EXPLAINING MAXIMUM POWERPOINT TRACKERS (MPPTs)
Low Voltage MPPTs are used to extract maximum power from solar panels to provide optimum charging of boat and RV batteries.
Why don't Solar panels deliver maximum charge to batteries?
So called 12 Volt solar panels have an output Voltage of around 20 Volts. Nominal 12 Volt batteries require charging Voltages that vary according to their state of charge. For example a half discharged AGM battery will require around 14.4 Volts to commence the charging cycle varying through to around 13.2 Volts as the battery approaches full charge. Conventional solar regulators manage this change in the required charging Voltage satisfactorily but do so in a manner that reduces available power from the solar panel(s). They place a load on the panels that forces the panel to operate away from its best operating condition and this can result in a loss of available solar power of as much as 30% or more, depending on conditions. The solar panels optimum operating condition is known as the Maximum Power Point. MPPTs overcome this difficulty by ensuring that the solar panel always operates at its Maximum Power Point.
How does an MPPT work?
The MPPT contains a small computer that controls an inverter. The MPPT monitors the battery Voltage and determines the appropriate charging Voltage required for the battery's particular state of charge and instructs the MPPT's inverter to provide this Voltage. The inverter is powered by the solar panel(s) and converts the available solar panel Voltage to the required battery charging Voltage. The MPPT keeps the solar panel operating at its Maximum Power Point thus overcoming the loss of available power caused by conventional solar regulators.
The preceding explanation of the MPPTs operation only provides an outline of the process. In reality the detailed operation is far more complex. The operation of an MPPT is a super juggling act. The available solar power can change dramatically and rapidly with cloud, shading, panel angles, time of day and temperature. The required battery charging Voltage also changes as the battery's state of charge increases or reduces. The Maximum Power Point therefore changes dynamically with operating conditions. Despite this, the MPPT manages these variables effectively and keeps the panel(s) operation at the Maximum Power Point thus ensuring that maximum solar power is always available as required.
MPPTs do require a small amount of power for their operation but this is almost negligible over a solar day. Improvements in MPPT design have resulted in lower internal losses and more rapid TRACKING of system changes. You will note that TRACKING does not refer to panels following the Sun.
MPPT FEATURES
A basic MPPT has a solar panel input and a battery connection. The battery should always be connected first because the MPPT may use the battery to determine a range of information including system Voltage (12V or 24V).
Most MPPTs have LOAD terminals. These are normally provided so that loads can be automatically switched at various times or in various circumstances such as current overload or low Voltage disconnect. The use of the LOAD terminals is therefore optional in many installations and loads can be connected directly to the battery. This may be especially important when high current loads such as pumps or inverters would cause LOAD terminals to reach overload conditions and turn off. One useful function of LOAD terminals is to switch night lights such as anchor lights on boats.
Another useful feature is metering. Many MPPTs have a range of metering functions including battery Voltage, charge current and load current (obviously not much use if load terminals are not used). Being able to monitor solar panel current and compare it to charge current allows users to actually see the improvement that MPPTs provide but it is quite counter intuitive to see that charge current is larger than panel current. Some MPPTs even have a data logging capability that permits users to look back at the past performance details.
When the installation is running you may notice that charge current briefly falls to zero every so often. This is quite normal because the MPPT needs to sample battery Voltage without the charging Voltage being applied.
MPPTs in USE
The following formula is used in this text and is included to assist readers who do not regularly it:
WATTS / VOLTS = AMPS
A variety of figures are used to describe the improvement in solar power recovery that is possible with an MPPT with manufacturers claiming between 10% and 40%. MPPTs produce the best results with significantly discharged batteries, low Sun conditions and cool temperatures. Some measurements I have taken indicate that 20% to 30% improvement is commonly seen but I have on occasions seen results both worse and better than these figures. I have no doubt that MPPTs do offer worthwhile improvements.
To take an example, consider a 100 Watt panel installation. At 14.4 Volts the panel should provide 100/14.4 = 6.94 Amps of charge current to the battery but the panel cannot operate at its Maximum Power Point without an MPPT. The panel may be capable of providing 18 Volts but because it is not operating at its Maximum Power Point it is operating at 18- 14.4 = 3.6 Volts less than optimum and this represents 3.6/18 = 0.2 or 20% reduction in panel power. This means that the 100 Watt panel is actually only producing 80 Watts in this example and therefore the charge current is now 80/14.4 = 5.56 Amps. Adding an MPPT would restore charge current to 6.94 Amps. Other cases can produce even greater improvements.
Consider also the strange situation where in an MPPT installation, the charge current can be greater than panel current. Our 100 Watt panel now operates at 18 Volts and the MPPT inverter converts this to the 14.4 Volts required to charge the battery. Since the MPPT allows the panel to operate at its Maximum Power Point the panel current will be 100/18 = 5.56 Amps whereas the charge current will now be 100/14.4 = 6.94 Amps! These figures do not take into account that there will be a small reduction in the charge current due to the small amount of operating current required by the MPPT (probably less than 3%), nevertheless the principle holds true.
Many solar panel installations on boats or RVs are around 400 Watts so that without an MPPT and an effective loss of around 20%, the installation is only capable of 320 Watts. If the loss is 30% this decreases to 280 Watts. In other words, in this installation adding an MPPT is like adding an extra 80 or 120 Watt panel.
BEFORE BUYING