Northern Arizona Wind & Sun
Electricity from the sun since 1979
Solar Charge Controllers
What is a
solar charge controller?
Why do I need one?
What does a charge controller do?
A charge controller, or charge regulator is similar to the voltage regulator in your car. It regulates the voltage and current coming from the solar panels going to the battery. Most "12 volt" panels put out about 16 to 20 volts, so if there is no regulation the batteries will be damaged from overcharging. Most batteries need around 14 to 14.5 volts to get fully charged.
Do I always need a charge controller?
Generally, there is no need for a charge controller with the small maintenance, or trickle charge panels, such as the 1 to 5 watt panels. A rough rule is that if the panel puts out 1/60th or less per day of the rated battery capacity, you don't need one.
For example, a golf car battery is around 200 amp-hours. So to keep up a series pair of them (12 volts) just for maintenance or storage, you would want a panel that would give you around 1/2 of 1% of the battery capacity per day. Assume 5 hours sun per day. 200 amp-hours divided by 200 (.5%) gives you around 1 AH per day loss. A 5 watt solar panel will supply around .3 amps (1.5 AH per day), so a 5 watt panel will give you more than enough to keep the batteries up, but not so much as to damage them.
Why 12 Volt Panels are 17 Volts
The obvious question then comes up - "why aren't panels just made to put out 12 volts". The reason is that if you do that, the panels will provide power only when cool, under perfect conditions and, full sun. This is not something you can count on in most places. The panels need to provide some extra voltage so that when the sun is low in the sky, or you have heavy haze, cloud cover, or high temperatures*, you still get some output from the panel. A fully charged battery is around 12.7 volts, so the panel has to put out at least that much under worst case conditions.
Detailed information on MPPT charge controllers.
The charge controller regulates this 16 to 20 volts output of the panel down to what the battery needs at the time. This voltage will vary from about 10.5 to 14.6, depending on the state of charge of the battery, the type of battery, and temperature. (see complete info on battery voltages in our battery section).
Contrary to intuition, solar panels work best at cooler temperatures - in fact, some of the so-called "self regulating" panels (low voltage) will not charge a battery in temperatures commonly found in hot climates.
Charger Controller Types
Charge controls come in all shapes, sizes, features, and price ranges. They range from the small 4.5 amp (Sunguard) control, up to the 60 amp Outback Power MX60 MPPT programmable with computer interface. Often, if currents over 40 amps are required, two or more 20 to 40 amp units are wired in parallel. The most common controls used for all battery based systems are in the 6 to 40 amp range.
Charge controls come in 3 general types (with some overlap):
Simple 1 or 2 stage controls which rely on relays or shunt transistors to control the voltage in one or two steps. These essentially just short or disconnect the solar panel when a certain voltage is reached. For all practical purposes these are dinosaurs, but you still see a few on old systems. Their only real claim to fame is their reliability - they have so few components, there is not much to break.
3-stage and/or PWM such Morningstar, Xantrex, BZ Products, Blue Sky, Steca, and many others. These are pretty much the industry standard now, but you will occasionally still see some of the older shunt/relay types around, such as in the very cheap systems offered by discounters and mass marketers.
The maximum power power tracking ones (MPPT), such as those made by Blue Sky Energy and Outback Power. These are the ultimate in controllers, with prices to match - but with efficiencies in the 96 to 98% range, they can save considerable money on larger systems since they provide 15 to 30% more power to the battery. For more information, see our article on MPPT.
Most controllers come with some kind of indicator, either a simple LED, a series of LED's, or digital meters. Some newer ones, such as the Outback MX60 and a few others now have built in computer interfaces for monitoring and control. The simplest usually have only a couple of small LED lamps, which show that you have power and that you are getting some kind of charge. Most of those with meters will show both voltage and the current coming from the panels and the battery voltage. Some also show how much current is being pulled from the LOAD terminals.
All of the charge controllers that we stock are 3 or 4-stage PWM types, including the MPPT units. (in reality, "4-stage" is somewhat advertising hype - it used to be called equalize, but someone decided that 4 stage was better than 3). And now we even see one that is advertised as "5-stage"....
What is Equalization?
Equalization does somewhat what the name implies - it attempts to equalize - or make all cells in the battery or battery bank of exactly equal charge. Essentially it is a period of overcharge, usually in the 15 to 15.5 volt range. If you have some cells in the string lower than others, it will bring them all up to full capacity. In flooded batteries, it also serves the important function of stirring up the liquid in the batteries by causing gas bubbles. Of course, in an RV or boat, this does not usually do much for you unless you have been parked for months, as normal movement will accomplish the same thing. Also, in systems with small panels you may not get enough current to really do much bubbling.
What is PWM?
Quite a few charge controls have a "PWM" mode. PWM stands for Pulse Width Modulation. PWM is often used as one method of float charging. Instead of a steady output from the controller, it sends out a series of short charging pulses to the battery - a very rapid "on-off" switch. The controller constantly checks the state of the battery to determine how fast to send pulses, and how long (wide) the pulses will be. In a fully charged battery with no load, it may just "tick" every few seconds and send a short pulse to the battery. In a discharged battery, the pulses would be very long and almost continuous, or the controller may go into "full on" mode. The controller checks the state of charge on the battery between pulses and adjusts itself each time.
Why PWM (a PDF file from Morningstar explaining the benefits of PWM)
What is a Load, or "Low Voltage Disconnect" output?
Some controllers also have a "LOAD", or LVD output, which can be used for smaller loads, such as small appliances and lights. The advantage is that the load terminals have a low voltage disconnect, so it will turn off whatever is connected to the load terminals and keep from running the battery down too far. The LOAD output is often used for small non-critical loads, such as lights. A few, such as the Xantrex C12, can also be used as a lighting controller, to turn lights on at dark, but the Morningstar SLC lighting controller is usually a better choice for that. Most systems do NOT need the LVD function - it can drive only smaller loads. Depending on the rating of the controller, this may be from 6 to 30 amps. You cannot run any but the smallest inverter from the LOAD output. On some controllers, such as the Morningstar SS series, the load output can be used to drive a heavy duty relay for load control, gen start etc.
What is a "Battery System Monitor"?
Battery system monitors, such as the Xantrex Link-10 and TM500, and the TriMetric 2020 are not controllers. Instead, they monitor your battery system components and give you a pretty good idea of what you are using and generating. They generally keep track of the total amp-hours into and out of the batteries, and the battery state of charge, and other information. They can be very useful for medium to large systems for tracking exactly what your system is doing with various charging sources. They are somewhat overkill for small systems, but are kind of a fun toy if you want to see what every amp is doing :-). TriMetric's new PentaMetric model also has a computer interface and many other features.
For a complete list of all our charge controllers, to check pricing, or to order online, see our Charge Controller page on our webstore. For battery monitors, meters, and shunts see our Meters & Monitors page there.
Solar Charge Control FAQ - copyright 1997-2005 NAWS
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and the respective manufacturers and may not be used without permission.
Last updated
Tuesday, April 19, 2005