PWM = Pulse Width Modulation Battery Charging with a Charge Controller
PWM is one of several approaches used by the manufacturers of charge controllers to float charge a battery. This method of "FLOATING" a battery involves pulsing the charge instead of using a continuous float charge. Pulsing the battery it is believed helps to break down the sulphate crystals that would like to form inside your battery.
When using PWM charge controllers, a series of short charging pulses are sent to the battery depending on it's state. This is similar to a consistent, and rapid "on-off" cycling of the power to the battery. The charge controller must continually check the state of charge inside the battery between each pulse to determine how fast or slow to send the pulses. Some charge controllers are capable or varying the length of the pulse(width), amperage pulsed (size of the pipe full of water), and voltage of the pulse (pressure of the water pulsed through the pipe) as well as the repetition.
A PWM Charge Controller in Operation
When a PWM charge controller is hooked up to a completely charged battery without a load attached, the PWM charge controller will only "pulse" the battery every few seconds. However, with a fully discharged battery, the pulse is longer, and the timeout between pulse are almost non-existent. To the naked eye, often this charging cycle looks continuous. When the cycle does become continuous, the charge controller will flip to "FULL" mode.
Feedback, Fuzz, and Snow with PWM
Some users notice a feedback in their speakers, some notice snow on their TV or computer screen. Either way, the cause is the same, EMI, Electro Magnetic Interference. When anything pulses, it creates a frequency, sensitive electronics pick up these frequencies, and tell you they saw it with the glitches listed, and some not.
How do I stop the BUZZ, FUZZ, and SKUZZ?
Every PWM charge controller I have ever installed had an abililty to shut the PWM function off. If you are having issues like those stated above, simply shut the system down, switch of PWM, and fire it back up. If the snow goes away, don't turn PWM back on. Power you can't use is worthless. The benefits of PWM do not outweigh the usage of the appliances you are trying to power if they sense feedback. If you must have the PWM please look into shielding EMI. EMI is difficult to stop, and requires a bit of material and engineering. EMI Shielding, EMI filtering, EMI cancelling, and EMI suppression are the phrases to google at this point.
To quote the engineers at Xantrex, one of the top Charge Controller Manufacturers....
"Interference from inverters will always be an issue. It's a difficult topic for many to understand and equally difficult to reduce. Note, you can reduce but not eliminate the interference."