Solar panels, battery bank voltage, and Charge Controller balancing are important in the Hybrid PCU or Off-grid Solar Application.
The major challenge Solar Installers face when installing the Solar Storage solution, or Solar off-grid or Solar hybrid PCU system is how to match the Solar Panel Voltages and Battery Voltage in Solar Hybrid PCU and the right Charge controller, as all these things are interlinked. As of today, new panels of 550Watt capacity are launched in the market where the per watt cost is lower than the other solar panels available, so the installer is very much fascinated by the price and wants to use these panels. But before doing this, one has to understand the basics of battery Voltage matching with the Solar Panel Voltages. As Solar panels are being made for higher wattages, the solar panel voltage is also increasing as the number of cells increases in any given Solar Panel. So nowadays, the 550 Watt solar panels have approximately 48 Volts as the VOC, which is way higher than the 330 Watt panel, which was close to 33 to 38 Volts. So using these 48v VOC panels in a 24v system will create a lot of challenges. Still, one has to understand that rather than using the 24V system, one can shift to a 36V Solar System, which will be ideal for designs, rather than trying to fit this panel in a 24 Volt System, which will give you the same output or nearby output of 330Watt panel, when the Solar panel manufacturer increasing the wattage in a single solar panel than he has two options to use the cells with higher wattage or increase the no of cells to make it a single solar panel with higher wattage. When we talk of mono or crystalline solar panels, the difference is the quality of cells used to make the panel. So if higher efficiency cells are used, we need to make the wattage higher without changing the voltage of the solar panel. But if the solar panel is made with the same normal cells, then the voltage of the solar panels will also increase as the no of cells is increased to make the higher wattage solar panel, which is not understood by the installer at times.https://suvastika.com/top-tips-for-buyers-looking-to-purchase-inverter-and-save-money-in-the-process/
To achieve the maximum performance from your solar panels, you should design your system such that the VOC (Voltage Open Circuit) of your solar panel(s) are between 1.4 and 1.8 times your nominal battery bank voltage. So here, we will avoid the Vmpp and any other voltages written on the solar panel.
So let us see the specs of panels to prove this concept
The panels available in the market are generally 150 watts and used in 12 Volt batteries and inverter circuits for off-grid solar or hybrid Solar PCU applications.
150 w panel generally has 22V, and the battery voltage is 12V, so the battery voltage and panel voltage fall in our formulae of 1.4 to 1.8 times the battery voltage if the 150Watt panel has 22 volts, so 12 X1.8= 21.6V.
Same way, we have to make sure that the panel we buy for our Hybrid PCU or solar off-grid system for a 24 V system varies between 24 X1.4 = 33.6 to 24 X 1.8 = 43.2V maximum so that we don’t lose much of the power by using higher voltage panels.
So let us take an example of a 48 Volt battery System, and based on this voltage, we can calculate the 48V panels and 48 X 1.4 = 67.2 to 48 x 1.8 = 86.4Volts. One should use these maximum limits while calculating and matching the panel voltages with the battery voltages.
Now the next question is, what will be the position if we increase the panel voltages more than 1.8 times the battery voltages? Or if we decrease the panel voltage from 1.4 times, what will be the outcome?
The power will start wasting if we increase the VOC more than 1.8 times. The wastage of power will create unnecessary heat in the charge controller as the voltage in the 48 Volt system crosses 87 Volts. The charge controller needs to work harder to make it 48 volts for charging the battery as the battery must be set between 42 volts to 57.6 volts required by the batteries to be charged. The moment the VOC of the solar panel crosses 100 volts, the pressure on the charge controller increases, and the efficiency of the charge controller will be compromised. So charge controller will lose its efficiency, and the heat sink on the charge controller will create extra heat to manage the voltage mismatch. So in the case of the PWM charge controller, the efficiency is further reduced, and the panel wattage will be diminished, and there is no benefit to the user other than that we are getting 550 Watt panels by getting only 300 Watts out of them.
