Installation Guide

MORNINGSTAR CORPORATION
41
4.0 OPERATION
40
4.0
High Voltage Strings and Grid-tie Modules
Another benet of TrakStar MPPT technology is the ability
to charge 12 or 24 Volt batteries with solar arrays of higher
nominal voltages. A 12V battery bank can be charged with
a 12 V, 24 V, or 36 V nominal off-grid solar array. Certain
grid-tie solar modules may also be used as long as the
solar array open circuit Voltage (V
oc
) rating will not exceed
the SS-MPPT 60V maximum input Voltage rating at worst-
case (coldest) module temperature. The solar module
documentation should provide V
oc
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 for systems with long wiring runs between
the solar array and the SS-MPPT.
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 below the
module’s V
mp
. In a 12 V system for example, the battery
voltage may range from 10 - 15 Vdc but the module’s V
mp
is
typically around 17 V. Figure 8 shows a typical current vs.
voltage output curve for a nominal 12V off-grid module.
CURRENT
VOLTAGE
10 V 15 V
MAXIMUM
POWER
POINT
{
BATTERY
VOLTAGE
RANGE
~17 V
Figure 8. Nominal 12 V Solar Module I-V curve
The array V
mp
is the voltage where the product of current
and voltage (Amps x Volts) is greatest, which falls on the
“knee” of the solar module I-V curve as shown in Figure 8.
Because traditional controllers do no operate at the V
mp
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.
TrakStar MPPT technology will always operate at the
V
mp
resulting in less wasted energy compared to traditional
controllers.