On the backside of the solar panel, you can see a label on which its technical specifications are noted. From that, we will get a lot of information about that piece of product. We will get Vmax and Imax, Voc and Isc, and Pmax.
The power curve or I-V curve will give you an elaborate understanding of the above measurements. But all the above data is measured under standard testing conditions. So the actual value may be different.
However, I am going to show you how to measure the Isc (short circuit current- maximum current a solar panel can supply under given solar radiation), and the Voc ( Open circuit voltage- Maximum voltage a solar panel can supply under given solar radiation.
Table of Contents
How to test short circuit current (Isc) of a Solar Panel
An ammeter is used to measure the current in a circuit. I am going to show you how to use a digital multimeter to measure the short circuit current of a solar panel. When we are connecting an ammeter (inside a multimeter) to the positive and negative terminals of the solar panel (in series) we are actually short-circuiting the solar panel. When we short circuit a maximum current flow will happen. At that time the voltage will become zero. First, connect the black test lead to the common input jack and the red test lead to the 10A jack. Then position the function switch to the A range (as shown). Now carefully connect the positive and negative test lead to the positive and negative output of the solar panel respectively. (Don’t hold it for more than 5 seconds since if there’s a large current it will damage the instrument.)
How to meassure the Open Circuit volatege (Voc)
A voltmeter is used to measure the voltage in a circuit. I am going to show you how to use a digital multimeter to measure the open-circuit voltage of a solar panel. When we are connecting a voltmeter (inside a multimeter) to the positive and negative terminals of the solar panel (in parallel) we are not short-circuiting the solar panel. There will be no flow of charge through the circuit. So we will get the maximum voltage the solar panel can supply under given solar radiation. At that time the current will be zero. First, connect the black test lead to the common input jack and the red test lead to the V jack. Then position the function switch to the V range (as shown). Now carefully connect the positive and negative test lead to the positive and negative output of the solar panel respectively.
Fixing and Connecting Solar Panels to for 12V Off-grid System
I have already fixed two of my panels on a stand. I am going to extend that stand to accommodate my new boys along with it. To do that I have to extend my existing stand. I already had a metal cutter and a small welding machine. So it’s not a big deal. After that, I have to connect all four panels in parallel or in series to get the desired voltage. Since I am going to build a 12v system I have to connect all these panels in parallel.
I have a total of 200W solar panels. Since I am living in India there’s a guaranteed 5 hrs of Peak Solar Radiation. It is called PSH or Peak solar hrs. According to the specifications of all these panels are connected in parallel we will get a Voc of 22V and Isc of 12A. Remember this is just the short-circuit current not the actual current and a value of 12A cannot be expected because of wiring and panel loss.
I made the frame this way so that I can later install a single axis solar tracking system that tracks the sun’s daily east-west movements. It’s not worth installing a tracking system these days because modern solar panels are highly efficient. I just wanted to show you how a tracking system works.
Let’s connect all the four 50W panels in parallel and check how much Voc and Isc are we getting. To do that we have MC4 four in one branch connectors. Using it we can easily connect all the panels in parallel as shown in the figure below.
You can Buy mc4 branch connectors from my amazon store. As we discussed earlier we should get 22V Voc and a maximum of 12A (not more than 8A expected). Let’s take the measurements using the multimeter. Then we can decide the AH of the battery we are going to use with this solar inverter.
Redesigning Control Panel
This is the control panel I am using for my existing solar power system. You can see the charge controller, Isolators and control board. This is a simple setup.
10A PWM Charge controller
16A Isolators
Ordinary AC Switchboard
The next step is designing the controls panel. We have to connect the charge controller, a DC MCB for the solar panel, another DC MCB for the battery and an AC isolator to disconnect the inverter from the AC control box.
PWM Charge Controller
This is a 20A PWM charge controller I am going to use with this system. Charge controllers are used to protecting the battery from getting overcharged or over-discharged. It also prevents the reverse flow of the charge from the battery to the solar panel.
Our panel size is not large enough to use an MPPT charge controller which is way more efficient than PWM controllers. In the next upgrade, I will replace this PWM with an MPPT charge controller.
This charge controller has six connectors on the bottom side. Two of them are for solar panels, another two for battery and the last two for connecting DC load.
In an MPPT charge controller, this is where we connect the inverter. Since our PWM controller is not rated enough to handle the possible current flow from the battery, we connect the inverter directly to the battery.
Most inverters have an over-discharge protection circuit built-in, so it’s not at all a big deal. Otherwise connecting the inverter directly to the battery may lead to frequent discharge which is unhealthy for the battery.
Inverter
This is the 800W sign-wave inverter I am going to use with this system. As you can see there are two input leads that we have to connect to the positive and negative terminals of the battery.
And there are input, output and neutral connectors. Since we are building an off-grid system we don’t need this Input connector. These are the phase and neutral of the inverter output.
We can use the input terminal to charge the battery using grid power and inverters inbuilt battery charger during unforeseen circumstances like continuous cloudy days or battery over-discharge etc.
Since we are constructing an off-grid system for a property where there is no electricity grid forget about the input connector on the inverter.
Battery
This 110Ah battery is going to store the solar energy and power the inverter. This is a lad acid tubular battery, which usually has a depth of discharge around 50%.
Let’s connect the inverter and the charge controller’s battery leads to the positive and negative terminals of the battery. The first and second DC MCBs are used to isolate and protect the charge controller from the solar panel and the battery respectively.
Here we can see an AC isolator which is used to isolate the AC control panel from the inverter. Make sure the inverter is off and connect the output phase and neutral to the AC isolator.
Lets Connect
Now we can connect the solar array to the first MCB. Make sure you are connecting positive and negative leads from the panel array to the same on the charge controller. You should pay attention when connecting the battery too.
Interchanging the polarity will damage the charge controller. I suggest you use different coloured wires for positive and negative polarity so that you will not get confused. You can use this Wire gauge calculator to determine the DC wire gauge.
Great. Sun is shining bright on the solar panel, which has already started producing power but the MCB blocks the power to reach the charge controller and then the battery.
Testing
Just turn on the first MCB and check how much voltage the solar panel array is generating. Great, 21.6 V. It is the open-circuit voltage of the solar panel array under current solar radiation.
Now let’s turn the second MCB between the battery and the charge controller. Keep this in mind. When turning on an offered system always turn on the battery first. It will help the charge controller to identify the battery and its voltage.
Turning on the MCB to the battery automatically turn on the charge controller output. There’s a switch on the charge controller to turn it off. The battery is a new one that’s why it already read 12.2V.
Then you can turn on the solar panel. If you turned on the battery while the solar panel is connected, the charge controller will malfunction. You can see that the battery voltage is rising which means the solar array has started charging the battery.
Now we can turn on the inverter and, AC isolator and test the AC loads using this control box. Perfect. Our 800W off-grid solar power system is ready to harness unlimited solar energy and power your office, workshop or outhouse. Enjoy.