|INCREASE SOLAR CHARGING WITH A POWER
TRACKING CHARGE CONTROLLER
by Windy Dankoff
A new feature is showing up in charge controllers. It's called maximum
power point tracking (MPPT). It extracts additional power from your PV
array, under certain conditions. This article explains the process by
a mechanical analogy, for people who do not understand basic electricity.
The function of a MPPT is analogous to the transmission in a car. When
the transmission is in the wrong gear, the wheels do not receive maximum
power. That's because the engine is running either slower or faster than
its ideal speed range. The purpose of the transmission is to couple the
engine to the wheels, in a way that lets the engine run in a favorable
speed range in spite of varying accelleration and terrain.
Let's compare a PV module to a car engine. Its voltage is analogous to
engine speed. Its ideal voltage is that at which it can put out maximum
power. This is called its maximum power point. (It's also called peak
power voltage, abbreviated Vpp). Vpp varies with sunlight intensity and
with solar cell temperature. The voltage of the battery is analogous to
the speed of the car's wheels. It varies with battery state of charge,
and with the loads on the system (any appliances and lights that may be
on). For a 12V system, it varies from about 11 to 14.5V.
In order to charge a battery (increase its voltage), the PV module must
apply a voltage that is higher than that of the battery. If the PV module's
Vpp is just slightly below the battery voltage, then the current drops
nearly to zero (like an engine turning slower than the wheels). So, to
play it safe, typical PV modules are made with a Vpp of around 17V when
measured at a cell temperature of 25°C. They do that because it will
drop to around 15V on a very hot day. However, on a very cold day, it
can rise to 18V!
What happens when the Vpp is much higher than the voltage of the battery?
The module voltage is dragged down to a lower-than-ideal voltage. Traditional
charge controllers transfer the PV current directly to the battery, giving
you NO benefit from this added potential.
Now, let's make one more analogy. The car's transmission varies the ratio
between speed and torque. At low gear, the speed of the wheels is reduced
and the torque is increased, right? Likewise, the MPPT varies the ratio
between the voltage and current delivered to the battery, in order to
deliver maximum power. If there is excess voltage available from the PV,
then it converts that to additional current to the battery. Furthermore,
it is like an automatic transmission. As the Vpp of the PV array varies
with temperature and other conditions, it "tracks" this variance
and adjusts the ratio accordingly. Thus it is called a Maximum Power Point
What advantage does MPPT give in the real world? That depends on your
array, your climate, and your seasonal load pattern. It gives you an effective
current boost only when the Vpp is more than about 1V higher than the
battery voltage. In hot weather, this may not be the case unless the batteries
are low in charge. In cold weather however, the Vpp can rise to 18V. If
your energy use is greatest in the winter (typical in most homes) and
you have cold winter weather, then you can gain a substantial boost in
energy when you need it the most!
Here is an example of MPPT action on a cold winter day:
Outside temperature: 20°F (-7°C) Wind is blowing a bit, so the
PV cell temperature rises to only around 32°F (0°C). Vpp = 18V
Batteries are a bit low, and loads are on, so battery voltage = 12.0
Ratio of Vpp to battery voltage is 18:12 = 1.5:1
Under these conditions, a theoretically perfect MPPT (with no voltage
drop in the array circuit) would deliver a 50% increase in charge current!
In reality, there are losses in the conversion just as there is friction
in a car's transmission. Reports from the field indicate that increases
of 20 to 30% are typically observed.
NOTE: MPPT has also been incorporated into some solar water pump controllers.
to Catalog for Current Listings.
Boost MPPT Charge Controllers
Manufacturer: RV Power Products
Article by Rick Cullen and Windy Dankoff
How MPPT Works
The maximum amount of power a PV array can produce varies with solar
intensity, solar cell temperature, and module design. With these factors
constant, the actual power that the module delivers varies with the voltage
at which it is allowed to operate. A PV module is a constant current type
device. As shown on a typical curve of PV module voltage vs. current,
current remains relatively constant over a wide range of voltage. A typical
75 watt module is specified to deliver 4.45 amps at 17 volts with a cell
temperature of 25°C.
A traditional PV controller connects the PV array directly to the battery
when the battery is not fully charged. (The only time it actually processes
the power is when it must be reduced to prevent overcharge.) When this
75 watt module is connected directly to a battery charging at 12 volts,
it still provides about the same current. But, because PV voltage is now
reduced to 12 volts by the battery rather than 17 volts, it can only deliver
53 watts to the battery. This wastes 22 watts of available power.
The patent pending MPPT technology used in the Solar Boost line of charge
controllers operates in a very different fashion. Under these conditions
Solar Boost calculates the maximum power voltage (VMP) at which the PV
module delivers maximum power, in this case 17 volts. It then operates
the PV module at 17 volts, which extracts the maximum available power.
Solar Boost continually recalculates the maximum power voltage as operating
conditions change. Input power, in this case 75 watts, feeds a power converter
which reduces the 17 volt input down to the battery voltage at the output,
and correspondingly boosts the current. The full 75 watts which is being
delivered at 12 volts would produce a charge current of 6.25 amps. A charge
current increase of 1.8 amps or 40% is achieved by converting the 22 watts
that would have been wasted into useable current. In reality, current
increase will be somewhat less as a bit of power is lost in the conversion
How Much Charge Current Increase Will You See?
For a particular installation, the actual charge current increase will
vary with PV temperature and battery voltage. Lower PV temperature increases
VMP and thus the potential current boost. Lower battery voltage also increases
the boost. In cool but comfortable temperatures with typical 75W modules,
current increase normally varies between 10 to 25%, with up to 30% or
more achieved in cold temperatures with a discharged battery. These controllers
work quite well with higher voltage BP Solar 85W modules since a large
portion of their power increase is due to higher VMP which traditional
controllers can't make use of. In hot weather, when PV voltage is lower
and current boosting may not be possible, Solar Boost will pass current
through with a very low voltage drop.
Solar Boost also features 3-stage charge control (bulk, absorption and
finish). This type of charging assures the most rapid charging but it
reduces water loss and battery damage during long periods of excess energy.
It has a manual equalize setting for periodic battery maintenance. It
is adjustable to the amp-hour size of the battery bank, to optimize both
energy transfer and battery life. You can also connect it to an external
shunt so that its decision to drop to finish charge is based on the net
current flow measured right at the battery. A temperature compensation
probe is additional.
MORE TO KNOW ABOUT SOLAR BOOST CHARGE CONTROLLERS
Array Capacity vs. Controller Rating
Solar Boost controllers are rated for their maximum OUTPUT capacity in
amps. In general, the PV array capacity can be as much as 80% of that,
but no more. This is because the controller will boost the current! Since
the boost effect tends to be greatest when sunlight is weak (cell temperature
low), that much margin is normally sufficient. In very cold climates,
75% array size should be the maximum, since large increases in charge
current can occur during peak charging conditions. If conditions are sufficient
to cause output current to exceed the rating of the controller, current
limiting will prevent controller overload without shutting it down.
Double the Array Voltage to Reduce Wiring Cost
The 48V Solar Boost can use a 48V PV array to charge a 24V system. Similarly,
the 24V model can use a 24V array to charge a 12V system. This way the
long wiring from the array to the control center can be reduced not to
half, but to ONE QUARTER! This is a major cost-saver if an array is far
from the batteries, or if an array is to be enlarged but the installed
wiring is not sufficiently large to carry the increased current. To determine
the array capacity of the controller, observe "Array Capacity vs.
Controller Rating" above, then cut the result in half. The output
current will be doubled AND boosted!
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