In this post, we are going to see how to select a solar charge controller for your Solar Power Systems.
We will discuss in detail about Solar Charge Controller and its functions in the off-grid / stand-alone solar PV systems.
You will find that Photovoltaic modules are highly reliable and virtually maintenance-free. But the modules alone do not solve customer power problems. Modules are usually connected to batteries to give power 24hrs a day.
Whenever batteries are included in a system, the additional facility must be built in, that will protect against overuse such as overcharge or over-discharge and that will send the appropriate information back to the user regarding the state of charge of the battery.
A Solar charge controller is a priceless protective instrument that protects your solar batteries from overcharging or over-discharging. This consequently will increase the lifetime of the batteries, save your investment and the trouble to replace those heavy and expensive batteries.
Selecting the right charge controller for your battery is an important step to build a highly efficient and quality off-grid solar energy system.
There are different types of solar charge controllers, that are designed to suit our electrical energy needs.
When buying a solar charge controller, most people get confused in the technical details and are not certain about the type of unit they should buy, hence they end up purchasing a wrong one.
Reading this detailed buying guide, you will be in a much better position to select the best solar charge controller suitable for your power system.
Now let’s discuss in detail the Solar Charge Controller and its role in a good Solar Power System.
What is a Solar Charge Controller?
The Charge controller is the energy manager in a stand-alone Solar PV system, which ensures that the battery is cycled under conditions which do not reduce its ability to deliver its rated capacity over its expected lifetime
Functions of Solar Charge Controller
Regulating Battery charging functions and control of the energy output produced by the Solar PV array is a critical function in PV Systems.
Listed below are the 7 most important charge controller functions:
- Preventing Battery Over Charge.
- Preventing over-discharge of Battery.
- Status Information to System Users/Operators
- Load Control.
- Interface and Control backup energy sources
- Divert PV Energy to an Auxiliary Load
- Serve as Wiring Centre
Now we will see in detail about each function of Solar Charge Controller
1. Preventing Battery Over Charge.
Charge regulation is the main function of a battery charge controller, and perhaps the single most important issue related to less battery performance and life.
When the solar array is operating under good weather conditions, energy output by the array often exceeds the electrical load demand.
Current from the array will flow into the battery proportional to the irradiance, even when the battery needs is full or when the battery does not require a charge.
If the battery is nearly full already, it will be overcharged. To prevent damage to the battery resulting from overcharge, a charge controller is used to protect the battery.
During overcharge, the voltage will rise, gassing will begin, the electrolyte will be lost, the internal heating will occur, and battery life will be reduced. If left uncontrolled, the battery could lose almost all its electrolyte and be permanently damaged, and the loads could fail.
The charge controller prevents excessive battery overcharge by controlling (limiting) the current flow from the array to the battery when the battery becomes fully charged.
Charge regulation is most often accomplished by limiting battery voltage to a maximum value, often called the Voltage Regulation(VR) Setpoint.
2. Prevention of Battery Over Discharge
If you leave the loads ON for too long, the battery can be over-discharged.
In the case of lead-acid batteries, the reaction of lead and lead dioxide will occur close to the lead grid material and weakens the bond between the active materials and the grid.
This can result in greater resistance and heat generation, loss of capacity and life will eventually occur.
Sometimes shallow cycling types of batteries are very difficult to recharge once they have severely discharged, especially with slow charge rates typical for remote photovoltaic systems( typically c/30or less).
Since Lithium-ion batteries, batteries do not have a charge memory like Ni-Cad batteries, deep-discharge cycles are not required.
When we discharge the battery below its recommended safe low voltage (exact number varies between manufacturers) some of the copper in the anode which is copper current collector can dissolve and mix with the electrolyte.
The copper ions (atoms) then, in turn, get attracted and stick on to the anode during charging and cause dendrites on the anode.
These dendrites can cause a short circuit inside the battery. So basically discharging below the recommended voltage level too much is as bad as charging too much.
If batteries are too deeply discharged, the voltage falls below the operating range of the loads and the loads will fail.
Over-Discharge protection in charge controllers is usually done by disconnecting the circuit connection which connects the battery and electrical load when the battery reaches a preset or adjustable Low Voltage Load Disconnect(LVD) Setpoint.
Most charge regulators also have an indicator light or alarm to alert the system user/ operator to the load disconnect condition.
Once the battery is recharged to a required level, the loads are reconnected to a battery.
3. Status Information to System Users/Operators
Nowadays charge controllers used in Photovoltaic systems can provide status information on the operation of the system and the condition of the battery.
These features of controllers can allow users to intelligently manage their use of energy and gain a better understanding of how the system operates to fully utilize its potential.
Battery voltage and State of charge is an essential piece of information that can be indicated by the charge controllers.
Digital readout of the exact battery voltage can also be provided for more sophisticated users. While random battery voltage can be provided, the user must know how to interpret this information.
For example, the user must know at what voltage levels they should begin to reduce their energy usage, to prevent battery over-discharge.
Similarly, other status information provided by the charge controller are :
- Whether the array is charging the battery or not.
- Whether the array/ load is disconnected or not.
- Load currents
- Net battery amp-hours
4. Load Control
Some Charge controllers have optional features that allow regulation or control of PV System electrical load.
The most popular feature of load control is the PV lighting system. This control can take place at Sunrise or Sunset as sensed by a photosensor or the array current or voltage output.
In other cases, the controller may have a timing function to cycle the load operation for a specified period or at a certain time of a day.
Sometimes load control functions may require adjustment and proper specification for the array type and site conditions such as temperature and background lighting.
Even when these features add to the extra cost and the complexity of the charge controller functions, they can greatly simplify the use and operation of the PV system.
