Maximum Power Point Tracking (MPPT) allows a PV array to
deliver up to 30% more power to a battery than it would if it were connected
directly to the battery. This type of charge control costs more, but under
certain conditions can be very cost-effective. A 20% power increase on a
1000-watt PV array is like getting 200 additional watts. A photovoltaic (PV)
array is a constant current device. The current (amps) from a PV module
remains relatively constant over a wide range of voltage. For example a
typical 75-watt module delivers 4.45 amps at up to 17 volts when the sky is
clear and temperature is cool. Traditional PV controllers connect the PV
array directly to the battery until it reaches a full charge voltage. When
this 75-watt module is connected directly to a battery in a low state of
charge it will begin charging at 12 volts. The PV panel still provides about
the same current, but, because PV output voltage is lower, it can only
deliver 53 watts to the battery. This wastes up to 22 watts or 30% of the
available power. MPPT charge controls use this extra voltage to boost
charging amps to the battery. Outback and Blue Sky charge controls also
allow the use of an array voltage higher than the nominal battery voltage so
smaller wire may be used.
Since the battery voltage rises when charging and the
maximum power point of the PV module falls as it warms up, a 10 to 20%
increase is more realistic. MPPT charge controls are most effective in cool
climates and/or when the PV array is charging batteries that are at a low
state of charge.
