ROGUE MPT-3024, Installation Manual And Owner'S Manual, Page: 18 / 52

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normal charger operation (see Section 15 for instructions). Upon selecting this
mode the controller will resume operation in the MPPT mode (if it’s presently in
any other mode, it will exit that mode and enter MPPT mode), and will begin an
attempt to charge the batteries until their voltage reaches the value determined
by the EQLZ setpoint. Once the equalize setpoint has been met, the controller
will transition to the equalize mode and will try to maintain the battery voltage
at the equalize setpoint for the amount of time specified by the user. The
controller will remain in this mode until either the timer has elapsed, the user
aborts the operation, or the controller enters the sleep mode. If the controller
enters the sleep mode before the equalize setpoint has been met, or before
the timer has fully elapsed, it will display an error when it reawakens and will not
attempt to resume operation in the equalize mode. For safety reasons,
equalize must always be initiated (or re-initiated) manually.
See Figure 5.2.1 for mode transitions during a typical day of operation.
6. Maximum Power Point Tracking
A PV array, for the most part, delivers a constant current over a wide range of
voltages. For this reason, it’s usually thought of as a
constant current
source.
By contrast, a battery will provide a relatively constant voltage over a wide
range of currents; it’s called a
constant voltage
source.
An array that is sold for 12-volt use, for example, actually generates anywhere
from zero to about 21 volts, depending on the load it’s connected to and the
number of individual cells comprising the array. A PV array will develop its
highest voltage without a load; this is the open-circuit condition, and the
voltage measured at this point is called V
oc
. Since there is no load in the
open-circuit condition, the resistance of the load is essentially infinite, and
therefore the array will produce no current at V
oc
. With no current flowing, and
because power equals voltage multiplied by current (P=VxI) the power being
produce by the array will be zero watts.
At the other extreme, the load connected to the array can be a short circuit, the
resistance of which is zero ohms. Under this condition the array will produce the
maximum amount of current of which it’s capable, called I
sc
. Since the load
under this condition (ideally) offers no resistance, there can be no voltage
across it. The array under a short circuit therefore generates no voltage, and
like the open-circuited array, produces no power.