A current, and by changing the jumper on the board, this value can be changed to 0.5 A, 1 A, 1.5 A or 2 A maximum.
By changing the value of the potentiometer on the board, we adjust the MPPT point because this charger uses a fixed voltage technique. We adjust the MPPT point by turning the potentiometer while the panel is in the strongest sun to which it will be exposed and the voltage at the “MPPT SET” point should be set to 2.8V.
Resistor calculation for different battery configurations (1S, 2S, 3S, 4S)
The value of the resistor using which we change the configuration (1S, 2S, 3S or 4S) can be calculated according to the formulas depending on how many batteries we have in series (max. 4 batteries).
First, we calculate the ratio RFB2 / RFB1 = 3.3 / (Vbat-3.3).
Depending on how many batteries we have, we must change the Vbat (1S> Vbat = 3.6V, 2S> Vbat = 2*3.6 = 7.2V).
For example, we have 3 batteries connected in series and Vbat = 10.8V
RFB2 / RFB1 = 3.3 / (10.8 – 3.3) = 0.44. The current passing through the RFB2 should be between 10 and 15uA, if we choose a current of 15uA at a voltage of 3.3V, the RFB2 resistance is equal to 220 kOhm.
RFB2 = 3.3/15uA
Now that we know the RFB2 and the relation between the RFB2 / RFB1, we can calculate the RFB1.
RFB1 = 220k / 0.44 = 500 kOhm
The total resistance seen by the Vfb pin should be equal to 250 kOhm and we will select RFB3 so that it is, together with the RFB1 and RFB2, equal to that value. RFB1 and RFB2 are connected in parallel so we will calculate their total resistance according to the following formula:
RFB1 || RFB2 = RFB1*RFB2 / (RFB1+RFB2)
And we will get RFB3 if we subtract RFB1||RFB2 from 250 kOhm.
RFB3 = 250k- 152.78 = 97.2 kOhm Since we have obtained a value of 97.2 kOhm, which is not the standard value, we will take 100 kOhm.
When calculating the values of all three resistors, we just need to replace them and in the following image, you can see where each resistor is located.
Products used in this tutorial
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