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Solar Charge Controllers & Inverters
This fourth blog post is about Solar Electric Components. We are going look at some of the main components in a solar electric system:
- solar charge controllers (SCC)
Technology is moving forward at a very fast rate. Most of the generalities are true today, but there are likely to be exceptions.
If you missed the earlier blog posts, I recommend that you read them to get a good foundation:
Solar Charge Controllers (SCC)
A charge controller is an important component in a battery based system. There are two (2) main functions of a charge controller:
- Prevents battery overcharge;
- Prevents battery discharge at night through the solar modules.
Charge controllers charge the batteries by sending different voltages and currents to the battery bank based on how full the battery is. Much like pouring a glass of water. When the glass is empty you can have the faucet on full blast. But when it starts to get full you want to turn down the faucet to prevent overflowing. Likewise a charge controller sends a lot of power to the battery when it is low but as it approaches full, it slows it down. Once it is full, it will send a smaller amount of power, a trickle charge, to keep it topped off.
Stages of Charging:
- Bulk Charging
BULK CHARGING – When the battery is low it will accept all the current provided by the solar system and send it to the battery.
ABSORPTION – The battery has reached the regulation voltage. The controller begins to hold the voltage constant. This is to avoid overheating and over gassing the battery. The current will taper down to safe levels as the battery becomes more charged.
EQUALIZATION – Done only with flooded batteries not with sealed batteries. Many batteries benefit from a periodic high-voltage boost charge. This is to stir the electrolyte, level the cell voltages and complete the chemical reactions. Your battery specs will tell you how often and at what rate it wants to be equalized.
FLOAT CHARGING – This is when the battery is fully recharged. The charging voltage is reduced to prevent further heating or gassing of the battery.
There are three main types of solar charge controllers:
- Shunt (rarely used nowadays)
- PWM (Pulse Width Modulated)
- MPPT (Maximum Power Point Tracking)
Shunt chargers just turn the flow to the batteries on or off. They are rarely used anymore so we won’t discuss them.
The two main types you’ll find these days are PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking).
PWM are the less expensive option. A PWM charge controller pulses the power sent to the battery bank. This allows it to do the different charging stages we discussed. When using a PWM charge controller, the voltage of the solar panel must be the same nominal voltage as the battery bank. If you are using a 12-volt battery you must use a 12-volt solar panel. If you have a 48 volt battery bank you must wire four (4) 12 volt panels or two (2) 24 volt panels in series to make 48 volts. Make sure the charge control you select is designed for that battery bank voltage. Some can support multiple voltage ranges. Others are designed for only one voltage. If a PWM charge controller says it can support 12V or 24V, both the panel and battery bank must be one or the other. It is not saying it can take a 24-volt panel to charge a 12-volt battery. It is saying it can work in either a 12 volt or 24 volt system.
Summary of PWM
Less $$$ than MPPT
Same volts in as out
Uses variable (duration and spacing) pulse charging
A MPPT charge controller is the more sophisticated and more expensive type of solar charge controller. It tracks the output of the solar array and adjusts itself so the output is always maximized. In doing so, it can increase the production of the array by up to 30%. Another great advantage is that most MPPT charge controllers can take a higher voltage array. For example, a 60V array to charge a lower voltage battery bank like a 48V. This is required if you have a 60 cell 20V grid-tied solar panel and then want to use it to charge a 12-volt battery. It is also very useful if you have to go a distance from your array to your battery bank. The higher the voltage of the solar array, the lower the current going across the wire. You can use smaller gauge wire which will cost less and have a lower voltage drop which gets more power to the batteries. There are also a few MPPT charge controllers that can take a lower voltage panel and charge a higher voltage battery bank. But most MPTs require either higher or equal to voltage panel. Be sure to read the specs carefully.
Summary of MPPT
Tracks the optimum voltage/current ratio from array
Can provide up to 30% increase in performance over other controller types
Supports different voltage in than out
SCC Optional Features
There is a wide range of features that are optional on some but not all charge controllers.
In most cases, a display does not automatically come with a controller but can be added separately for remote display.
A few even have ethernet connections, allowing you to monitor your system across the web.
Temperature compensation will improve the battery bank charging by adjusting its output based on the temperature.
