Photovoltaic (PV) systems are usually installed with battery backup systems, and they require a device to control how batteries are charged and discharged, regulating the current and voltage. The best device for this task is the solar charge controller.
If you are looking to learn more about solar charge controllers, the different types available, and learn how to choose them, this article is for you. Here you will learn everything you need to know about them, including their best features and a large list of tips to help you choose the best solar charge controller.
What is a solar charge controller? Why do you need it?
The solar charge controller is a device that works as a protection system for batteries and loads in solar PV systems. Without this device, due to the instability of the solar panel’s output, the voltage could exceed permissible values for the loads or the battery, potentially causing damage to any of these. Providing this protection is the most important purpose of charge controllers, however, there are other reasons why it is necessary, as can be seen below.
Some modern solar charge controllers include a battery-voltage temperature compensation system. Since the ideal voltage for a battery varies when the temperature increases from 25ºC, the solar charge controller can measure the temperature difference and compensate accordingly. Depending on the device, the charge controller might compensate -3mV/°C/cell, -4mV/°C/cell, or -5mV/°C/cell.
Solar charge controllers allow you to monitor battery specs. With this information, you can easily find out the state of charge of your batteries and even detect if there is an anomaly.
Protection From Reverse Currents
PV systems with batteries lacking a solar charge controller would regularly have reverse currents, especially overnight. This occurs when the voltage from the batteries is larger than the solar panels, which mainly happens during the night when the modules are not generating electricity. In other words, having reverse currents on a PV system means that the batteries would discharge to “charge” the panels, which would not make any sense.
By installing this device, you will protect your system from this phenomenon, extending the lifespan of your panels and keeping an optimum charge on your batteries. Some panels are designed to withstand small reverse currents, but under regular circumstances, could be dangerous damage for them.
Battery Set Points
Solar charge controllers use a multi-stage charging system designed to charge batteries with the right voltage and current for each stage. Depending on the battery electrolyte, the charge controller might use different charging stages:
- Lead-Acid Batteries: (1) Bulk, (2) Absorption, (3) Float, and (4) Equalization (only for flooded batteries)
- Li-Ion Batteries: (1) Trickle Charge, (2) Constant Current, (3) Constant Voltage, and (4) Charge termination
- NiMH Batteries: (1) Trickle Charge, (2) Constant Current, (3) Top-off, and (4) Charge termination
Aside from properly charging batteries, a charge controller protects the batteries by including a series of protections systems. Most charge controllers will protect batteries from over-load, over-charging, over discharges, and they will cut them off due to low voltage.
MPPT vs. PWM solar charge controllers: What’s the difference?
With many different solar charge controllers on the market, it is difficult to know which the best option is, but in truth, every model belongs to one of two types: MPPT or PWM. Here, we explain how each of these technologies works.
How do PWM solar charge controllers work?
A Pulse Width Modulation (PWM) works as a switch connecting solar panels to batteries, and not as a DC to DC converter, which is why this charge controller does not fully take advantage of the I-V curve of the panels. This charge controller modulates a pulse coming from the panels to the battery, limiting the voltage according to the charging stage of the battery, instead of converting it.
Since the PWM charge controller only limits the maximum voltage, this translates from power losses. The reason for power losses is that the voltage set point for the battery may not be the most optimum point in the I-V or P-V curve of the solar panel. In other words, setting the voltage to 12V without adjusting the current to match the maximum power point of the curve, will result in power losses.
How do MPPT solar charge controllers work?
The Maximum Power Point Tracking (MPPT) solar charge controller maximizes the power extraction from the solar panels by following an algorithm that allows it to track the maximum power point of the I-V curve (point generally marked as Pm in the I-V curve). To match this Pm value (which varies across the day) at the voltage of the battery, the electrical current is increased. In other words, this type of charge controller will always be delivering the maximum possible amount of power from the panels.
PWM vs. MPPT solar charge controllers
PWM and MPPT charge controllers are always being compared. While PWM controllers are cheaper, MPPT is much more advanced, making them better in the long run. Here we compare some of the pros and cons of each one:
The best advantage of MPPT solar charge controllers against PWM is the efficiency. While the PWM solar charge controller reduces the voltage of the I-V curve, causing power losses of up to 25%, MPPT uses advanced microcontrollers to track the maximum power point on the I-V curve. This can be done by making a DC to DC conversion that matches this power value to the corresponding voltage and current values for the batteries, barely causing 5-8% power losses to the system.
