Blue Sky Solar Boost 3048L - Manual (Page 13)

Installation and Operation Manual
12
TEMPERATURE AND OUTPUT POWER
Over temperature protection is provided to protect the unit from damage due to high output power at high ambient
temperatures. When mounted vertically as described in the installation section, the unit can deliver full output in an ambient
temperature of up to 40
°C (104°F). If an over temperature condition exists, the unit will cycle on/off, reducing average power
delivery to within safe limits. During periods of thermal shutdown the Charge Status Indicator will display an “off” condition.
MAXIMUM POWER POINT TRACKING (MPPT)
MPPT and associated current boost operation is fully automatic and will function whenever sufficient PV voltage and current
are available. The percent increase you will receive in output charge current relative to PV array current is variable, and will change
with operating conditions. When conditions are such that insufficient PV power is available to produce an increase in output current,
the unit will stop switching power conversion and operate as a conventional series pass PWM controller with very low forward
voltage drop. Current boost performance can be monitored using the digital display. When output charge current is greater than
solar panel current, current boost is functioning. A minimum PV current of just under 2.0 amps is required for MPPT current boost to
operate.
The principal operating conditions which affect current boost performance are battery voltage and PV array temperature. At
constant solar intensity available PV array power changes with array temperature. A PV array’s power vs. temperature characteristic
is such that a cool PV array can produce a higher voltage and more power, than a hot PV array. When PV voltage is sufficiently high
for MPPT to operate, a constant power output is delivered to the battery. Since output power is constant while MPPT is operating, a
decrease in battery voltage produces an increase in charge current. This means that the greatest current increase occurs with a
combination of cool ambient temperature and low battery voltage. The unit delivers the greatest charge current increase when you
need it most, in cold weather with a discharged battery.
Because output power is constant while boost is operating, anything that leads to lower battery voltage will produce an increase
in output charge current. While a discharged battery is one way to produce lower output voltage, and therefore higher output current,
other normal conditions may produce lower voltage as well. Any 24/48 volt power consumption during the day will decrease net
battery charge current, which decreases battery voltage. Operating a large inverter or application of other heavy loads can produce
substantial drops in output voltage leading to significant increases in output current. Additionally, anything that can be done to lower
PV array temperature will also lead to increased charge current by increasing PV power production. Installing modules in a breezy
location for example will cool the PV array due to increased air circulation.
HOW MPPT WORKS
A PV module is a constant current type device. As shown on a typical PV module voltage vs. current curve, current remains
relatively constant over a wide range of voltage. A typical single 75 watt module is specified to deliver 4.45 amps @ 17 volts @ 25
°C.
Conventional PV controllers essentially connect the PV array directly to the battery when battery voltage is low. When a pair of 75
watt modules are connected directly to a battery charging at 24 volts, the modules still provide approximately the same current. But,
because output voltage is now at 24 volts rather than 34 volts (2x17V), they only delivers 106 watts to the battery. This wastes 44
watts of available power.
Solar Boost 3048’s patented MPPT technology operates in a very different fashion. Under these conditions Solar Boost 3048
calculates the maximum power voltage (V
MP
) at which the PV module delivers maximum power, in this case 17 volts. It then
operates the PV module pair at 2x17 volts which extracts maximum power from the modules. Solar Boost 3048 continually
recalculates the maximum power voltage as operating conditions change. Input power from the peak power tracking controller, in
this case 150 watts, feeds a switching type power converter which reduces the 34 volt input to battery voltage at the output. The full
150 watts which is now being delivered at 24 volts would produce a current of 6.25 amps. A charge current increase of 1.8 amps or
40% is achieved by converting the 44 watts that would have been wasted into useable charge current. Note that this example
assumes 100% efficiency to illustrate the principal of operation. In actual operation, boost will be somewhat less as some available
power is lost in wiring, connections, and in Solar Boost 3048.
TYPICAL CURRENT BOOST PERFORMANCE
As described above, current boost performance for a particular installation varies primarily with PV array temperature and
battery voltage. Two other factors which affect boost performance include system wiring and PV module design. The effect
wiring has on performance is power wasted heating undersized wiring is unavailable for charging. This is discussed further in the
Battery And PV Wiring section. The effect PV module design has on performance is modules with a maximum power voltage
(V
MP
) of 17 volts or higher will tend to produce more boost, whereas modules with V
MP
less than 17 volts will tend to produce less
boost. Panel types with a high V
MP
value in the range of 17.0
−18.5V provide the best boost performance since there is more
typically untapped power to be extracted. For a 24 volt system using eight 75 watt modules with peak power specifications of
4.45 amps @ 17 volts @ 25
°C, representative boost performance under a variety of operating conditions is shown in Table 2.
Your current boost performance will vary due to a variety of factors. What you can be sure of is that Solar Boost 3048 will deliver
the highest charge current possible for a given installation and operating conditions.