Low voltage disconnect (LVD) is a great feature that allows you to connect your DC load straight to the charge controller. If the battery voltage gets low, it will turn off the load preventing the batteries from becoming too low and getting damaged.
Some controllers can be used as a diversion or dump load controller turning power onto a heater to burn off excess power. There are others that have light control functions turning lights on and off automatically.
Solar charge controllers are rated by both voltage and amps. As we said, PWM charge controllers support the same voltage in as out. Check the specs to make sure what voltage that controller supports. MPPT specs will list the maximum Voc voltage it can support. This is higher than the nominal voltage. A typical 150V charge controller can support up to three (3) 38V panels in series. We must remember that cold weather increases the voltage output of a solar panel. If we say that the Voc of a panel is 38 volts, three (3) in series equals 114V. If we also figure in the cold temperature in winter, we increase the voltage. You can see why at least in cold climates three (3) 38V nominal panels would max out the 150V charge controller.
Voc x in series = Max Voltage
38V x 3 = 114V
Cold weather compensation (winter)
114 x 1.2 (temperature compensation) = 136Voc (close to max of 150V SCC)
There are now higher voltage charge controllers available with some accepting as much as 600V. This is very useful if the array is a long distance away from the battery bank.
Charge controllers are also rated by the current range they are able to support.
PWM charge controllers just pass the power through from the panels to the battery bank. There is no adjustment, current in equals current out. You would select the charge controller based on the Isc (short circuit current) of the solar array.
MPPT charge controllers are rated by their current output not their input. If you are inputting 60V and outputting 24V, the voltage is going to drop. This is because power (watts or W) equals volts times amps (W = V x A). To keep the power constant, anything done to the Volts, the opposite needs to be done on the Amps. When the volts drop on the output of the charge controllers, the amps will increase. To figure out what the output will be from the charge controller, take the total watts of the solar array and divide it by the voltage of the battery bank. For example, a 1000W solar panel array into a MPPT SCC that is connected to a 24V battery bank will output 41A. You need to find a MPPT SCC that can output at least 41A.
The primary job of an inverter is to convert the direct current (DC) power (red line in the picture above) from the battery bank to alternating current (AC) power needed for most appliances. In order to do that, we must take the constant DC voltage and change it to a sine wave curve that goes above and below zero volts. When inverters first came out, the most common way to do this was to make the voltage go straight up and down creating a blocky signal. This is called a modified sine-wave (blue line in the picture above). More advanced modified sine waves make multiple steps trying to get as close as possible to a pure sine wave. You can see the output of a modified sine-wave on an oscilloscope (bottom left of picture above). The bottom right of the picture is of a pure sine-wave.
Other than how the signal looks, let us see the difference between the two outputs.
Modified Sine Wave Inverter
A modified sine wave inverter can be used for simple systems that don’t have any delicate electronics. CRT TVs and motors with brushes are usually fine with modified sine-wave but your digital clock will likely act funky and battery rechargers quite often just plain won’t work. Some equipment may seem to be working fine but it may run hotter than with a pure sine wave and reduce the life of it. It is very difficult to say exactly what will and won’t work with modified sine wave inverters.
For simplest systems
Fine for older TVs, incandescent lights, motors with brushes
Generally not good with: electronics, audio, induction motors, rechargeable batteries, or digital clocks
Pure Sine Wave Inverters
Pure sine wave is always needed for grid-tied systems. It is generally needed for the newer LED TVs, CFL and LED light bulbs and inductive loads like brushless motors.
Mandatory for grid-tied
Preferred for off-grid homes or larger systems
Generally more expensive than modified sine wave inverters
Necessary for electronics, fluorescent lights and dimmers, inductive loads
Generally, modified sine wave inverters are less expensive than a pure sine wave inverter so they are still commonly used in simple systems. As technology advances the cost of pure sine wave inverters is coming down making them much more affordable and really the favorite option these days.
