MPPT Solar Charge Controllers

The PWM Company do not agree with the common used definition of MPPT controller.

Definition of MPPT controller for solar & wind electricity from the PWM Company

The PWM Company say: The Maximum Power Point is the maximum available & allowed voltage for accumulators batteries and for inverter as well. The maximum voltage will give the maximum power and current as well for the system output.

Many sources in Internet say that the Maximum Power Point is the specially calculated value. The PWM Company do not agree with this. Only maximum rated battery (or inverter) voltage is the definition of it.

Besides performing the function of a basic charge controller, an MPPT charge controller also includes a DC to DC voltage converter, converting the voltage of the array to that required by the battery bank, with very little loss of power.

The best MPPT charge controller will adjust its input voltage to harvest the maximum power from the solar array and then transform this power to supply the varying voltage requirement of the battery plus load. Thus, it essentially decouples the array and battery voltages, so that there can be a 12-volt battery on one side of the MPPT charge controller and four 12V panels wired in series to produce 48 volts on the other.

If connected to a solar panels array with a substantially higher nominal voltage than the battery voltage, an MPPT charge controller will therefore provide charge current even at very high cell temperatures or in low irradiance conditions when a PWM charge controller would not help much.

As array size increases, cable length will increase. The option to wire more solar panels in series and thereby decrease current is a compelling reason to install an MPPT controller as soon as the array power exceeds a few hundred Watts (12V battery), or several 100 Watts (24V battery), several 1000 Watts (48V battery).

Some of the best MPPT charge controllers, such as the Midnite Solar Classic 150 Charge Controller, are designed to work when the PV array is up to 150VDC. The Midnite Solar Classic 150 has a maximum output current of 96 amps, an operating voltage of 150 volts and works with 12 to 72 volts battery systems.

Types of Battery Chargers.

There are several types of electronic circuitry used within battery chargers for the marine market

FERRO-RESONANT (or CVT) These use a low-frequency MAGNETIC control system, which makes them very HEAVY, very BULKY and are also only available with a poor FLOAT charge characteristic, therefore very SLOW recharging. They can also generate a large magnetic field which can upset other equipment on board. On the plus side, they are CHEAP and RELIABLE due to the low number of components used and they tend to appeal to boat-builders who put price at the top of their list of priorities.

LINEAR CHARGERS These also use a low-frequency transformer to reduce the input voltage to a lower level, but they then use transistors to control the current and voltage fed to the battery. This technique can be used for either FLOAT or 3-STAGE chargers but is very IN-EFFICIENT and therefore HOT, HEAVY and BULKY. The biggest drawback is a LIMITED INPUT VOLTAGE range – not ideal for running from a generator or some marina supplies.

MPPT SWITCHED MODE CHARGERS. These are more complicated than the previous two types and use the techniques perfected for and now universally used in computers and televisions. The AC input is first turned into high-voltage DC. It is then turned into high-frequency AC using special types of transistor and a high-frequency transformer (one thirtieth the weight of a low-frequency transformer!) reduces the voltage to the exact level needed to charge the battery. A sophisticated control circuit produces an overall design with HIGH-EFFICIENCY, SMALL SIZE and LIGHT WEIGHT. The extra complexity adds to the initial cost but results in lower running costs and the ability to run from a SMALLER (and cheaper) GENERATOR if required. Switched mode chargers can be either FLOAT or 3-STAGE types.

There are many types of battery but batteries on boats are nearly always LEAD-ACID types – similar to car batteries but heavier duty. A BATTERY is made up of a number of CELLS. A LEAD-ACID CELL generates around 2 volts. Small batteries contain 6 cells in a container which add together to give 12 volts at the terminals. Larger cells are quite heavy and individual CELLS are connected together ‘in series’ to make batteries of either 12 volt (6 cells) or 24 volt (12 cells). Although a battery is called a ’12 volt’ battery, its voltage varies from about 12.6 volts down to 10 volts when it is discharging and can rise to 15 or 16 volts during charging. It is very important, however, to limit the maximum battery voltage during charging otherwise the battery will be damaged. The battery voltage should not exceed 13.8 volts for long periods and 14.4 volts for short periods (8 hours maximum).

Stages of Battery Charging

There are two basic types of lead-acid battery charger: FLOAT chargers and 3-STAGE CHARGERS. Float chargers also come in two versions – good ones and bad ones! A good FLOAT charger charges the battery at a constant current until the ‘FLOAT’ voltage (13.8V or 27.6V) is reached, then it progressively reduces the current to maintain that voltage. This gets about 75% capacity back into the battery quickly but then takes a long time to restore the other 25%. A bad FLOAT charger commences charging at the rated current but as the battery takes the charge, its voltage rises and the current drops off long before the float voltage is reached. Thus, even 75% capacity takes a long time to restore and full charge takes forever. By far the best type of charger is the 3-STAGE CHARGER. This starts charging like a good float charger but continues charging at constant current until the ‘BOOST VOLTAGE’ (14.4V or 28.8V) is reached. Then, the current is progressively reduced until it drops to one quarter of its maximum. This corresponds to 90% capacity in the battery. The charger voltage now changes automatically to the float voltage where it then remains, slowly restoring the last 10% capacity. Thus almost full charge is restored quickly and safely.

