Shunt regulators function by short circuiting the solar array when the battery reaches a set voltage. When the battery voltage drops, the array is un-shorted and current is allowed to flow to the battery again.

This is also sometimes referred to as a pulse regulator, since the current can be “pulsed” to the battery as the array current is regulated. As the charge regulation is either on or off, it’s simply a single stage charge controller. As the regulator sees the full current from the solar array during regulation, the shunt regulators get hot and are generally only used for small solar arrays.

Shunt regulators are generally solid-state and contain a blocking diode and a transistor. The solar array is shorted by a transistor (or relay) and the blocking diode prevents the battery from being shorted at the same time. Shunt regulators are generally for negatively grounded systems only as the block diode is usually in the positive line.

Shunt regulators are on/off type controllers. This means the solar array is either on or off; the battery sees the full charge current available or none. The regulator allows current from the array to flow to the battery until the disconnect voltage is reached, at which time the solar array is shorted, preventing any further current to flow to the battery. Without any charge current, the battery voltage will drop until the reconnect voltage is reached at which time the regulator will allow current to flow to the battery again. The battery voltage will rise and the cycle will repeat.

When the shunt regulator shorts the array during regulation, measuring the array voltage during this time will yield an array voltage that should be less than 1V. During normal charging, the array voltage should be slightly higher than the battery voltage (battery voltage + the voltage drop from diodes or transistors). If array open circuit voltage was ever measured during normal operation, this would indicate a problem.

Series regulators function by open circuiting the solar array when the battery reaches a set voltage. When the battery voltage drops, the array is reconnected and current is allowed to flow to the battery again.

Series regulators generally use a relay or transistor to connect and disconnect the solar array. As the relay (or transistor) can be placed in either the positive or negative line, Series regulators can be used in positive and negative ground systems.

Series regulators (similarly to shunt regulators) are on/off type controllers. The solar array is either on/off, so the battery sees the full charge current or none. The regulator allows the current from the array to flow to the battery until the disconnect voltage is reached, at which time the solar array is disconnected (open circuited) and prevents any further current to flow to the battery.

Without any charge current, battery voltage will drop until the reconnect voltage is reached, at which time the regulator will allow current to flow to the battery again. The battery voltage will rise, and the cycle will repeat. It is sometimes referred to as a pulse regulator, since the current can be “pulsed” to the battery as the array current is regulated. The duration of the pulses can be from hours to seconds depending on: battery SOC & health, load current, temp., etc.

Unlike shunt regulators, some series regulators can control multiple relays (or transistors), allowing for multiple disconnect/reconnect set points and stepped charge current. If the series regulator has a single relay, it is simply a single stage charge controller. Additional relays with different set points can make the regulator a multi-stage controller.

As the regulator opens the solar array to regulate the battery voltage, series regulators run much cooler than shunt regulators (especially if a relay is used instead of a transistor). For this reason, series regulators are well suited for large solar arrays.

When the series regulator opens the array during regulation, measuring the array voltage during this time will yield an array voltage that should be close to the open circuit value. During normal charging, the array voltage should be slightly higher than the battery voltage (battery voltage + the voltage drop from diodes or transistors). If an array voltage value is less than the battery voltage was ever measured during normal operation, this would indicate a problem.

PWM regulators are similar to series regulators, but they use a transistor instead of a relay to open the array. By switching the transistor at high frequency with various modulated widths, a constant voltage can be maintained. The PWM regulator self-adjusts by varying the widths (lengths) and speed of the pulses sent to the battery. Unlike the on/off charge controllers which instantaneously cut off the power transfer to minimize battery overcharging, PWM regulators act like a rapid on/off controller constantly.

When the width is at 100%, the transistor is at full ON, allowing the solar array to bulk charge the battery. When the width is at 0% the transistor is OFF, open circuiting the array preventing any current from flowing to the battery when the battery is fully charged.

Like the series regulator, the transistor can be placed in either the positive or negative line, allowing the regulator to be used in positive and negative ground systems.The difference between the series regulator and the PWM regulator is the PWM of the transistor. When the modulation width is at 100% or 0%, the regulator is essentially a series regulator, it is that modulation width variation that allows the PWM regulator to create a constant voltage to the battery as opposed to the on/off of the series regulator. The below figure shows an example of a PWM regulator regulating with a 70% on 30% off duty cycle.

Some PWM regulators have provisions for converting to a series (on/off) regulator. This could be needed for sensitive loads that have an issue with the noise created by the frequency of the PWM. Some PWM regulators have provisions for converting to a series (on/off) regulator. This could be needed for sensitive loads that have an issue with the noise created by the frequency of the PWM. Because PWM charge controllers require transistors, they are always solid-state; this means heat dissipation can become a problem, especially in larger solar arrays.

As with series regulators, because the PWM regulator regulates by opening the array during regulation (at high frequency), if you were to measure the array voltage during this time, the array voltage can be anywhere between battery voltage and open circuit voltage depending on the regulator’s charging stage. If an array voltage value less than the battery voltage was ever measured during normal operation, this would indicate a problem.

The Maximum Power Point Tracking (MPPT) charge controller takes the PWM to the next level, by allowing the array voltage to vary from the battery voltage. By varying the array input, the charge controller can find the point at which the solar array produces the maximum power. The MPPT process works like this. Imagine having a battery that is low, at 12 V. A MPPT takes a voltage of 17.6 volts at 7.4 amps and converts it down, so that what the battery gets is now 10.8 amps at 12 volts. MPPT controllers takes the DC input from the solar panels, convert it to high frequency AC, and then change it once again to a different DC voltage and current. The point is the voltage will exactly adhere to the requirements of the battery. As the MPPT charge controller uses the negative line as a reference and then switches the positive line, they can be used in negative ground systems only. It is crucial to understand that voltage is a potential difference; the ‘difference’ refers to the difference between ground potential and some potential. This means that the starting point is below zero, but this is only used as a reference point.

Since MPPT charge controllers can vary the charge current to the battery, the regulator can be a multi-stage charger with bulk, absorption, and float settings. They are always solid state; this means heat dissipation can become a problem, especially in larger solar arrays. MPPT controllers are typically step-down converters, so the array voltage always needs to be higher than the battery voltage. Therefore, an array voltage value less than the battery voltage during normal operation would indicate a problem.