31
TriStar MPPT Operator’s Manual
An Advantage Over Traditional Controllers
Traditional controllers connect the solar module directly to the battery when recharging. This re-
quires 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.
P
OW
ER
V
OL
TAGE
1
0
15 1
7
TrackStar
Maximum
Power
Point
Traditiona
l
Contro
ll
er
O
perating
Range
CURRENT
V
OL
TAGE
1
0
15 1
7
Maximum
Power
Point
Typica
l
Battery
Vo
l
tage
Range
12
Vo
l
t
Modu
l
e
Current
vs
.
Vo
l
tage
12
Vo
l
t
Modu
l
e
O
utput
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.
