Charge Controller Overview
In general, charge controllers regulate the power of solar panels and charge your batteries safely. They have sensors that determine battery status and keep them from over-charging or adjusting the charge when the environment plays a role. Some also come with predetermined algorithms for major battery types commonly used such as Sealed Lead Acid, Gel, and Flooded.
Two major types of controllers exist for usage in the solar power industry: PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking). In order for these controllers to commence charging a battery, the voltage going into the controller needs to be higher than the battery so that the charge controllers can regulate the voltage. Both PWM and MPPT controllers charge deep cycle batteries but handle the charging algorithm differently which contributes to their efficiency differences. The main differences between PWM and MPPT are the charging efficiency and cost. MPPT (90% and above efficient) controllers are more efficient than PWM (70%-80% efficient) at battery charging, and PWM controllers are more inexpensive than MPPT controllers (About 1/3 price).
Summary
Charge controllers are a complex electronic equipment that is essential to the operation of an off-grid solar system.
The following summarizes the key points of this section:
- Charge controllers typically come in two major varieties, Pulse Width Modulated (PWM) andMaximum Power Point Tracking (MPPT)
- MPPT controllers are much more efficient than PWM controllers and are sometimes the only logical option especially when preventing excessive power losses in larger systems.
- PWM controllers work the best when panel array voltages are paired closely with the battery charging voltages
- Battery pairing with controllers is important to ensure battery health
- Charge controllers typically exhibit three-stage charging which allows for the best battery health compensation and power conservation for charging
PWM Charge Controllers
PWM charge controllers are the least efficient controllers of the two main types used in the solar industry. The reason for this being is the method of regulation. For a PWM controller, it acts as a constant voltage regulator for reducing the voltage required to a battery bank. Since the PWM controller is a voltage regulator, it does not change the output current until the battery is close to being full. Additionally, the output voltage will depend on the charging voltage required by a battery. As with current output, this varies throughout the charging cycle, but for the bulk of this cycle, it remains constant. PWM controllers emit a pulsing signal to the battery to determine the level of charge(setpoint) to completely charge the battery and maintain it.
Based on the setpoints within the controller, it can determine the general state of charge of the battery, charge it to full, and maintain it. In PWM, once the controller reaches the set-point, it ceases proportional control and enters the pulse width modulation. Pulse width is a type of on/off control meaning once the set-point is achieved the signal is shut off to the plant. However, it turns the signal on and off frequently rather than shutting completely off. This is a great feature for battery charge control because once charging ceases, a battery will tend to slowly discharge itself. By continuously pulsing a signal to the battery once it is full, it will maintain the battery at the set-point. This would be akin to holding a cup under a faucet that has a leak in the bottom.
You would then attempt to maintain the full water level by rapidly shutting the faucet on and off. Essentially this signal approximates keeping the battery at a constant voltage while ensuring that it is not overcharged. When PWM regulate the voltage, the step down to match the battery bank is lost to heat, hence why they are less efficient. PWM controllers generally have a lower input voltage which means you have to wire solar panels in parallel.
Lastly, PWM controllers are typically used on smaller systems where applications are not so critical. A rule of thumb is 400W or less should use a PWM charge controller.
MPPT Charge Controllers
With an MPPT controller, rather than regulating voltage, it actually behaves as a DC voltage converter. By doing this it essentially acts as a power regulator. This allows the controller to accept any input of power (within its voltage/current range) and convert it to the appropriate voltage for the battery bank.
DC voltage converters typically have an efficiency above 90%, depending on the level at which the converter is run. Depending on the input, the efficiency can actually range from90% to 99%. The output current of an MPPT controller will always produce more current flow to the battery than a PWM controller will. Since we know on average that the PWM controllers have an average efficiency of around 79% (max) and MPPT has an average efficiency of 94%, an MPPT will produce a current of about 1.2 times that of the array current or 20% higher than the array. It maintains a higher efficiency and boosts the array current to more quickly charge the battery rather than wasting energy as heat.
In addition, MPPT controller typically supports a higher voltage input which allows for the wiring of the panels in series. This can be advantageous in systems with long panel to-controller wire runs as it will overcome voltage losses in the wiring. MPPT controllers attempt to ensure maximum power conversion and for that reason are typically used in critical power applications and are essential for bigger systems. 500W solar power systems and more should use an MPPT charge controller.
