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I am trying to build a subsystem for a project that I am undertaking in university. An overview of the subsystem can be seen in the attached image. solar panel subsystem

The only difference from my subsystem and the one in the image is that I wish to connect to a relay instead of just a single AC appliance, and also that one of the batteries is supposed to act as the grid. Battery A is supposed to be used to store solar power while Battery B is supposed to act as the grid for the purposes of demonstration of this project.

My questions are related to component selection. The relay is supposed to power 2 appliances; a fan and a lamp. It is also supposed to power 3 Arduinos and a Raspberry Pi.

I did some initial power calculations and I came to a conclusion that I would need a car battery to power the appliances for at least 3 hours worth of demonstration. I did not include the micro-controllers for the calculations. So the first question is: Is the power consumption from the micro-controllers big enough to need a recalculation of how much power is needed? If not, does a car battery sound reasonable for the purposes of this project?

I am trying to figure out what type of power inverter I need to fulfill my purposes. I recently found out that most grid-tied inverters switch between the backup battery and the grid when the battery is full/empty. Please correct me if I'm wrong on this. However, I have to remind you that I am planning to use a battery to represent the grid. Will a grid-tied inverter also work in this case? Also, I was hoping to be able to decide when the inverter switches back and forth between the batteries. For example, I wish to use the solar powered battery only during peak electricity rate hours. Is there a way to program an inverter to switch at only certain times?

Since I am not planning on using large solar panel cells, I am confused as to if I will need a charge controller to actually regulate the battery. The reason I am not using large solar panel cells is because I just have to demonstrate that the battery can be charged, and it does not matter how long it will take to charge it. I am just connecting a solar panel for demonstrative purposes to show the subsystem on a smaller scale than in actual implementation. Under what conditions should I use a charge controller?

Thank you for taking the time to read this. This project is being undertaken virtually unsupervised so I am trying to take advantage of any resources that I can get.

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    \$\begingroup\$ Explain what you mean by using one battery as the grid. \$\endgroup\$ – Daniel Jan 26 '16 at 19:54
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    \$\begingroup\$ Microcontroller power consumption should be negligable compared to everything else. But the thing about your switchover plan is that a second battery is nothing like the grid: the grid is AC, and grid-tied inverters often have "anti-islanding" systems that cut out for safety if the grid is not present. Also, assembling a bunch of commercial units sounds both expensive and not very educational for an undergrad project .. \$\endgroup\$ – pjc50 Jan 26 '16 at 19:58
  • \$\begingroup\$ The micro boards will not use significant energy compared to the appliances. Will you be building the inverter? IF you are buying one then ability to control it is one of the requirements for selecting it. If you build it you make it do what you need. When you say that "most grid-tied inverters switch between the backup battery and the grid when the battery is full/empty" not sure what you mean, they usually supply power from the DC (battery) side to the grid (AC) side. \$\endgroup\$ – user1582568 Jan 26 '16 at 20:02
  • \$\begingroup\$ You can start with this: raspberrypi.stackexchange.com/questions/5033/… This also might me useful: Arduino: arduino.cc/en/Tutorial/ArduinoZeroPowerConsumption RasbPi: raspi.tv/2015/raspberry-pi2-power-and-performance-measurement As you can see, everything depends on device's implementation. \$\endgroup\$ – kjote Jan 26 '16 at 21:05
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Lots of questions.

In general, the microcontrollersellers won't use much energy compared an AC apliance.

A car battery is a bad idea. They aren't designed for deep discharges. They are designed to supply a large amount of amperage for a second or two and then be recharged. You would do much better with a deep discharge lead acid battery which looks just like a car battery, but can sustain deeper discharges and are made for your type of project.

If your solar panel is small and around 1/20 or less of the size of your battery, then you can get away without a charge controller, but it will take a very long time to fully charge the battery when it is empty. I.E. if you have a 100 Ah battery and a 20 watt solar panel a charge controller is probably not necessary as you will not likely damage the battery from overcharge.

A grid tie inverter takes the dc voltage from the solar panel and inverts it to 120 volts AC - how will this work when you are attaching it to a battery that is 12 volts DC? As far as programming the inverter to supply power as you want, I believe it is possible, but you will have to shop around to find the right one and they are not cheap.

Good luck

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