Inverters are used in three different types of solar power systems:
- Off grid
- Grid tied with battery backup
A grid-tied inverter only connects the solar panel directly to the electric companies grid through fuses and breakers. Through net metering, if you make more power than you use, your electric meter spins backwards. More often than not, your electric meter will spin slower as your house uses all the power you make. You buy less power from the grid. At night or during cloudy days, you buy your power from the grid same as usual. If the grid goes out, so will your house power even if the sun is shining. The only noticeable difference is that your electric bill will be lower than before you got solar. When selecting a grid-tied inverter, base it on the size of your solar array. A 5KW array will use about 5KW inverter. You need to match the electrical service you get from the grid.
Microinverters have become very popular for grid-tied systems. Instead of one or two large string inverters, microinverters install in the back of the solar panels, generally one per panel. They convert the DC power to AC at the panel. The advantage of this is that if you have partial shading on some of the panels, it doesn’t affect the output of the whole string of panels like it would with a string inverter. You can also have a deeper view into the system monitoring down to the panel level instead of on the whole inverter. Microinverters are very helpful if you have future expansion in mind. You add more panels and more inverters as needed. You don’t even need to match the panels as you would with a string inverter. The inverter is outside but shaded by the solar panel. It is more exposed to the weather than a string inverter that’s installed inside a building. Microinverter systems can also be a little bit more expensive.
Off-grid inverters are for stand-alone systems where the grid is not available. Once the charge controller has charged up your battery bank, the off-grid inverter converts the 12, 24 or 48V battery bank to AC voltage. The AC output depends on your requirements. Depending on how you wire the output of the inverter and which inverter you get, you could have a dual output. You need to determine what your loads need and select and configure the inverter. An off grid inverter cannot sell extra power back to the grid. An inverter charger can connect to the grid if available, to act as a battery charger. If you have an inverter charger, you can connect to grid power and use the grid to charge your battery bank when the solar doesn’t provide enough power. That AC connection is one directional, it will only take power from the grid but not send it back. Likewise you can often connect a generator to the AC input of an inverter charger to top off the batteries when needed. This is a common configuration for off-grid homes that need more power than the sun can provide. When selecting an inverter you must determine what your maximum wattage draw will be if all your appliances that may be on at the same time are on. If you have an 800W well pump, a 100W refrigerator, five (5) 10W light bulbs and a 50W laptop, you will need to add the wattages together to get at least a 1000W inverter. You also have to make sure the inverter is able to handle the surge as motors turn on. For example, if your refrigerator and well pump both turned on at once, the surge could be three times the rated wattage. You must make sure the inverter can handle the surge. Inverters are rated both by continuous wattage and the surge capability. Additionally you need to select the battery bank size and buy the inverter to match. Inverter voltage is not field selectable, they are either 12V, 24V or 48V.
A grid-tied battery backup inverter is the best of both worlds. Under normal circumstances it converts the power from the DC battery bank and provides AC power to the house selling any extra back to the grid. At night, your house gets its power from the grid same as usual. But when the grid goes out that is when this is the best. When installing a grid-tie battery backup system, you select which appliances in your house you want to back up with battery power and take these off your main breaker panel and connect them instead to a critical loads panel. When the grid is out, only the items wired to the critical loads panel get powered. So you can have your selected appliances and lights remain on while the rest of your house is off . This allows you to have a smaller battery bank and PV array than if you were to have an off-grid system because you’re only powering a small subset of your house when the grid is out. You reduce your electric bill but you can also keep important things running when the grid is out without having to run a generator. When selecting a grid-tied battery backup inverter you need to base it both on the battery bank voltage as well as the size of your array. You also have to base it on the wattage of your critical loads including the surge. There is a bit of planning when selecting the right inverter.
Battery based inverters have a lot of options to choose from. Not all inverters have the same features. You need to decide the required features and select the inverter based on which has them. Some of the features are the ability to charge a battery bank from an AC source like the grid or generator. Some can start the generator when the batteries are low and turn it off when charged. Some can use the generator to assist with high loads. The inverter is often installed in an out-of-the-way location near the batteries. A remote controller or display in the living area is useful to keep an eye on the system. Some inverters even have the ability to monitor the system via the web. This is very useful for part-time locations that you are not always there to keep an eye on it. Many inverters are stackable to increase either the voltage or the current or both. This allows you to use multiple inverters in a master-slave configuration. Inverters turn on as needed conserving battery power.