How MPPT charge controller works

MPPT charge controllers employ DC to DC conversion, which allows the solar array voltage to be higher that the voltage required to charge the batteries. Many years ago solar modules were designed to charge 12 volt batteries. These solar modules were built with 36 cells in series to have a peak power voltage of 16- 18 volts. Two of these modules could be wired in series to charge a 24 volt battery and four could be wired in series for a 48 volt battery. Modules with 72 cells were also available for 24 volt charging. When all modules were made this way, the simple PWM charge controllers described previously worked fine. As connected off grid solar power systems became the prominent use for solar modules, cost became the most important factor in module design and off the grid homes. This lead to larger 60 cell modules, manufactured in a convenient size that could be carried by one person. These modules have a voltage too high for 12 volt battery charging and too low for 24 volt battery charging and are usually wired in series to get approximately 400 volts DC which is easy to convert to 240 volts AC for connection to the utility grid.

36 cell modules are available with power outputs up to 140 watts and 72 cell modules are available with power outputs up to 300 watts. These modules cost more per watt, but can be used with lower cost PWM charge controllers. It makes economic sense to use these modules on systems that required less than 700 watts. If the power required is greater than 700 watts, the cost savings on the solar modules and the wire between the modules and the batteries to pay the added cost of a MPPT charge controller.

MPPT charge controllers from Schneider (formerly Xantrex), Outback, Morningstar and Midnite Solar can operate with a maximum input voltage from a solar array of 150 volts, which can be used with three 60 cell modules in series. Other models from Midnite Solar can be used with arrays up to 250 volts allowing the use of six 60 cell modules in series. An added advantage of using a higher voltage solar array made from modules wired in series is that much smaller wire can be used between the solar array and the charge controller without significant power loss. Every time you double the voltage in a wire, you can carry 4 times as much wattage with the same loss. This saving increases tremendously when you go from 15 volts in a wire to 150 volts.

When choosing an MPPT charge controller, the amp rating of the controller is the maximum amperage that it can supply to the battery being charged. To find this amperage, add the wattage of all modules in the array and divide by the battery voltage. For example, six 240 watt modules have a total wattage of 1440 watts. If you are charging a 12 volt battery, the charge controller amps required is 120 amps (1440 / 12 = 120). In this case you would need controllers with 60 amp or greater rating. If you were charging a 24 volt battery, only one 60 amp charge controller would be required. If you are using a 48 volt battery you could have an array of 2800 watts and still use a single 60 amp charge controller.

Solar cells are neat things. Unfortunately, they are not very smart. Neither are batteries – in fact, batteries are downright stupid. Most PV panels are built to put out a nominal 12 volts. The catch is “nominal”. In actual fact, almost all “12-volt” solar panels are designed to put out from 16 to 18 volts. The problem is that a nominal 12-volt battery is pretty close to an actual 12 volts – 10.5 to 12.7 volts, depending on state of charge. Under charge, most batteries want from around 13.2 to 14.4 volts to fully charge – quite a bit different than what most panels are designed to put out.

Maximum Power Point Tracking is electronic tracking – usually digital. The charge controller looks at the output of the panels and compares it to the battery voltage. It then figures out what is the best power that the panel can put out to charge the battery. It takes this and converts it to best voltage to get maximum AMPS into the battery. (Remember, it is Amps into the battery that counts). Most modern MPPT’s are around 93-97% efficient in the conversion. You typically get a 20 to 45% power gain in winter and 10-15% in summer. Actual gain can vary widely depending weather, temperature, battery state of charge, and other factors.

Grid tie systems are becoming more popular as the price of solar drops and electric rates go up. There are several brands of grid-tie only (that is, no battery) inverters available. All of these have built in MPPT. Efficiency is around 94% to 97% for the MPPT conversion on those.

The Power Point Tracker is a high-frequency DC to DC converter. They take the DC input from the solar panels, change it to high-frequency AC, and convert it back down to a different DC voltage and current to exactly match the panels to the batteries. MPPT’s operate at very high audio frequencies, usually in the 20-80 kHz range. The advantage of high-frequency circuits is that they can be designed with very high-efficiency transformers and small components. The design of high-frequency circuits can be very tricky because of the problems with portions of the circuit “broadcasting” just like a radio transmitter causing radio and TV interference. Noise isolation and suppression becomes very important.

There are a few non-digital (that is, linear) MPPT’s charge controls around. These are much easier and cheaper to build and design than the digital ones. They do improve efficiency somewhat, but overall the efficiency can vary a lot – and we have seen a few lose their “tracking point” and actually get worse. That can happen occasionally if a cloud passed over the panel – the linear circuit searches for the next best point but then gets too far out on the deep end to find it again when the sun comes out. Thankfully, not many of these around anymore.

