Also why is a charge controller required when between a PV module and a battery. It was mentioned somewhere else that without a charge controller the PV module or current source would take on the voltage of the battery and therefore would not output its maximum power capacity. Why is this?
The maximum power point voltage doesn't match the battery voltage across the entire range of battery charging voltages.
Here's a real example. The MPP voltage for my array is something on the order of 93 volts and the nameplate rating is 2800 watts, or roughly 30 amps. The charging range is about 50 to 58 volts, and the inverters make power whenever the voltage is at or above about 53 volts.
If you take those 30 amps and you multiple them by all those different voltages you will come out with a number of different powers. 30 amps times 53 volts is 1,590 watts, or about half of what I have, and without the charge controller, I'd be wasting around 1,200 watts of solar panels (at phat dollars per watt).
But it's more complex than that because the 93 volts is at 25 degrees C at the solar panel. I could have designed the system with panels that had a Vmpp of 58 volts -- that would have wasted less power -- but what happens in the summer when the array is 60 degrees C (140F)? Solar cells lose voltage as temperature increases -- -0.37% / degree C, in the case of my panels -- so Vmpp in the summer is (60 - 25) * 0.37%, or 13% (12 volts) lower. Now instead of being a 93 volt array, it's an 81 volt array. And had I chosen 58 volts, the new Vmmp would be less than 52 volts -- not enough to recharge the batteries.
The Maximum Power Point Tracking serves two purposes -- the first is it converts the voltage to the battery's voltage, and the second is it adapts to changes in the array's performance. In the winter the array voltage is higher, and in the summer it is lower.
The charge controller performs another important job -- it controls the charging process itself. My batteries will be around 50 volts in the morning. They need to reach about 58 volts in order to be fully charged. Anything more than 58 volts and the batteries could be damaged. So the charge controller will start outputting power and the voltage will rise to 58 volts. At that point the charge controller reduces the current to hold the voltage at 58 volts. At some point in time the system will decide the batteries are fully and it will sell surplus power so long as the batteries are at or above about 53 volts.
