PWM Charge Controller (Pulse Width Modulation)
A simple controller type that connects the panel directly to the battery and regulates the charge voltage by switching that connection on and off rapidly. Because the panel is pulled to the battery voltage during this, it does not operate at the maximum power point; in return its circuit is plain, its cost low and its probability of failure small.
A PWM controller has no converter. While the switch is closed the panel current flows straight to the battery, and the panel voltage settles at the battery's momentary voltage. The controller regulates the average charge current, and therefore the voltage, by varying the proportion of time the switch stays open; in this way it can apply the absorption and float stages.
The loss arises where the panel's maximum power point voltage is higher than the battery voltage. If the panel gives its highest power at around 18 V but is pulled to 13 V, the difference goes unused. The difference grows in cold weather and shrinks on a hot summer day. That is why the real efficiency difference between PWM and MPPT is not a fixed figure but a range depending on conditions.
PWM is still a reasonable choice in small RV installations with a single panel and short cabling where the panel voltage is close to the battery voltage. As the number of panels grows or the cabling gets longer, moving to MPPT becomes meaningful. In both types the correct battery chemistry setting must be made.
The context in which this term is used: RV automatic levelling guide