The power point tracker (and all DC to DC converters) operates by taking the DC input current, changing it to AC, running through a transformer (usually a toroid, a doughnut looking transformer), and then rectifying it back to DC, followed by the output regulator. In most DC to DC converters, this is strictly an electronic process – no real smarts are involved except for some regulation of the output voltage. Charge controllers for solar panels need a lot more smarts as light and temperature conditions vary continuously all day long, and battery voltage changes.

Using battery voltage for delivering maximum power

Current and Voltage are inversely proportional to each other. With other words, if the current increases, the voltage drops and vice versa.

By lowering the current by introducing some resistance in the path of the current, the MPPT charge controller can boost up the voltage.

This voltage to current ratio adjustment is called Maximum power point tracking. MPPT typically increases the current to the battery by approximately 25% to 30%.

Important to keep in mind is that a 80% discharged battery will charge faster than a 50% discharged battery.

The reason for this is that when the battery starts to discharge, its voltage also reduces. The larger the gap between the solar panel output voltage and the battery voltage, the more current will flow into the battery, and the faster the battery will be charged.

Combined techniques for optimum battery charging

MPPT charge controllers use both principles mentioned above to deliver the maximum amount of power.

This type of solar charge controllers come pre-programmed with adjustable set-points which can be edited and adjusted according to your needs.

If you need to choose between a standard and a MPPT charge controller, usually paying a bit more for a proper MPPT controller is the way to go.

MPPT vs simple PWM (Pulse Width Modulation) Solar Controllers

The most basic charge controller simply monitors the battery voltage and opens the circuit, stopping the charging, when the battery voltage rises to a certain level. Older charge controllers used a mechanical relay to open or close the circuit, stopping or starting power going to the batteries.

More modern charge controllers use Pulse Width Modulation (PWM) to slowly lower the amount of power applied to the batteries as the batteries get closer and closer to fully charged. This type of controller allows the batteries to be more fully charged with less stress on the battery, extending battery life. It can also keep batteries in a fully charged state (called “float”) indefinitely. PWM is more complex, but does not have any mechanical connections to break.

The most recent and best type of solar charge controller is called Maximum Power Point Tracking or MPPT. MPPT controllers are basically able to convert excess voltage into amperage. This has advantages in a couple of different areas.

Most solar power systems use 12 volt batteries, like you find in cars. (Some use other voltages and the same advantages apply to these systems as well.) Solar panels can deliver far more voltage than is required to charge the batteries. By, in essence, converting the excess voltage into amps, the charge voltage can be kept at an optimal level while the time required to fully charge the batteries is reduced. This allows the solar power system to operate optimally at all times.

Another area that is enhanced by an MPPT charge controller is power loss. Lower voltage in the wires running from the solar panels to the charge controller results in higher energy loss in the wires than higher voltage. With a PWM charge controller used with 12v batteries, the voltage from the solar panel to the charge controller typically has to be 18v. Using an MPPT controller allows much higher voltages in the cables from the panels to the solar charge controller. The MPPT controller then converts the excess voltage into additional amps. By running higher voltage in the cables from the solar panels to the charge controller, power loss in the cable is reduced significantly.

When using high voltage “Grid Connect” panels with VOC voltages above 35v to charge a 12v battery bank, the only controller option is an MPPT charge controller.

The final function of modern solar charge controllers is preventing reverse-current flow. At night, when solar panels are not generating electricity, electricity can actually flow backwards from the batteries through the solar panels, draining the batteries. You’ve worked hard all day using solar power to charge the batteries; you don’t want to waste all that power! The charge controller can detect when no energy is coming from the solar panels and open the circuit, disconnecting the solar panels from the batteries and stopping reverse current flow.

When assessing which type of solar charge controller to purchase, you need to know about their functionality and features but it’s also helpful to see a straightforward comparison of your options. To that end, we’ve put together a comprehensive look at the pros and cons of both PWM Type Solar Controllers and MPPT Solar Charge Controllers for your convenience!

Sizing Your MPPT Charge Controller

Sizing solar charge controllers is pretty simple really. Solar charge controllers are rated by current and voltage. These ratings depend on your solar array’s current (amps) and the battery voltage. Therefore, solar charge controller sizing basically involves “getting a charge controller big enough to handle the amount of power and current produced by your solar energy system”.

To get the right charge controller size, first, limit your choices to controllers that work with your battery bank voltage, which will usually be 12V, 24V, or 48V. Then, calculate the approximate maximum amperage your controller will need to handle. Divide the PV array watts by the system voltage to get amperage, then add a 25% safety margin to account for higher irradiance conditions. However, sizing MPPT charge controllers isn’t that simple.

All MPPT charge controllers have an upper voltage limit. This refers to the maximum amount of voltage they can handle from the solar array. Meaning that if you exceed this voltage, the charge controller can be permanently damaged. However, the fact is the solar PV arrays wiring in series produce higher voltages in cold weather, so an array of four 12V modules in series for 48V might be just fine during the summer for an MPPT charge controller with a 48 V maximum rating—but could damage the controller when it is cold.

Therefore, the best MPPT charge controllers for you should have upper voltage limit that is higher than the voltage your PV array produces.

Below we listed some of the best MPPT solar charge controllers on the market that we think are best for you