Introduction to Charge Controllers


Introduction to Charge Controllers

What Is A Charge Controller?

The basic functions of a controller are quite simple. Charge controllers block reverse current and prevent battery overcharge. Some controllers also prevent battery over-discharge, protect from electrical overload, and/or display battery status and the flow of power. Let’s examine each function individually.

Blocking Reverse Current

In most controllers, charge current passes through a semiconductor (a transistor) which acts like a valve to control the current. It is called a “semiconductor” because it passes current only in one direction. It prevents reverse current without any extra effort or cost.


Preventing Overcharge

When a battery reaches full charge, it can no longer store incoming energy. If energy continues to be applied at the full rate, the battery voltage gets too high. Water separates into hydrogen and oxygen and bubbles out rapidly. (It looks like it’s boiling so we sometimes call it that, although it’s not actually hot.) There is excessive loss of water, and a chance that the gasses can ignite and cause a small explosion. The battery will also degrade rapidly and may possibly overheat. Excessive voltage can also stress your loads (lights, appliances, etc.) or cause your inverter to shut off.
Preventing overcharge is simply a matter of reducing the flow of energy to the battery when the battery reaches a specific voltage. When the voltage drops due to lower sun intensity or an increase in electrical usage, the controller again allows the maximum possible charge. This is called “voltage regulating.” It is the most essential function of all charge controllers. The controller “looks at” the voltage, and regulates the battery charging in response.
Some controllers regulate the flow of energy to the battery by switching the current fully on or fully off. This is called “on/off control.” Others reduce the current gradually. This is called “pulse width modulation” (PWM). Both methods work well when set properly for your type of battery.
A PWM controller holds the voltage more constant. If it has two-stage regulation, it will first hold the voltage to a safe maximum for the battery to reach full charge. Then, it will drop the voltage lower, to sustain a “finish” or “trickle” charge. Two-stage regulating is important for a system that may experience many days or weeks of excess energy (or little use of energy). It maintains a full charge but minimizes water loss and stress.

Control Set Points vs. Temperature

The ideal set points for charge control vary with a battery’s temperature. Some controllers have a feature called “temperature compensation.” When the controller senses a low battery temperature, it will raise the set points. Otherwise, when the battery is cold, it will reduce the charge too soon. If your batteries are exposed to temperature swings greater than about 30° F (17° C), compensation is essential.

Control Set Points vs. Battery Type

Sealed batteries use saturated pads between the plates. They are also called “valve-regulated” or “absorbed glass mat,” or simply “maintenance-free.” They need to be regulated to a slightly lower voltage than flooded batteries or they will dry out and be ruined. Some controllers have the means to select the type of battery. Never use a controller that is not intended for your type of battery.

Typical Set Points For 12V Lead-Acid Batteries at 77°F (25°C)

  • (These are typical, presented here only for example.)
  • High limit (flooded battery): 14.4 V
  • High limit (sealed battery): 14.0 V
  • Resume full charge: 13.0 V
  • Low voltage disconnect: 10.8 V
  • Reconnect: 12.5 V
  • Temperature compensation for 12V battery:
  • -.03 V per ° C deviation from standard 25° C

Low Voltage Disconnect (LVD)

Similarly, if you wait until your lights look dim, some battery damage will have already occurred. Every time this happens, both the capacity and the life of the battery will be reduced by a small amount. If the battery sits in this over discharged state for days or weeks at a time, it can be ruined quickly.
The only way to prevent over discharge when all else fails, is to disconnect loads (appliances, lights, etc.), and then to reconnect them only when the voltage has recovered due to some substantial charging. When over discharge is approaching, a 12 volt battery drops below 11 volts (a 24 V battery drops below 22 V).
If you have any DC loads, you should have an LVD. Some charge controllers have one built in. You can also obtain a separate LVD device. Some LVD systems have a “mercy switch” to let you draw a minimal amount of energy, at least long enough to find the candles and matches! DC refrigerators have LVD built in.
If you purchase a charge controller with built-in LVD, make sure that it has enough capacity to handle your DC loads. For example, let’s say you need a charge controller to handle less than 10 amps of charge current, but you have a DC water pressurizing pump that draws 20 amps (for short periods) plus a 6 amp DC lighting load. A charge controller with a 30 amp LVD would be appropriate. Don’t buy a 10 amp charge controller that has only a 10 or 15 amp load capacity!

Overload Protection

A circuit is overloaded when the current flowing in it is higher than it can safely handle. This can cause overheating and can even be a fire hazard. Overload can be caused by a fault (short circuit) in the wiring, or by a faulty appliance (like a frozen water pump). Some charge controllers have overload protection built in, usually with a push-button reset.

Displays and Metering


Have It All with a Power Center


Charge Controllers for Wind and Hydro

A charge controller for a wind-electric or hydro-electric charging system must protect batteries from overcharge, just like a PV controller. However, a load must be kept on the generator at all times to prevent the turbine from over-speeding. Instead of disconnecting the generator from the battery (like most PV controllers) it diverts excess energy to a special load that absorbs most of the power from the generator. That load is usually a heating element, which “burns off” excess energy as heat. If you can put the heat to good use, fine!

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