Why PWM? Whitepaper The technology for solar photovoltaic battery charge controllers has advanced dramatically over the past 5 years (1995-2000). The most exciting new technology, PWM (Pulse Width Modulation) charging, has become very popular. Some Frequently Asked Questions about PWM battery charging are addressed here. © 2000-2012. Morningstar Corporation. All Rights Reserved. www.facebook.com/MorningstarCorporation www.linkedin.com/company/morningstar-corp.
Why PWM? Whitepaper 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 Are all PWM Solar Charge Controllers the same? ••• current from the solar array tapers according to the battery’s condition and recharging needs.
In addition, this new method of solar battery charging promises some very interesting and unique benefits from the PWM pulsing; these include: The result is a higher 1. Ability to recover lost battery capacity and de-sulfate a battery. charging efficiency, 2. Dramatically increase the charge acceptance of the battery. rapid recharging and 3. Maintain high average battery capacities (90% to 95%) compared to on-off regulated state-of-charge levels that are typically 55% to 60%.
Ability to Recover Lost Battery Capacity: According to the Battery Council International, 84% of all lead acid-battery failures are due to sulfation. Sulfation is even more of a problem in solar systems, since “opportunity charging” differs significantly from traditional battery charging. The extended periods of undercharging, common to solar systems, cause grid corrosion, and the battery’s positive plates become coated with sulfate crystals.
Increase Battery Charge Acceptance: Charge acceptance is a term often used to describe the efficiency of recharging the battery. Since solar batteries are constantly recharging with a limited energy source (e.g. opportunity charging with available sunlight), a high charge acceptance is critical for required battery reserve capacity and system performance. Solar PV systems have a history of problems due to poor battery charge acceptance.
Maintain High Average Battery Capacities: A high battery state-of-charge (SOC) is important for battery health and for maintaining the reserve storage capacity so critical for solar system reliability. An FSEC Test Report (reference 6) noted that “the life of a lead-acid battery is proportional to the average state-of-charge,” and that a battery maintained above 90% SOC “can provide two or three times more charge/discharge cycles than a battery allowed to reach 50% SOC before recharging.
Equalize Drifting Battery Cells: Individual battery cells may increasingly vary in charge resistance over time. An uneven acceptance of charge can lead to significant capacity deterioration in weaker cells. Equalization is a process to overcome such unbalanced cells. The increased charge acceptance and capacity recovery capabilities of PWM pulse charging will also occur at lower charging voltages.
Self-Regulate for Voltage Drops and Temperature Effects: With PWM constant voltage charging, the critical finishing charge will taper per the equation I = Ae-t. This provides a self-regulating final charge that follows the general shape of this equation. As such, external system factors such as voltage drops in the system wires will not distort the critical final charging stage. The voltage drop with tapered charging current will be small fractions of a volt.
References: 1. Lam, L.T., et al, ‘Pulsed-current charging of lead/acid batteries—a possible means for overcoming premature capacity loss?’ CSIRO, Australia, Journal of Power Sources 53, 1995. 2. Hund, Tom, ‘Battery Testing for Photovoltaic Applications,’ Sandia National Laboratories, Albuquerque, NM, presented at 14th NREL Program Review, Nov. 1996. 3. Morningstar test results, 1999. 4.