This difference in efficiency is highly associated with the capability of MPPT solar charge controllers of tracking the maximum power point of operation, functionality that PWM models do not have.
PWM solar charge controllers are cheaper because they are a much simpler technology; however, they leave behind higher power losses. PWM solar charge controller is better used for small and portable PV systems, where losses are not as considerable as in large systems and cost reductions are important. Meanwhile, for large or home-size types of systems, an MPPT is definitely required despite the increased price.
Voltage of the Panels
In PWM solar charge controllers, the voltage of the array has to be close enough to the voltage of the batteries in order to avoid substantial power losses. This is not the case for MPPT controllers, where the voltage of the solar array can exceed that of the batteries by far.
Most MPPT solar charge controllers include built-in temperature compensation systems, regulating the output voltage of the device to the ideal voltage for battery charging. Only modern PWM charge controllers include this option, but they usually require an external battery temperature sensor.
How to choose a solar charge controller? Things to consider
When looking to pick the best solar charge controller for you, it is important to consider several aspects:
The type of charge controller you will need to pick might depend on the type of solar system you are going to install.
Off-Grid Solar System? Grid-Tied System?
Off-grid solar systems will always require a solar charge controller. Depending on the size of the solar PV system you may require to go with an MPPT model, or if the system is very small you may opt for a PWM charge controller. If you are thinking of turning your grid-tied system into a grid-tied with battery backup system, then you will definitely need an MPPT.
Solar Light (Garden Light or Street Light?)
Solar lights generally come with an added solar panel to power an LED light, for this type of system a PWM charge controller will probably do the work quite well. Solar street lights are generally not electronic sensitive components and demand low amounts of electricity, besides, since the source is only a single module, they are perfect for PWM technology. But for robust performance and longer lifespan, many modern solar street lights have switched to MPPT controllers as their costs have been going down.
RV (Recreational Vehicle)?
RVs will always require a solar charge controller. If you have a very small PV system (maybe 1-2 panels) with the output voltage being close to the battery’s voltage, you might be good having a PWM charge controller, however, if the system intends to cover a large part of the loads in the RV, then an MPPT would be a better choice.
Unlike popular beliefs, PV systems are more efficient in cool temperatures. This is because voltage increases with cold temperatures and therefore, power increases as well. Similarly, charge controllers generally work better in cold temperature conditions. Choosing one or another based on weather alone can be a hard decision, but what matters the most is to verify that the ambient temperature is within the operating temperature conditions of the charge controller.
The Voltage of the Panels and Battery
Most battery storage systems operate at a voltage ranging from 12-48V. If you are looking to install a PWM charge controller, you have to match the voltage of the panels to the battery bank. If you want to install a solar array with a much higher voltage, you should pick an MPPT solar charge controller.
Size of the Loads to Power
When choosing a solar charge controller, you should consider the size of the load concerning how many amps the charge controller can handle. Most PWM controllers are better suited for small PV systems, handling small loads of up to 240W and operating at 20A 24V. MPPT solar charge controllers, on the other hand, can handle higher loads, being a better option for off-grid homes.
Specifications and Additional Features for Solar Charge Controllers
While the aforementioned aspects are some of the most important ones, you should also consider other specs as can be seen below
- Maximum Current Output
- Nominal Battery Voltage
- PV Open Voltage
- MPPT voltage range (For MPPT charge controllers only)
- Maximum PV Array Power under STC
- Temperature compensation parameters
- Operational temperatures
- Voltage regulation stages
- Type of compatible battery
Aside from the technical specifications, there are other features that you should consider:
- Type of screen and user interface
- Additional protection features for the batteries and the load
- Remote controlling
The solar charge controller is one of the most vital components for battery-based and off-grid solar systems. This device will protect your batteries, solar panels, and control many aspects of the system. When installing a solar charge controller, always consider between PWM and MPPT, depending on the size of your system, budget, and the power losses that you expect for the system.
To choose the best solar charge controller for you, compare each option against the aspects and tips in the last section of the article. This section will help you choose a solar charge controller that will perfectly adapt to your needs, showing you the best performance and extending the life of your batteries.