5. Interface and Control backup energy sources
In the case of Hybrid PV Power systems using one or more backup energy sources in addition to the PV array, we need more advanced charge controllers to interfere between the different alternate power sources with PV systems.
One scenario would be a controller that activates a backup generator at a low battery state of charge. The charge controller would start the generator at a preset low battery voltage, and turn it off when the battery is recharged or reaches a higher voltage limit.
In some cases, stand-alone inverters used in Photovoltaic systems will start a backup generator or divert the loads to utility power when the battery reaches a low state of charge.
Once the battery has been recharged to a preset level, the loads are again connected to and powered by the battery bank.
6. Divert PV Energy to an Auxiliary Load
Batteries in Solar Photovoltaic systems are often fully recharged by the middle of the day during the summer.
The charge controller would normally disconnect the PV array to prevent battery overcharging, the array can continue to produce power for some other use.
So we can have a charge controller which includes a diversion function.
When the battery bank is full, the array power is connected to another auxiliary load. There is no voltage control because there is no battery holding the voltage steady.
Typical Auxiliary loads used in PV systems are:
- Backup battery
- DC Water Pump
- A resistive element in the water heater
- A simple motor that operated without voltage regulation.
When the battery voltage falls, and array power is needed to run the regular loads, the auxiliary load will be disconnected from the PV array.
7. Serve as Wiring Centre
In most cases, the charge controller serves as the termination and connection point between the conductors leading to the various components in a Photovoltaic system.
For example, the charge regulator in a small residential lighting system has the PV array, battery, and load, all connected to the charge controller terminals.
A Fuse or Circuit breaker for array and battery protection can also be included in the charge controller design.
Large PV systems generally have overcurrent protection and disconnect devices included as part of the charge controller.
With these devices included in the controller, the conductors leading to the PV array, battery and loads are connected at a centralized point in the power system.
The controller may also be the principle grounding point in the system and include surge suppression devices.
With this centralized configuration for the power system connection and controls, the installation, operation, and maintenance of the system are greatly simplified.
Types of Solar Charge Controllers
In the current trend, there are 2 main types of Charge Controllers based on the working principle.
- PWM – Pulse Width Modulation ‘
- MPPT- Maximum Power Point Tracking
Let’s see the basic difference between the two types of Solar Charge Controllers.
Comparison of MPPT & PWM Charge Controller
|MPPT Solar Controllers||PWM Solar Controllers|
|High charging efficiency (94%-98%)||They are nearly 30% - 40% less efficient than MPPT|
|With an MPPT controller, the current is drawn out of the panel at the panel “maximum power voltage”||With a PWM controller, the current is drawn out of the panel at just above the battery voltage.|
|Able to harvest solar energy below 45°C or above 75°C.||Work best between 45°C-75°C.|
|Provide more boost than PWM especially during cold days and/or when the battery voltage is low||Often chosen for very hot climates which will not yield as much MPPT boost|
|Generally operate below Vmp||Operate at Vmp|
|Multiple solar modules can be wired in series to boost the output voltage.||Multiple solar modules have to be wired in parallel to maintain 12V to the controller.|
|More expensive & suitable for large module configurations that have a lower cost per watt||Affordable price & Suitable for small module configurations|
How to select a Solar Charge Controller?
Below are the few important factors that you need to consider when selecting a suitable charge controller. Let’s look at each one right now:
1. Energy Needs:
If you need a solar energy system to power up your home or an office building or factory , i.e. for lighting, cooling and heating purposes then an MPPT charge controller would be a more ideal choice.
MPPT charge controllers can comfortably handle higher current and utilize it effectively.
They also come in handy when future plans involve scaling up the solar power system by acquiring more solar panels.
However, if you are using solar power for a smaller solar power system such as charging handheld devices, and lighting a single or few rooms, then a PWM charge controller is recommended.
If you are budget conscious and your electric energy needs are not really high, then going for a PWM would be more cost-effective since MPPT charge controllers tend to be more costly.
Usually, different manufacturers offer different prices and more features based on the money you choose to spend on a charge controller.
Temperature plays a major part in the efficiency of charge controllers. PWM performs best in warm climatic conditions since they are designed to operate at battery voltage. But, MPPT works best in cold and hotter climatic conditions and when the battery is low since they can operate above the battery voltage.
PWM charge controllers have a less complicated connections when compared to MPPT charge controllers. Due to their complex nature of MPPT charge controllers, you may find it hard to install when compared to PWM charge controllers.
If you find it difficult to install MPPT charge controllers, you will have to hire an expert which means extra costs added to your budget.
5. Product Warranty
You have to make sure that your charge controller is covered under warranty before obtaining it so that, in case if anything goes wrong, you have the option to take it back or have it freely repaired or get replaced based on the warranty.
1 year or more is considered ideal. Nowadays, most of the charge controllers come with a 1-year warranty.
During purchase, find out the terms and conditions governing your warranty to avoid violating them.
Remember, always your safety is the main priority. Some people consider solar energy as less dangerous when compared to mains electricity power supply, hence don’t consider it as life-threatening – but actually is more dangerous!
There have been reports of several accidents in the past resulting from solar power. To avoid this, if you are not sure about how to handle the technical details involving a charge controller, battery and solar panel, seek professional services.
I understand that it is an extremely daunting task to choose the technology product with which you are unfamiliar, especially when there are a wide range of technical details and brands that are available in the market.
But do remember, as long as your solar power system has rechargeable batteries, you definitely need a charge controller to protect it from being overcharged or over-discharged.
But now you are now provided with information that you need to choose the best solar charge controller for your solar power system.
Remember, your solar panel, battery, and energy needs plays a major role in determining which type of charge controller will work best for you.
Please go through the manufacturer’s manual clearly to get the technical details of your solar power system and use the above guide to choose the most ideal solar charge controller that meets your budget and energy needs.
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