ROGUE MPT-3024, Installation Manual And Owner'S Manual, Page: 21 / 52

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lesser current from the array and convert it to a lower output voltage at a
higher current for the battery.
For an ideal converter without any losses, the power in equals the power out,
or V
in
x I
in
= V
out
x I
out
. This formula is useful in calculating the approximate
relationship between the PV array and the batteries. For example, consider a
75 watt module. It’s rated for its maximum output (the full 75 watts) when
operating at a V
mp
of 17.4 volts, and will produce about 4.31 amps of current
(17.4 x 4.31 =75.0). V
in
therefore equals 17.4 volts and I
in
equals 4.31 amps
(the MPT-3024 will automatically find and track these values). If the battery
voltage is 12.0 volts while charging, V
out
equals 12.0 volts. We can easily solve
for I
out
, which is about 6.25 amps (12.0 x 6.25 = 75.0). This is a 45%
improvement (“boost”) in current over direct connection to the battery.
It can be seen that boost increases as the difference between battery voltage
and the V
mp
of the array becomes greater, so that a highly discharged battery
affords much greater benefit when using maximum power point tracking. If the
battery was charging at 11.0 volts (almost completely discharged), I
out
would
be 6.82 amps, a 58% improvement.
In reality, external factors such as array temperature and the voltage drop in
wiring will produce boost of less than these idealized amounts. The efficiency of
the charger will also have a limiting effect on boost. The MPT-3024, like any
DC-DC converter, is not 100% efficient. This is due to the resistance of circuit
elements, the switching losses of the conversion, and the small amount of
energy used in simply powering the electronics that operate the converter. We
can approach 99% efficiency under certain conditions. A converter that is 99%
efficient would take a 75 watt input and render 74.25 watts at the output,
having consumed 0.75 watts of that power in the conversion process and
converted it to heat. For the above example, this would leave us with about
6.19 amps for charging our 12 volt battery, which is still a 44% improvement
over direct connection to the battery. 99% efficiency is exceptional and may be
approached under certain conditions. Typical efficiencies range from 95-97%
under most conditions. As a rule of thumb, higher efficiency occurs as PV
voltage increases (which causes a corresponding decrease in current for a
given amount of power), and as the difference between PV and battery voltage
decreases. Typical efficiency for the MPT-3024 is shown in Figure 6.1.2.
The DC-DC conversion process described above is only part of maximum power