Do I Need a PWM or MPPT Solar Charge Controller?

Author: Solar 4 RVs

Read more

Importance of a Solar Charge Controller

A solar charge controller, or regulator, functions similarly to a battery charger. Its primary role is to manage the current from the solar panels to the battery bank, preventing overcharging. For those uninterested in the details, feel free to skip to the end for a handy flow chart. Like a top-quality battery charger, various battery types are supported, and settings for absorption voltage and float voltage can usually be adjusted. They are especially tailored for lithium-iron-phosphate batteries, maintaining a holding voltage of approximately 13.6V after full charge for the remainder of the day.

The standard charge profile follows a similar sequence as most quality mains chargers: bulk mode > absorption mode > float mode. The transition to the bulk charge mode typically occurs at:

• Sunrise
• Whenever the battery voltage dips below a specified point for a set duration, such as 5 seconds

This mechanism functions efficiently with lead-acid batteries, which tend to experience greater voltage drop and droop compared to lithium-based batteries, which usually maintain more stable voltages throughout their discharge cycle.

Understanding Lithium Batteries

Lithium batteries (LiFePO4) do not gain from re-entering bulk mode during the day, as their internal resistance increases at both high and low states of charge. The balancing of cells can only occur around the absorption voltage. Avoiding sudden voltage changes is crucial, as large loads turning on and off can cause notable fluctuations.

These batteries lack a defined float voltage; hence, it is recommended that the float voltage of the controller is set slightly below the charge knee voltage of the LiFePO4 profile (around 3.4V per cell or 13.6V for a 12V battery). The controller must maintain this voltage throughout the day following bulk charging.

Comparative Analysis of PWM and MPPT Solar Charge Controllers

The fundamental distinction is:

• With a PWM controller, current is extracted from the panel just above battery voltage.
• Conversely, an MPPT solar charge controller draws current from the panel at its maximum power voltage, similar to a smart DC-DC converter.

Marketing claims often suggest that MPPT controllers yield 20% or more in energy harvesting. However, this increase can fluctuate significantly. For illustration, let’s assume perfect sunlight and the controller in bulk mode, ignoring any voltage drops:

• Panel maximum power current (Imp) = 5.0A
• Panel maximum power voltage (Vmp) = 18V

Battery voltage = 13V (which fluctuates between approximately 10.8V when fully discharged up to 14.4V during absorption). At 13V, panel amps slightly exceed maximum power amps, potentially reaching 5.2A.

Using a PWM controller, the energy harvested from the panel is 5.2A * 13V = 67.6 watts, remaining consistent as long as panel voltage stays above battery voltage.

Contrarily, with an MPPT controller, the panel provides power calculated at 5.0A * 18V = 90 watts, translating to a 25% increase. Nevertheless, this increase may be optimistic since voltage droops with rising temperatures; with a panel temperature increase of 30°C from standard conditions, dropping 12% in voltage results in an MPPT output of 5A * 15.84V = 79.2W—an increase of 17.2% over PWM.

To summarize, MPPT controllers do enhance energy harvesting, though the increase in percentage can vary greatly throughout the day.

Operational Differences Between PWM and MPPT

PWM Operation:

A PWM (pulse width modulation) controller operates akin to an electronic switch between the solar panels and the battery:

• Switch is ON during bulk charge mode
• Switch is pulsed ON and OFF as needed to maintain battery voltage at the absorption setting
• Switch is OFF when the battery voltage settles to the float voltage
• Switch is again pulsed to uphold battery voltage at the float voltage

When OFF, the panel voltage approaches open-circuit voltage (Voc), and when ON, it equals battery voltage plus any drops between the panel and the controller.

Optimal Panel Compatibility for PWM:

The ideal panel for a PWM controller has a voltage sufficiently above the battery charging requirement, typically around 18V for a 12V battery (36V for a 24V battery).

