Design - 4

• Batteries

Connecting my panels directly to the inverter to power my appliances means that I can have power only while the sun is shining, so I need to store the excess power for using it at night (or in my case at the time of power outage).

Battery bank should have enough capacity for the designed working hours without charging. The capacity is described as Ah (Amperes hours) which means the amount of amperes I can extract from the battery bank in a time of some hours. If I have a battery of 12 Ah, that means I can get a current of 12 Amperes for one hour, or I can get a current of 1 Ampere for 12 hours, o a current of 6 Amperes for 2 hours, ...etc.

If I'm going to power my 575 Watt/Hour system for 4 hours after sunset, then I will need my batteries to store a power of 575 x 4 = 2.3 kWh. That means 2300 Watt/Hour / 12 V DC = 192 Ah and that is the capacity of the battery bank needed (without calculating losses)

According to my previous calculation in Design - 1, my requirements are day is 3.2 kWh. 3200 / 12 = 266 Ah. Adding losses of 20%, the result is 320 Ah.

If I used a battery bank of 320 Ah, then I will use all of this capacity every day, and I will need to fully recharge the batteries also everyday.

Discharging the batteries to the last drop will damage them, therefore I have to decide on the DOD (Depth Of Discharge). To know how much I can discharge my battery bank, I need to know about:

• Battery types

At first I thought that I can just buy enough car batteries and install them as my battery bank, but then I figured out that they can be used only on certain conditions and they are not recommended. Car batteries are designed to give a very high current in a very short time (cranking the car to start it up), and are designed to be discharged not less than 80% of their capacity or else their life expectancy is shortened much. In the car, after cranking, the dynamo is responsible of charging the battery to 100%.

If I want to use car batteries, then I will use them for a DOD of 20% only, then I need a battery bank of 320 / 20% = 1600 Ah. If the average 100 Ah car battery price is 1,000 EGP, then I need 16,000 EGP.

In my research  I will categorize batteries into two sections; Deep cycle and Shallow cycle (car batteries).

Deep cycle batteries are designed to give constant current while discharging, and to be charged more quickly. These batteries have many types, some of them are:

1. Flooded: These are positive and negative plates which have liquid acid to act as an agent for
   
   moving charges (current). This is the cheapest type of the deep discharge batteries. It has thicker plates to withstand deep discharging without corrosion. The bad thing about these batteries that they produce hydrogen gas when being charged. This makes them not suitable for mounting in a confined area because hydrogen is flammable and poisonous. Also these batteries need constant distilled water refill to compensate the hydrogen (Charging the batteries decomposes the water into hydrogen and oxygen).
2. (Semi)Sealed Lead-Acid: These are as the above type, but are sealed, hydrogen gas is
   
   significantly reduced (not eliminated) and does not need water refill. These are also called VRLA (Valve regulated Lead Acid). These have a moderate price.
3. AGM and GEL: These two have the acid absorbed in a fiberglass mat (AGM) and in the form of gel (GEL). These have the advantage of mounting horizontally or vertically.

Deep cycle batteries can withstand regular DOD of 40% and occasionally DOD of 80%. It is adviced not to go below 50% to lengthen their life expectancy.

• Battery bank capacity

To cope with a DOD of 50%, I need to double my calculated battery bank size, therefore I have to get 320 x 2 = 640 Ah batteries. If the average VRLA battery price of 100 Ah is 2,000 EGP, and I need 640/100 = 7 batteries, they will cost me 14,000 EGP.

Comparing this to a normal car battery, I will need 640/20%= 3200 Ah which will cost me 32,000 EGP.

• Wiring the batteries together

In my design, I went for 12 V DC battery charger and inverter (which was not the best choice). To have

 a battery bank of 12 V DC, just connect all the positive terminals together and all the negative terminals together. I will have then a 12 V DC, 640 Ah battery bank.

To have a  24 V DC battery bank, connect each half of the batteries as a standalone 12 V DC bank, then

connect one positive terminal from bank #2 to one negative terminal from bank #1. I will have a left one positive terminal from bank #1 and negative terminal from bank #2 which will have a potential of 24 V DC, 320 Ah. Note that voltage and current change by changing the battery setup, but the constant is the power. P = V x I (Power = Voltage x Current).

These batteries should be connected to the battery charger and the inverter.