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.
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.
P(W)
10.8V
15V
17V
VP curve
0
I(A)
V(V)
MPP
10.8V
15V
17V
Typical
Battery
Voltage
Range
VI curve
0
V(V)
Px
Pmax-Px
MPP
PWM
Controller
Operating
Range
Nominal 12 Volt Solar Module I-V curve and output power graph.
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