MPPT Operation:

MPPT controllers function as sophisticated DC-DC converters, adjusting panel voltage to match battery charging needs. The current increases in proportion to the reduced voltage, analogous to a standard step-down converter.

The intelligence of the DC-DC converter lies in tracking the panel’s maximum power point, which changes throughout the day based on sun intensity, angle, temperature, shading, and the health of the panels. This versatility allows the system to adapt and optimize the input voltage accordingly.

Additional resources:
How Big Are Residential Solar Panels?

Visit KINGSUN to discover more.

Optimal Panel Matching for MPPT:

To align a panel with an MPPT controller, consider the following:

1. Ensure the panel open circuit voltage (Voc) remains below the maximum permitted voltage.
2. Voc should exceed the starting voltage for the controller to activate.
3. The maximum panel short circuit current (Isc) should be within the specified range.
4. The maximum array wattage—some controllers permit oversizing; e.g., the Redarc Manager 30 can support up to 520W.

Selecting the Right Solar Controller/Regulator

PWM: An Economical Choice:

• Ideal for smaller setups
• When system efficiency is less critical, such as trickle charging
• Or with panels featuring a maximum power voltage (Vmp) up to 18V for 12V batteries (36V for 24V batteries, etc.)

MPPT: The Superior Choice:

• Best for larger systems
• Where the extra energy—20% or more—is valuable
• When the voltage of the solar array significantly exceeds the battery voltage, like with house panels used for charging 12V batteries.

*An MPPT controller will outperform a PWM controller as panel voltage increases. For instance, a 160W panel with 36 traditional monocells yielding about 8.6A at 12V can see greater output due to additional cells. A PWM controller remains static in its harvest, while an MPPT can extract an additional 11.1% from upgraded panels. Hence, using SunPower cells with over 32 cells necessitates an MPPT charge controller for maximum energy extraction.

Features and Choices for Solar Charge Controllers

Victron SmartSolar Controllers

These controllers come equipped with built-in Bluetooth, allowing remote monitoring of your MPPT via smartphones through the Victron app.

Boost MPPT Controllers

These controllers enable charging of batteries at higher voltages than those of the panels.

Combined MPPT and DC-DC Chargers

With a natural blend of MPPT and DC-DC functionalities, several reputable brands offer this. A single unit can efficiently transition between alternator and solar charging. For larger installations, it is often best to utilize a separate MPPT controller for permanent roof-mounted panels, while using combined systems for portable panels. This requires an Anderson connector on RV exteriors for solar input into the MPPT/DC-DC device.

Also, ensure that battery capacity accommodates combined charging from both alternator and solar without exceeding the recommended maximum current.

Budget Options

Some cheaper controllers advertised as MPPTs may actually be PWM devices. Be cautious as they might lack over-voltage protection, risking battery damage from overcharging. Typically, MPPT solar charge controllers are larger due to their complex circuitry compared to PWM counterparts.

Multiple Solar Chargers

If wired correctly, one can integrate multiple solar chargers (of varying types and ratings) for battery charging. This entails each charger connecting directly to battery terminals to prevent interference with voltage readings among controllers. Modern controllers ensure minimal reverse current flow.

Quick Reference Flow Chart for Selecting a Solar Charge Controller

If you require a solar charge controller

Yes

No

The solar panel Vmp exceeds:
- 19V for a 12V battery
- 34V for a 24V battery
- 49V for a 36V battery
- 64V for a 48V battery

The solar panel Vmp fits within:
- 17-19V for a 12V battery
- 30-34V for a 24V battery
- 43-49V for a 36V battery
- 56-64V for a 48V battery

Yes

A PWM and MPPT will perform comparably A boost controller is necessary

No

An MPPT controller will provide optimal performance

The solar panel Vmp is lower than:
- 13V for a 12V battery
- 26V for a 24V battery
- 41V for a 36V battery
- 43V for a 48V battery

Interested in exploring further about PWM MPPT Charge Controller? Contact us today for an expert consultation!