Why PWM?

The technology for solar photovoltaic battery charge controllers has advanced dramatically over the past five years. The most exciting new technology, PWM charging, has become very popular

What is PWM?

Pulse Width Modulation (PWM) is the most effective means to achieve constant voltage battery charging by switching the solar system controller’s power devices. When in PWM regulation, the current from the solar array tapers according to the battery’s condition and recharging needs.

Charging a battery with a solar system is a unique and difficult challenge. In the “old days,” simple on-off regulators were used to limit battery outgassing when a solar panel produced excess energy. However, as solar systems matured it became clear how much these simple devices interfered with the charging process. The history for on-off regulators has been early battery failures, increasing load disconnects, and growing user dissatisfaction. PWM has recently surfaced as the first significant advance in solar battery charging. PWM solar chargers use technology similar to other modern high quality battery chargers. When a battery voltage reaches the regulation setpoint, the PWM algorithm slowly reduces the charging current to avoid heating and gassing of the battery, yet the charging continues to return the maximum amount of energy to the battery in the shortest time. The result is a higher charging efficiency, rapid recharging, and a healthy battery at full capacity. In addition, this new method of solar battery charging promises some very interesting and unique benefits from the PWM pulsing. These include:

  1. Ability to recover lost battery capacity and desulfate a battery.
  2. Dramatically increase the charge acceptance of the battery.
  3. Maintain high average battery capacities (90% to 95%) compared to on-off regulated state-ofcharge levels that are typically 55% to 60%.
  4. Equalize drifting battery cells.
  5. Reduce battery heating and gassing.
  6. Automatically adjust for battery aging.
  7. Self-regulate for voltage drops and temperature effects in solar systems.

Pulse-Width Modulation (PWM) comes into play when the battery bank is full. During charging, the controller allows as much current as the PV panel/array can generate in order to reach the target voltage for the charge stage the controller is in. Once the battery approaches this target voltage, the charge controller quickly switches between connecting the battery bank to the panel array and disconnecting the battery bank, which regulates the battery voltage holding it constant. This quick switching is called PWM and it ensures your battery bank is efficiently charged while protecting it from being overcharged by the PV panel/array.

PWM controllers will operate close to the maximum power point but often slightly “above” it. An example operating range is shown below.