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What & Why: Charge Controllers

PWM & MPPT charge controllers

Charge Controllers & Your Renewable Energy System

If you’ve checked out our Renewable Energy System Diagram, you know all alternative energy systems use a charge controller between the power source (solar, wind or hydro) and the battery bank. In simple terms, the charge controller regulates the power delivered to your battery bank, to prevent damage to either the batteries or the power source. But when you actually go to buy a charge controller for your system, choosing the right one from all the models available can be daunting.

Of course it needs to be sized appropriately for the amount of power you’ll be generating, but beyond that there’s load-diverting controls, pulse-width-modulated (PWM) models, and something called Maximum Power Point Tracking (MPPT). Knowing which controller is best for you may seem confusing. In this post, we’ll explain all that terminology, and make sure you know everything you need to choose the right type of charge controller for your needs!

Load-Diverting vs. Solar Controllers

Solar panels and turbine-based generators (wind or hydro) require different kinds of charge controllers. In short, solar panels only produce power when power is actually needed (when an electrical load is connected), so a solar controller just needs to be able to shut off the flow of electricity when your battery bank is fully charged, to prevent damage to the batteries. Wind and water turbines, on the other hand, produce power whenever they are in motion. Having an electrical load connected helps control the speed of the generator, and the turbine can be damaged if it’s left to spin uncontrolled when no power is needed.

Having a diversion load to direct power to provides that control when the battery bank is full, thereby preventing damage to the generator. If you’re installing a wind or hydro turbine by itself, many turbine manufacturers make load diverting controls designed specifically for their product line, making your choice of charge control very easy!

Pulse Width Modulated Controllers

Pulse Width Modulated (PWM) controllers are most commonly used with solar PV systems (not all turbines can handle the varying electrical load they create). PWM is simply a bunch of electrical “gates” that open and close at varying speeds to regulate the amount of power that can pass through the controller. PWM controllers are relatively simple and inexpensive, and often the best choice for small solar photovoltaic systems.

PWM technology has one major drawback though: Because of the nature of solar panels, pulse width modulated controllers can cause significant losses in your system. Solar panels operate at a fixed voltage, and have a fixed maximum amount of current they can produce. A 100W solar panel operating at 17V can produce 5.88 amps of power. A PWM controller will take that 17V power and step it down to safe charging voltage (14.5V for a 12V battery bank), but the panel still only produces 5.88 amps. Instead of being able to use the full 100 watt output of the panel (17V x 5.88 amps), you’re only able to use (14.5V x 5.88 amps) 85 watts of power.

In a small and relatively inexpensive system, that power loss may be acceptable compared to the much higher purchase price of a Maximum Power Point Tracking (MPPT) charge controller, but if you’re building a large PV system, investing in an MPPT controller will pay for itself in increased power production…

Maximum Power Point Tracking Controllers

Maximum Power Point Tracking (MPPT) controllers avoid the drawbacks of PWM models. By being able to adjust the amperage as well as the voltage of the incoming current, an MPPT controller puts out the same amount of power that comes in (minus small, unavoidable, internal losses). To use our earlier example, the 17 volts at 5.88 amps the solar panel sends to the MPPT controller is output at 14.5 volts at ~6.9 amps. If you have 100 watts input, you get ~100 watts output.

MPPT controllers were originally developed for use with solar systems, but if you have a multi-source power system they can also work well with wind and hydro turbines (with a diversion load integrated into the system). The primary drawback to MPPT charge controllers is their price, but if you’re already investing in a large solar array & battery bank, the extra expense of one of our MPPT charge controllers will be well worth it, compared to the system losses of a PWM model. If, however, you’re building a small solar system with one or two panels, a PWM charge controller is a much more economical choice.

For a more detailed explanation of both PWM and MPPT controllers, and the electrical concepts behind them, read our Charge Controller Overview white paper.