The battery and the charger

I have purchased a cheap Chinese solar battery charger for my system. As I mentioned before this
was not chosen by calculation, but by price. I have a 20A MPPT charger.
20A means that is can handle a max of 20A solar panels input. My panel outputs 8A (by design), so I'm okay if I added another panel, then the current will be 16A, but I can't add a third panel except when I buy a bigger charger. This charger works with input power from the solar panel and charges the battery by regulating this power.

There are two types of battery chargers, PWM and MPPT as mentioned before.

As long as the sun is bright, the battery is charged, and when the sun goes down, the battery charger consumes the battery and routes it to the load.

This type of charger has an instruction that I should first connect the battery then connect the solar panels, but I ignored this instruction and connected only the solar panels, then I had an over current. Fortunately it has an electronic fuse and I haven't burnt anything. I wired the output to my inverter to test the output AC power, but the charger refused to power the load directly from the panels, so I thought the charger was faulty.

I then assumed that the battery has something to do with the operation of the charger, so I went down and bought a test battery of 7Ah (a cheap one of 175 EGP). The make of the battery is MxVolta and it's made in Korea. It is a VRLA (Valve Regulated Lead Acid) battery that can be discharged up to 60% DOD (Depth of discharge) for 2000 cycles. This suits the solar application.
I connected the battery to the charger and to the inverter and I was very pleased when the system worked flawlessly.

Sure a 7Ah has a power of 12V DC x 7A DC = 84 Watts/hour when it is fully charged. I was powering the PC, monitor and fan and the power consumption was 150 Watts, so 84/150=0.56 hours. Adding a losses of 20% then the time this battery can supply is 0.45 hours. But I must take into consideration that I don't want to fully discharge the battery (actually it's a function in the inverter and the battery charger as well). I will aim a 50% DOD, so this battery will have a time of 0.45/2=0.23 hours which is 13 minutes on condition that the battery is fully charged.

The charging time of the battery also counts to decide how big my battery bank will be. I now have a charging capacity of 120 Watts per hour. Considering 10 hours of bright sun I  can get 1.2 Kw of energy everyday. Thus I can charge a battery bank of 1200/12=100Amperes. I intend to buy a 65Ah battery for a start (easily available at my area) and it costs 1150 EGP.

Next time I should write about the cabling and the ATS (Auto Transfer Switch)

Building solar panels

I have stopped writing on my blog for more than two months due to my work heavy load, so I was very tired, but now, I have received the rest of my solar system's components and I again have the urge to write. At this moment when I'm writing these lines, I have my PC fed from solar energy. YES! I have completed my solar UPS and I have made a successful test run.

But first I should resume from where I stopped earlier. I will talk about building my solar panel.

I purchased a a pack of 108 solar cells from ebay (on the right picture and I have to say I expected them to be larger) and I have also purchased tabbing wire to connect solar cell together (this was a time consuming job). The solar cells I bought has a dimensions of 6"x6" each produces 0.5V DC and 8A DC (that what was written on ebay).

I wanted to assemble some of them to produce 18V DC (as standard panels produce) so the calculation will be: 18V DC/0.5V DC = 36 cells. 

Note: These cells are extremely fragile and brittle, I broke about a dozen, but they are still functional.

First thing I did is to put one in the sun and measure how much power does it produce. I measured a 0.55V DC and 1.1A DC. I thought something was wrong as it should produce 8A DC, but I think I could not hold it right or my hand shadow affected the current. Anyway I decided to go on with experimenting. I connected two of them and measured 1.1V DC and also 1.1A. I thought that since they are imported from China, they should be not good, but I couldn't stop at this phase. In the picture the measurement is away from the sun.

I had to connect those 36 cells in a rectangular pattern of 9x4 by connecting each positive side (dark side) to its neighbor's negative side (shiny side)

The frame is made of aluminium angles (3cm x 3cm thickness 3mm) joined together and in the middle is a glass sheet (thickness 6mm) glued to the frame with silicone. I had to line up the cells to be neat and have a nice look. I welded all the 36 cells and extracted the far terminals to be the positive and negative. I covered the back of the cells with a PVC tape to isolate them from humidity (I should use a piece of plexiglass or any isolating sheet for the back of the panel because the PVC tape is not solid, thus sagging a little bit).

The final panel look is in the picture on the right. I measured the voltage output and it was 18.8V DC. I measured the short circuit current and I was very pleased to read 6.5A DC.

My panel initial design was 144 Watts, but practically it is 122 Watts. It's okay for me and this result satisfies me for my first home built solar panel.

After doing some research about which direction to point my panel and which angle is the most suitable to extract the biggest amount of juice from the sun, I reached some very easy numbers. Since I live in Egypt in the Northern hemisphere, then I should point my panel to the true South. Also in Cairo the suitable tilting angle in summer is 15 deg and in winter is 45 deg (I have made an extension leg to increase the tilting angle to 45deg). Sure a tracking device will be much more efficient, but I don't want to bother myself with mechanical designs at this stage yet.