Design - 3

• Battery charger controller
  

The DC voltage coming out from the panels must be higher than the DC voltage of the batteries, so that panels will have more emf than the batteries to push the charges through them. But if our typical 18 V DC panels are connected directly to the batteries, they will eventually overcharge and will cause severe damage to the batteries. Therefore I need a device to control the charges going into the battery bank to prevent overcharging. The charger is required to be three stage charge cycle controller. That means the batteries when they are under a certain voltage are charged applying bulk voltage and the battery draws the maximum current, then absorption voltage is applied and the current gradually decreases as the battery is being charged, then floating voltage is applied.

There are two type of solar battery charge controllers; PWM and MTTP.

• PWM charge controllers

These are the cheaper ones which uses a duty cycle (on time/off time) to regulate the voltage from the panels to the battery bank

• MTTP charge controllers

These are more expensive but usually gives 30% more power than PWM. They reduce the voltage coming out of the panels to reach that of the battery so that maximum current draw is possible

The charger is rated by input voltage (12 V DC in our case) and max current that it can handle ( we have 7 panels each 100 Watts, therefore 700 / 12 = 58.3 Amperes). We must be ready for some phenomena like edge of cloud effect or very bright circumstances, so I will calculate a 30% spare capacity. 58.3 x 1.3 = 75.8 Amperes.

In this case I need either one 80 A charge controller or two 40 A charge controllers wired in parallel.

Another option is raise the voltage to 24 V DC and use only one charge controller of 40 A.