What is a charge controller? A 2 way DC Battery Charger.
How does a charge controller work?
A charge controller, or charge regulator is similar to the voltage regulator in your car. The charge controller's job is to regulates the voltage and current coming from somewhere, and going somewhere else. Charge controllers are always hooked up to a battery, the question is whether you are filling it up (charging the battery) or unloading the power (discharging the battery). Let's look at each case individually:
Charging a battery with a charge controller:
When you are filling your battery up with juice, or charging it, you want to put power into the box, but you don't want to hurt the battery. Batteries are designed to be charged in voltage and amperage ranges, to protect the materials inside. Apply too much power and you cook the inside of the battery. Apply too little power and the battery will not charge. So a charge controller takes the power from somewhere (usually a solar panel installation, wind, or solar, and pushes it into the battery at the right levels.
Working example: You buy 12 volt panels and you plug them directly into your battery. When there is perfect sun, they produce between the rated 13.1 and 14.4 volts. With just a little bit of shade, the voltage drops down to about 11.4 volts. On overcast days the voltage is roughly 10.5 volts.
The result of the above installation is that your battery only charges on perfect days. A 12 volt battery will never charge with a voltage of less than 12 volts. WHY? Well, think of water pressure like voltage, our battery like a tank, and the wire from the solar is like a pipe.
If we have 11.4 psi in the pipes and the tank is at 12.2 psi, the water will never make it into the tank. In fact, the tank will want to discharge into the pipe. A charge controller not only acts as a valve in the water scenario, it also boosts the pressure on the pipe to push the water into the tank. The charge controller takes the 11.4 volts and changes it to 13.1-14.4 volts so the battery can be charged.
As you can see in the example above, without a charge controller, this solar install would rarely charge the batteries. The system would not have the juice to push power into the battery with even one cloud in the sky.
Discharging a battery with a charge controller:
Discharging a battery, or taking the power out of it, can be controlled by a charge controller, and the point of doing so is to protect the battery. Batteries are not designed to be fully discharged. If you do run your batteries all the way down, you ruin their life expectancy. (See our battery pages for more info). So a charge controller can be used to protect your batteries by sensing when they are full, ½ way done, and need to be shut off to remain protected. Since the batteries are the most expensive part of many autonomous, remote, solar systems, spending a few bucks to protect them is usually a good idea.
You want to pump water with the wind generated power, but you don't want the pump to keep killing those batteries. You are eating them every 60-90 days. You have a water pump, connected to an inverter, and then to a battery. The battery is charged by a wind generator that is very erratic, but charges the battery via a charge controller. You are willing to switch out the battery every 60 days if you can get it to run on just one, and while not in service, the other can sit on a battery charger.
In this installation the battery will be charged by the wind, and we never know when it is going to come. The charge controller between the wind generator and the batteries normalizes that side, but we install a second charge controller on the load side (right above the water pump) to protect the batteries from discharging too much. When the second charge controller senses that the battery voltage is too small, it shuts down the pump, preserving the batteries. By protecting the low end of the batteries with a discharge control, you take the life expectancy of the battery up by a factor of 3-4. For instance, a Sun Xtender battery fully discharged should see roughly 500 cycles, or a little under a year, 2 times a day. On the other hand if you only discharge to 50% (1/2 way down), you get 1000 cycles, which is nearly 1.5 years longer with the same battery.