98% of the time produce their maximum power at a voltage higher than the battery
voltage (in practice about 16.5-17v).
When a panel is connected directly to the battery
, or via a simple regulator
it has to be at the same voltage as the battery. When the panel is at the battery voltage it will produce less power than at its optimum voltage.
controller adds a bunch of electronics
that allows the panel to operate at its is optimum voltage and converts this higer voltage to the lower battery voltage. When it lowers the the voltage the current
is increased.(electronics can convert a high voltage low current
to a low voltage high current. The overall power stays the same)
So more current can be extracted from the panel than the solar
panel was forced to be the lower battery voltage.
Unfortunately their are losses in the conversion and the controller has a hard time picking the optimum panel voltage as it is constantly changing. Good circuits that track the optimum panel voltage are complex and the better controllers burn some power tracking this voltage.
The net gain has been debated on CF a few time, but my belief is for the best controllers it is about 5-15% above a non MPPT
For the poor cheap
MPPT controllers the gain is less and may be negative.
The expensive controllers that offer MPPT often also offer sophisticated battery management (which is important) and detailed displays of the solar
output etc, but these features can also be found on some MPPT controlers
Another way of looking at the controller is to think of like a gearbox
. The gearbox
in your car allows the engine
to spin at suitable revs while the wheels turn at at completely different revs.
A MPPT controller keeps the solar panel at its optimimum voltage no matter what the battery voltage is.
I thinks Marks definition is technically more accurate