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©1999 Windy Dankoff
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 an 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. This can be illustrated by an analogy:
The Energy Chef is watching a pot of water on a gas burner, which is fed by a tube coming from the sun. He has one hand on the gas valve. He’s thinking, “I need to get this water as close to a boil as possible before the sun goes down, but I must never boil the water.”
In this analogy, the temperature of the water represents battery voltage; the flow of gas represents charging current; boiling represents overcharge; and the energy chef manipulating the valve is like the charge controller.
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, called pulse width modulation (PWM). Both methods work well when the voltage set points are properly selected for your type of battery.
Acharge controller is an essential part of nearly all power systems that charge batteries, whether the power source is PV, wind, hydro, fuel, or utility grid. Its purpose is to keep your batteries properly fed and safe for the long term.
The basic functions of a controller are quite simple. Charge controllers block reverse current and prevent battery overcharge. Some controllers also prevent battery overdischarge, protect from electrical overload, and display battery status and the flow of power. Let’s examine each function individually.
Blocking Reverse Current
Photovoltaic (PV) panels work by pumping current through your battery in one direction. At night, the panels may pass a bit of current in the reverse direction, causing a slight discharge from the battery. (Our term “battery” represents either a single battery or bank of batteries.) The potential loss is minor, but it is easy to prevent. Some types of wind and hydro generators also draw reverse current when they stop, but most do not, except under fault conditions.
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 in only one direction. It prevents reverse current without any extra effort or cost.
In some controllers, an electromagnetic coil opens and closes a mechanical switch. This is called a relay. It switches off at night, to block reverse current. As it turns on and off, there is an audible clicking sound.
If you are using a very small array relative to the size of the battery, then you may not need a charge controller. This is a rare application. An example is a tiny maintenance PV module that trickle-charges a battery and compensates for battery discharge in a parked vehicle but will not support significant loads. In this situation, you can install a simple diode to block reverse current. A diode used for this purpose is called a blocking diode.
The Energy Chef
68 Home Power #72 • August / September 1999
Chef & valve = charge controller
Boiling = overcharge
Water temperature = battery voltage
Gas = charging current
Image | What is a charge controller
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