Would it be a right choice to use a 6V sealed lead acid battery with 12V, 50W solar panel? This way I can save some money on battery and the panel. After all, it's going to be just a school project.
Would it be a right choice to use a 6V sealed lead acid battery with 12V, 50W solar panel? This way I can save some money on battery and the panel. After all, it's going to be just a school project.
Once this is done, select a controller that is fast enough and has all the necessary modules - you will need at least 4 ADC channels, may be a CPP/PWM module, enough pins for all of this plus whatever user interface you want.
Don't I need 2 ADC channels - one for current sensor and the other for voltage sensor?
I believe many microcontrollers come with built-in CCP/PWM module.
If you're going to measure voltage at the panel, current at the panel, voltage at the battery, and current at the battery - that is 4.
But I had thought that if I'm not going to implement battery charging algorithm then I can do away with those sensors. I could use this circuit between the output terminal of buck converter and the battery. You can see that the circuit uses dummy load. Could that circuit be used?
For the small cost of voltage and current sensor circuits, and a trivial amount of coding, you can monitor the output voltage and current and add additional protection features to your design.
I didn't study that circuit in great detail, but my quick look at it says that it monitors the battery voltage and charges when the voltage is in the specified range. Comparitors are comparing the voltage to threshold set points. If the battery is out of range, the charging is shut off and the dummy load is switched in by the relay.Q1: To start with, I don't even understand this circuit. I have assumed that when a battery is being charged up and the current flowing into the battery is above certain value, the relay keeps connection to the battery bank. When the current falls below a certain below which would mean battery is fully charged then the relay makes connection with the dummy load. Is that how it works? What's about those LEDs? When will yellow LED be lit?
I still don't know what battery you intend to use. But, if the one you use has a complicated charge profile or limits on voltages and currents, the the charger has to be programmed to obey the restrictions. If MPPT will overcharge or overstress the battery, you need to disable MPPT. This is one reason why we keep saying to monitor the output voltage and current. You need to monitor and limits those values to be in the safe range. If safe, MPPT can proceed. If not safe, MPPT is disabled and the limiting function can be performed.Q2: Now I will try to understand what you have been trying to warn me about. Let's say that the 12 V battery has entered Stage 2. In stage 2, the voltage remains constant but current flowing into the battery gradually starts decreasing. The battery limits the current flowing into it or the system should be able to regulate the flow of current into the battery. I think if the voltage is kept constant then the battery can limit the current itself. Right?
Yes, exactly. The battery could get damaged or a capacitor could explode if you don't design it correctly and put in multiple safeguards.Q3: I have assumed that the battery is limiting the current flowing into it. Please have a look on this figure. The tracker which is functioning at maximum power is continuously pumping current into the converter. Let's say that the voltage at the input terminal of the converter is 17V which will be Vmp because the tracker is working at MPP and the voltage at output terminal is almost 14.4V. If the MPP tracker is continuously pushing more energy (in form of current) then this energy needs to go somewhere. But where does it go? It looks like that this continuous throwing of energy into the converter will tend to increase the overall voltage until the voltage of the converter is also 17 V. Let's see how. Let's suppose the tracker is continuously supplying an average of one ampere into the converter. This might force more current into the battery than is actually allowed or recommended, and further this might raise the overall voltage of converter by raising the voltage of the battery little by little until the voltage of 17 V is reached. This might lead to two things. The battery gets damaged or capacitor gets exploded or both gets damaged. Do you agree with me, at least to some degree?
OK, the capacitor may be safe, depending on your design. You are free to choose the working voltage of the capacitor and make it high enough. Also, the buck converter can only charge to the input voltage. But, again you have not specified anything yet, so how can I know you will do what needs to be done? You might use a 18 V solar cell and drive a 5 volt charger, and you might put a 10 V capacitor on the output. In that case, "capacitor may go boom!".Q4: Okay. The battery might be damaged but why the capacitor would explode. The Cin of the tracker doesn't explode when it is charged up to Voc which is greater than Vmp; in this case Vmp is 14.4 V. Further, when the voltage of converter is reached 17 V, the whole system will stop functioning assuming the battery is not present at the output terminal because when the voltage of Cout is 17 V, the tracker is no longer able to throw in any more energy. Do I have it correct?
It's your job to design it so that the requirements are met. But, choosing a battery that is too restricted on the charging is going to be harder to code and might prevent you from demonstrating MPPT. Imagine that you limit the output current for charging. This then limits the charging power. If the charging power is limited, then MPPT is not the function you want. If power input is below the required charging power, you can't even charge at that current. And if input power is above that power, you will want to deliberately operate at the lower power point not the maximum power point. Why not choose a battery with a simple charging requirement and oversize the battery so that it can accept just about any power the cell might deliver. Then you only need to shutdown MPPT when the voltage on the battery is near its maximum value.Q5: You can notice once a battery enters stage 3, its current falls to zero and its voltage also falls somewhat. Does this voltage falls on its own, or, we need to reduce it to that level for some reason?
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