Suppose we reduce the 48 X 1.4V = 67.2 volts. In that case, the same problem will be seen as the cable losses and temperature increment on the solar panels will come to such a level that 57.6 volts required to charge the battery are reduced one day as the losses in wires connected to the solar panels and charge controller will have more losses throughout times and according to the temperature outside as most of the times these wires are exposed. They will reduce the voltage over a while.
In the case of MPPT, the same problem will be experienced as the MPPT charge controller’s efficiency depends on the voltage range, and the peak efficiency is only on a small voltage range. So if we increase the Voltage range of the MPPT of 48 V, then the efficiency of the charge controller will be reduced, and maybe the MPPT charge controller will behave like a PWM charge controller.https://en.wikipedia.org/wiki/Solar_inverter
Another challenge we face is panel wattage and battery capacity.
Let us take the example of a 150 Ah battery, and if we want to install the charge controller for the same, then how much charge we can pump into the batteries is an important question.
So let us say we have four 150 Ah batteries of tubular type Lead Acid batteries to be installed in a solar system, so how do we calculate the charge controller and solar panel capacity?
So we know that Tubular lead Acid batteries can take 10% of their maximum capacity charge. So if we have a 150 Ah battery, then we can give a maximum of 15 Amp charging which has been a standard being followed by the industry as most of the Inverters UPS sold in the market have kept 15Amp maximum charging while charging the Tubular lead Acid battery.
If we try to increase this charging current, there will be two challenges: the battery water will evaporate fast. The battery will get fast charging as we are supposed to charge it in more than 10 hours, and if we charge it more quickly, the water will evaporate. The battery plates will start getting hardened, and the life of the battery will be reduced if we charge it will 50 Amps compare to 15 Amps. Then, the possibility of a battery blast will increase as the battery has vents for extra gassing, and in case they are not being cleaned properly, the gasses will accumulate. The chances of a battery blast will increase. Also, the battery being charged at 50 Amps will create lead fumes in the house, which will be four times of fumes created in the battery when charged at 15 Amps.
Let us come to the main point of Charge controller sizing with the Solar panel and battery capacity sizing.
So if we are using a tubular lead Acid battery of 150 Ah and the maximum current we can feed is 20 Amps as the solar has the maximum intensity for 2 to 3 hours only as the solar panels are installed on one side only, so the full power of solar panels will be for 2 to 3 hours only so we can afford the fast charging during that time which we can take it to 30 Amps maximum because after that the sun will change its position. We will not get the best wattage after and before those 2 to 3 hours. So after 30 Amps, the charging current going to the battery will harm the life of the battery, and the water topping will also increase over time.
So in 48Volt System, we can attach 30 Amps panels, so the maximum capacity we can install is 1500 watts. With the PWM charge controller, we can go up to 2000 Watts as the maximum wattage given by the 2000 watts from the PWM is only 1500 watts practically. In my next article, I will forward this data for your reference for 2 hours. So in the case of a good MPPT charge controller, 1500 watts will give more output than the PWM charge controller having a 2000 Watts panel.
Tip: Solar panels lose efficiency when they become hot. The sun’s rays lose power on a slightly overcast day. Dirt, dust, and other buildup on the panels will cause a loss of power. Panels are rated at 25 degrees/centigrade (77 Fahrenheit in the panel interior, not ambient air temperature) for 1000m/w2 (bright sunny day, no clouds), and no dust or dirt on the panels! Which is very difficult to achieve. That’s why I have suggested 2000Watt panels with 150/200Ah four Tubular batteries. or a lithium battery bank of 48V/100Ah capacity.
If you follow these simple design rules, you will achieve maximum power from your panels. Any quality charge controller will pass the power from your panels to the batteries. No downshifting or up-shifting of voltage is necessary. A PWM charge controller is a straightforward circuitry, so VOC variations can create havoc as it can increase the losses MPPT is a very complicated circuitry and minor changes in VOC voltage range can disturb its efficiency, which is the main role of the MPPT charge controller.