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Your Position: > Knowledge >> Solar controller >>> What’s a PWM solar charge controller?

What’s a PWM solar charge controller?

Amy / 2011-05-09
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In recent years, the technology for solar photovoltaic battery charge controllers has advanced dramatically, especially the new technology, PWM charging, has become very popular. Many people asked about MPPT. But most of the time, they only need a good PWM battery charging in fact.


What is PWM?

PWM (Pulse-Width Modulation) 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.


Why people like PWM so much?

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) Dramaically increase the charge acceptance of the battery.

3) Maintain high average battery capacities (90% to 95%) compared to on-off regulated state-of-charge 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. 


What’s the benefits of PWM technology?


The benefits noted above are technology driven. The more important question is how the PWM technology benefits the solar system user.

Jumping from a 1970's technology into the new millennium offers:


    • Longer battery life:
      • reducing the costs of the solar system  
      • reducing battery disposal problems


    • More battery reserve capacity:
      • increasing the reliability of the solar system  
      • reducing load disconnects
      • opportunity to reduce battery size to lower the system cost


    • Greater use of the solar array energy:
      • get 20% to 30% more energy from your solar panels for charging  
      • stop wasting the solar energy when the battery is only 50% charged  
      • opportunity to reduce the size of the solar array to save costs

Greater user satisfaction:
    get more power when you need it for less money!!


Are all of these benefits tested and proven?

A great deal of testing and data supports the benefits of PWM. More information is attached that describes the technology and various studies.


Wellsee will continue our ongoing test programs to refine the PWM charging technology (charger controller, solar light controller). Over time, each of these benefits will be improved and more clearly defined with numbers and graphs.


Are all PWM chargers the same?

Buyer beware! Many solar charge controllers that simply switch FETs differently than the on-off algorithm claim to be a PWM charger. Only a few controllers are actually using a Pulse Width Modulated (PWM) constant voltage charging algorithm. The rest are switching FETs with various algorithms that are cheaper and less effective.

Wellsee did this research for over ten years. And its PWM controller has updated for 4 times according to the market demand. It is a highly effective battery charging algorithm based on true PWM switching and constant voltage charging. All Wellsee products use this patented algorithm.



1. 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 causes grid corrosion, and the battery's positive plates become coated with sulfate crystals.

Wellsee's PWM pulse charging can deter the formation of sulfate deposits, help overcome the resistive barrier on the surface of the grids, and punch through the corrosion at the interface. In addition to improving charge acceptance and efficiency, there is strong evidence that this particular charging can recover capacity that has been lost in a solar battery over time. Some research results are summarized here.


2. 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.

A number of tests and studies have demonstrated that Wellsee’s PWM algorithm provides superior battery charge acceptance. Wellsee WS-C series PWM controller is a leading on-off regulator. The PWM controller put 20% to 30% more of the energy generated by the solar array into the battery than the on-off regulator.


3. 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 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.

However, as noted in the previous section, many solar controllers interfere with the recharging of the battery. The FSEC study noted at the end of the report that the most significant conclusion is that some controllers did not maintain the battery SOC at a high level, even when loads were disconnected.

In addition, a comprehensive 23 month study of SOC factors was reported by Sandia in 1994 (reference 7, page 940, attached). It was learned that the regulation setpoint has little effect on long-term SOC levels, but the reconnect voltage is strongly correlated to SOC. Five on-off regulators and two quasi constant voltage regulators were tested. A summary of the SOC results follows:

  • 3 on-off regulators with typical hysteresis averaged between 55% and 60% SOC over the 23 month period  
  • 2 on-off regulators with tighter hysteresis (risking global instability) averaged about 70% SOC  
  • the 2 constant voltage controllers with hysteresis of 0.3 and 0.1 volts averaged close to 90% SOC

Sandia concluded that the number of times a system cycles off and on during a day in regulation has a much stronger impact on battery state-of-charge than other factors within any one cycle.

It would be expected that batteries charged with Wellsee's PWM algorithm will maintain a very high average battery state-of-charge in a typical solar system. In addition to providing a greater reserve capacity for the system, the life of the battery will be significantly increased according to many reports and studies.


4. 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.


5. Reduce battery heating and gassing

Clearly battery heating/gassing and charge efficiency go hand in hand. A reduction in transient gassing is a characteristic of pulse charging. PWM will complete the recharging job more quickly and more efficiently, thereby minimizing heating and gassing.

The ionic transport in the battery electrolyte is more efficient with PWM. After a charge pulse, some areas at the plates become nearly depleted of ions, whereas other areas are at a surplus. During the off-time between charge pulses, the ionic diffusion continues to equalize the concentration for the next charge pulse.

In addition, because the pulse is so short, there is less time for a gas bubble to build up. The gassing is even less likely to occur with the down pulse, since this pulse apparently helps to break up the precursor to a gas bubble which is likely a cluster of ions.


6. Automatically adjust for battery aging

As batteries cycle and get older, they become more resistant to recharging. This is primarily due to the sulfate crystals that make the plates less conductive and slow the electro-chemical conversion.

However, age does not affect PWM constant voltage charging.

The PWM constant voltage charging will always adjust in regulation to the battery's needs. The battery will optimize the current tapering according to its internal resistance, recharging needs, and age. The only net effect of age with PWM charging is that gassing may begin earlier.


7. 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. In contrast, an on-off regulator will turn on full current with the full voltage drop throughout the recharging cycle (one reason for the very poor charge efficiency common to on-off regulators).

Because Wellsee controllers are all series designs, the FET switches are mostly off during the final charging stages. This minimizes heating effects from the controller, such as when they are placed inside enclosures. In contrast, the shunt designs will reach maximum heating in the final charging stage since the shunt FETs are switching almost fully on.

In summary, the PWM constant voltage series charge controller will provide the recharging current according to what the battery needs and takes from the controller. This is in contrast to simple on-off regulators that impose an external control of the recharging process which is generally not responsive to the batterys particular needs.


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