& Control
Max Power Point Tracking
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 Tracker.

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.

Link to Catalog for Current Listings.

Solar 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 process.

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.

Other Features

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.


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!

Link to Catalog for Current Listings.