opti-solar SC MPPT series User Manual page 5

High Voltage Strings and Grid-Tie Modules
Another benefit of MPPT technology is the ability to charge batteries with solar arrays of higher nominal voltages. For
example, a 12 Volt battery bank may be charged with a 12-, 24-, 36-, or 48-Volt nominal off-grid solar array. Grid-tie solar
modules may also be used as long as the solar array open circuit voltage (Voc) rating will not exceed the maximum input
voltage rating at worst-case (coldest) module temperature. The solar module documentation should provide Voc vs.
temperature data.
Higher solar input voltage results in lower solar input current for a given input power. High voltage solar input strings
allow for smaller gauge solar wiring. This is especially helpful and economical for systems with long wiring runs between
the controller and the solar array.
An Advantage Over Traditional Controllers
Traditional controllers connect the solar module directly to the battery when recharging. This requires that the solar
module operate in a voltage range that is usually below the module's Vmp. In a 12 Volt system for example, the battery
voltage may range from 10.8-15 Vdc, but the module's Vmp is typically around 16 or 17V.
Because traditional controllers do not always operate at the Vmp of the solar array, energy is wasted that could otherwise
be used to charge the battery and power system loads. The greater the difference between battery voltage and the Vmp of
the module, the more energy is wasted.
Nominal 12 Volt Solar Module I-V curve and output power graph
Contrast with the traditional PWM controller, MPPT controller could play a maximum power of the solar panel so that a
larger charging current could be supplied. Generally speaking, the MPPT controller's energy utilization efficiency is
15%~20% higher than PWM controller.
Conditions That Limit the Effectiveness of MPPT
The Vmp of a solar module decreases as the temperature of the module increases. In very hot weather, the Vmp may be
close or even less than battery voltage. In this situation, there will be very little or no MPPT gain compared to traditional
controllers. However, systems with modules of higher nominal voltage than the battery bank will always have an array
Vmp greater than battery voltage. Additionally, the savings in wiring due to reduced solar current make MPPT worthwhile
even in hot climates.
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