A Primer on Solar Charge Controls

A primer on solar charge controls

A brief search on eBay turned up some 4700 listings for solar charge controls. Prices on 12 /24V units range from about 7 to $250. With all the specifications and buzzwords that are thrown around, many are understandably confused. This discussion should be helpful in making the best selection.

Why a solar charge control is required

The typical 12V (nominal) solar panel puts out about 18V NL (No-Load) in full sunlight. If this solar panel is connected directly across a 12V battery, it will attempt to charge it to the no-load voltage, and if the available current is sufficiently high, it will boil the battery electrolyte and damage the battery due to overcharging. The loss of electrolyte is also a consideration as it is not normally necessary to top off liquid electrolyte more than once or twice per year.

Solar controls also include a reverse polarity diode to prevent the battery from discharging into the solar panel under low light conditions.

One exception (there are always exceptions) is low power charging using 2 to 5W solar panels with large 6 or 12V batteries –not recommended for small batteries. In this case, the solar panel is acting as a low current trickle charger and such needs no voltage regulation. This is a compromise and I believe that battery life may suffer somewhat.


Charge Control Matrix

Charge Control Matrix

Linear controls

The linear solar charge regulator is simply a series voltage regulator that drops the panel voltage to the desired battery maximum charge voltage. These are the only controls recommended for charging small batteries. They may also be used with large batteries. Most of my electroschematics designs fall into this category. Note that there are very few linear charge regulators available commercially –likewise, there are very few 6V charge controls available commercially. For hobbyists who want to make their own control, I recommend the first of the following list.


3A Solar Charge Control Schematic

3A Solar Charge Control Schematic

3A 6/12V solar charge control

My other linear charge controls on electroschematics

  • 6V LDO solar charge control
  • 12V LDO solar charge control
  • solar battery charger circuit

One of my designs is a low voltage shunt voltage regulator –good for solar panels with marginally low output voltage. 6v solar charge shunt regulator

Switch mode solar charge controls

Switch mode controls fall into two basic categories: low frequency (e.g. 0.2HZ) or high frequency PWM (Pulse Width Modulation (e.g. 10kHZ). (Note that the MPPT control may also fall into this category, but I have chosen to put it into a special category.)

Low frequency solar charge controls fall into two categories: Relay based and SSS (Solid State Switching). While the relay based control is considered by some a dinosaur, it is actually the workhorse of the industry. It functions by picking up a relay every few seconds and remaining picked up until the voltage reaches the cut-out level, at which time it drops and waits for the next cycle. The SSS control works in similar fashion, but uses a MOSFET instead of relay contacts. The MOSFET has the advantages of being able to work on both 12 & 24V systems, and offers longer life because there are no moving parts to wear out –this is the most common type of control available. They are well suited for charging large batteries at a relatively low rate (e.g. charge time is greater than perhaps 4hours). They are not suited for charging small batteries.

Switch based solar charge controls on electroschematics

  • 6A 6V relay based solar charge control
  • 6V SSS based solar charge control
  • 12V relay solar charge control
  • SSS solar charge control

PWM Controls use buck regulator topology (switching regulator) and operate at a high frequency –the higher the frequency, the smaller the series inductor. The duty cycle of the (on-time /period) may be set to optimize performance by increasing the output current so that it exceeds that of the solar panel.

Some approach MPPT performance by regulating solar panel voltage to a relatively high level, but generally it is a one size fits all method to avoid tricky adjustments. Also, it cannot efficiently operate over wide operating conditions. In general, these are not recommended for small battery applications.

Basic buck regulator schematic

Basic Buck Regulator

Basic Buck Regulator

I have seen cheap PWM controls that omitted the inductor –while they basically function; I consider them inferior. However, not having the buck inductor avoids the requirement of a free-wheeling diode, and that is one lossy component.

MPPT charge controls

MPPT (Maximum Power Point Tracking) controls squeeze as much power as possible out of the solar panel by optimizing the PWM duty cycle. In such controls, the output current actually exceeds the input current, but do not be deceived, it is not a (false) above unity device because the output power does not exceed the input power. To optimize the PWM duty cycle, it continually calculates the output power while modifying the PWM duty cycle –if the new power calculation indicates an increase, it continues to vary the duty cycle in the same direction –if the power calculation indicates a decrease, it instead changes the duty cycle in the opposite direction. As a result, the duty cycle is always dithering around the point of maximum power transfer. This technology provides a marginal current increase of about 10 to 30%. While the current increase is limited, the number of solar panels may be reduced by the same factor –the cost of solar panels is significant!

I have experimented with this technology and have devised a simple algorithm that enables MPPT control without the requirement of arithmetic multiplication or a microcontroller. One such control is a 6 to 12V boost converter –somewhat of an anomaly in this field. solar boost converter MPPT charge controller

In the future, I hope to publish a similar control that uses buck converter technology.

High end grid tie charge controls

GTI (Grid Tie Inverter) units range from 300 to $1200. Most of these are designed for 24 and 48VDC applications. To run the electric utility meter backwards (sell energy to the electric utility) the inverter must force current back into the line –when the line voltage is positive, the ‘load’ current is NEGATIVE, and vise versa when the line voltage is negative, the current is POSITIVE. This is accomplished by inverting the low voltage to 175VDC (115VAC mains) or 350VDC (230VAC mains). When the IGBT (Insulated Gated Bipolar Transistor) power devices switch on, the current conducts from the higher inverter source voltage to the lower instantaneous line voltage. The current (sine wave for high power factor units) is subsequently controlled via PWM techniques. While I am familiar with the technology, I am otherwise inexperienced in this type of control.

Synchronous rectification

Efficiency is a major goal, so the elimination of lossy devices such as silicon rectifiers is in order. Schottky rectifiers reduce losses to half, but synchronous rectifiers (use of MOSFETs as rectifiers) reduce losses to perhaps 10%. Some of the high end controls make good use of this technology. I hope to post some circuits using synchronous rectifiers in the future.

A missing specification

When evaluating controls, remember this important detail: What powers the control? the solar panel? or the battery? If it is the battery, it will tend to discharge the battery when the sun is not shining (or most of the time). Many controls do not specify quiescent battery current drain because it is embarrassingly high, so make sure you measure it –it should be a required specification.