Batteries, Electricity, Solar

Chapter 8: Solar Charge Controllers

Solar Charge Controllers are specialized battery chargers that regulate the energy produced by solar panels, while charging the battery bank. While solar charge controllers come in a vast range of prices, power ratings and features, they all fall into one of two basic categories: Pulse-Width Modulation (PWM) and Maximum Power Point Tracking (MPPT).

PWM (Pulse Width Modulation) Chargers

PWM types are relatively simple, using a switch between the PV array and the battery. The switch is able to open and close rapidly, thus being able to pulse or “throttle back” the electricity coming from a solar panel in order to taper off the charge current as the batteries become full.

Since PWM controllers operate with a switch only, the array voltage during operation is equal to the battery voltage. This means that you need to use nominal voltage solar panels with a PWM controller (36-cell panels for 12 V nominal and 72-cell panels for 24 V nominal).

Even with a nominal voltage array, a PWM controller will operate below the maximum power voltage (Vmp). When it’s cold outside or when the battery voltage gets low, a PWM controller will operate well below Vmp and the max power (Pmp) rating of the solar array. To take full advantage of a PV array’s maximum power output, you need an MPPT controller.

PWM chargers have their place, and are often included with “solar kits” directed at new buyers. These chargers are the lower to mid-range quality chargers.

MPPT (Maximum Power Point Tracking) Chargers

MPPT controllers are comparatively more sophisticated than PWM chargers; and, are the more recent and best style of solar charge controller available. MPPT solar charge controllers can adjust (or track) the input voltage and current of the PV array to find the optimum operating voltage that will generate the most power at a given moment. By continuously tracking and operating at Vmp, an MPPT controller will be able to generate more power than a PWM controller during bulk charging.

There are a variety of MPPT algorithms, but most will have some ability to sweep the entire operating range of the solar panel to find where maximum power is produced.  Maximum power, which in the context of solar panels is the ability to extract as much power as possible from the solar panel without collapsing the panel voltage. Ideally, any system using a solar panel would operate that panel at its maximum power output.

Key Features of Solar Charge Controllers

Multistage charging of battery bank – changes the amount of power set to the batteries based on its charge level, for healthier batteries.

Reverse current protection – stops the solar panels from draining the batteries at night when there is no power coming from the solar panels.

Low voltage disconnect – turns off attached load when battery is low and turns it back on when the battery is charged back up.

Lighting control – turns attached light on and off based on dusk and dawn. Many controllers are configurable, allowing settings for a few hours or all night, or somewhere in between.

Display– may show voltage of battery bank, state of charge, amps coming in from solar panel. 

Final Considerations When Selecting A Solar Charge Controller

It’s important to choose the right charge controller for your application. This can vary in terms of size and features.

For remote systems, reliability is likely the most important consideration, with performance a close second. Generally speaking, a lower cost solar charge controller will be less reliable and may not meet vital charging requirements. Purchasing an inexpensive charge controller and dealing with poor performance and/or reliability can end up costing many times over the cost of the more expensive solar controller, in terms of: replacing your battery bank, site visits for power, and loss of operating time.

Solar charge controllers must be designed to take a beating, since they deal with a lot of heat and have to manage it properly. Small charge controllers have the advantage of being fanless — they get rid of heat by simple passive cooling.

Some larger controllers (including all Morningstar controllers) also use passive cooling with no fans, incorporating very advanced thermal mechanical design and software. They are preferred in remote, mission-critical installations where maintenance is infrequent and replacements are difficult.

Smaller charge controllers will often only have preset charge settings. If the battery charging requirements are not sufficiently met with these presets, a controller with more settings options can be selected. Custom settings can be simple adjustments to voltage set points, particular applications or environments. For example, a system which does not cycle much can be set up with reduced daily absorption time, which is the amount of time before it puts the battery into float.

The life of your batteries will be longer and happier if you charge them correctly. The best chargers on the market are 3-stage chargers. Use of a good quality 3 stage charger will significantly improve your battery’s performance and lifespan. These chargers can be purchased separately or are included as part of many of the better quality inverters. When using a 3 stage charger, battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.

Bulk Charge – The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no “correct” voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take.

Absorption Charge – The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts.

Float Charge – The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2 volts) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it’s main purpose is to keep an already charged battery from discharging.

Okay So Which Is Better?

If there is not a long wire run and nominal voltage solar modules are being used, a PWM controller often is the best choice. The same is true in locations that may also have a lot of constant, dependable sunshine — in deserts or the tropics. In these locations, PWM controllers are the right tool for the job since some wasted solar electricity isn’t critical. Any advantage in using an MPPT controller may be minimal since the array voltage is lower in warm conditions.

In places with variable sunshine, fluctuating temperatures and shading, in northern or southern latitudes with snowfall in winter, MPPT is by far more desirable since it can maximize output under challenging conditions.

Another consideration is the size of the system. PWM controllers are often used in smaller, cost-sensitive systems where the added cost with MPPT is not worth it.

Our Choice

While MPPT charge controllers are easily double the price of their PWM counterparts, for our purposes, traveling throughout various regions in The United States, the pros outweigh the cons for us. MPPT chargers are up to 30% more efficient then PWM chargers, and are better suited for the varying environmental conditions we expect to encounter. Additionally, most MPPT chargers will have better battery management capabilities then a PWM charger.

We have gone ahead and purchased the Outback FM-80, for a detailed review, CLICK HERE!

Next: Chapter 9: Power Inverters & 110v Off-Grid

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