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)