35
TriStar MPPT Operator’s Manual
34
Operation
Operation
4.0
The TriStar MPPT 150V operation is fully automatic. After installation is completed, there are few
operator tasks to perform. However, the operator should be familiar with the operation and care
of the TriStar MPPT 150V as described in this section.
4.1 TrakStar
TM
MPPT Technology
The TriStar MPPT 150V utilizes Morningstar’s TrakStar
TM
Maximum Power Point Tracking
(MPPT) technology to extract maximum power from the solar array. The tracking algorithm is fully
automatic and does not require user adjustment. TrakStar
TM
technology tracks the array
maxi-
mum power point
as it varies with weather conditions, ensuring that maximum power is harvested
from the array throughout the course of the day.
Current Boost
Under most conditions, TrakStar
TM
MPPT technology will “boost” the solar charge current. For
example, a system may have 36 Amps of solar current flowing into the TS-MPPT and 44 Amps
of charge current flowing out to the battery. The TriStar MPPT 150V does not create current!
Rest assured that the power into the TriStar MPPT 150V is the same as the power out of the
TriStar MPPT 150V. Since power is the product of voltage and current (Volts x Amps), the follow-
ing is true*:
(1) Power Into the TriStar MPPT 150V = Power Out of the TriStar MPPT 150V
(2) Volts In x Amps In = Volts Out x Amps Out
* assuming 100% efficiency. Losses in wiring and conversion exist.
If the solar module’s
maximum power voltage
(V
mp
) is greater than the battery voltage, it follows
that the battery current must be proportionally greater than the solar input current so that input
and output power are balanced. The greater the difference between the V
mp
and battery voltage,
the greater the current boost. Current boost can be substantial in systems where the solar array
is of a higher nominal voltage than the battery as described in the next section.
High Voltage Strings and Grid-Tie Modules
Another benefit of TrakStar
TM
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
(V
oc
) rating will not exceed the TriStar MPPT 150V 150 Volt
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 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
V
mp
. In a 12 Volt system for example, the battery voltage may range from 10 - 15 Vdc, but the
module’s V
mp
is typically around 16 or 17 Volts. Figure 4-1 shows typical current vs. voltage and
power output curves for a nominal 12 Volt off-grid module.
PO
WER
VOLTAGE
10
15 17
TrackStar
Maximum
Power
Point
Traditional
Controller
Operating Range
CURRENT
VOLTAGE
10
15 17
Maximum
Power
Point
Typical Battery
Voltage Range
12 Volt Module
Current vs. Voltage
12 Volt Module
Output Power
Figure 4-1. Nominal 12 Volt Solar Module I-V curve and output power graph.
The array V
mp
is the voltage where the product of output current and voltage (Amps x Volts) is
greatest, which falls on the “knee” of the solar module I-V curve as shown on the left in Figure
4-1.
Because traditional controllers do not always 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 V
mp
of the module, the more energy is wasted.
TrakStar
TM
MPPT technology will always operate at the maximum power point
resulting in less
wasted energy compared to traditional controllers.
Conditions That Limit the Effectiveness of MPPT
The V
mp
of a solar module decreases as the temperature of the module increases. In very hot
weather, the V
mp
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 V
mp
greater than battery
voltage. Additionally, the savings in wiring due to reduced solar current make MPPT worthwhile
even in hot climates.
