Charge Controllers for Solar Systems

Top view of solar panels on houses - solar charge controllers

This article is excerpted from Lonny Grafman and Joshua Pearce’s book, To Catch the Sun: Inspiring Stories of Communities Coming Together to Harness Their Own Solar Energy, and How You Can Do It Too!

A charge controller regulates battery voltage and current to control the charging rate, or the state of charge, for batteries and/or loads. As discussed in Section 5.1, “Panels,” solar panels output varying voltage. Under full sun, the voltage from a solar panel may be quite a bit higher than the nominal voltage of the system. The need for a charge controller arises because many electronic devices are sensitive to voltage fluctuation, and overvoltage can destroy them. In addition, batteries can quickly be destroyed by overvoltage. In addition, some charge controllers use charging algorithms that can prolong the life of a battery (i.e. 3-stage charging or “smart charging” for lead acid batteries).

Charge controllers are available in many voltages, currents, and configurations, such as: no controller, voltage regulator, basic charge controller with low voltage disconnect (LVD), pulse width modulation (PWM) controller, and maximum power point tracking (MPPT) controller.

A quick note on charge controllers: most charge controllers have a prescribed order of connection, and disconnection, that must be followed. That order of connection is often Battery first, Panels second, Loads third. That order of disconnection is usually exactly opposite, i.e., Loads first, Panels second, Battery last.

No Controller


In some cases, a charge controller is not necessary. The two primary cases are when (a) a DC load is not sensitive to the range of voltages from the solar panels and (b) extremely low current panels are connected to a large battery. The first case may occur with a solar fan or solar pump, for instance. The second case is called a trickle charger.

Voltage Regulator


A voltage regulator (Figure 5.9) is a simple charge controller that caps the voltage at a maximum voltage. A voltage regulator is usually best paired with a photovoltaic system that contains no battery, since many more complicated charge controllers need a battery. Voltage regulators are usually less expensive than more complex charge controllers. Often the power with any voltage above the cap is lost as waste heat, so there can be significant power loss if the input voltage is much higher than the output voltage. These may be as simple as a shunt or relay that redirect the energy to a big resistor.

Figure 5. 9 - solar charge controllers
Figure 5.9
Adjustable voltage regulator (left) that accepts inputs between 8-22 V and adjustable output between 1-15 V. USB charger (right) that accepts input voltages 9-32 V and outputs 5 V.

Basic Charge Controller with a Low Voltage Disconnect (LVD)


A basic charge controller (Figure 5.10) may combine voltage regulation with a low voltage disconnect (LVD). The LVD disconnects loads when the battery voltage is low, at which point the battery may be damaged. These will also often have an integrated display that shows vital measurements of the system. All charge controller systems will generally require a battery.

The maximum current is often quite limited through a low voltage disconnect. Therefore, higher power devices should still be connected directly to the battery instead of being controlled through the LVD.

Figure 5. 10 - solar charge controllers
Figure 5.10
Basic charge controller with a LVD. Simple LEDs indicate system status.

Pulse Width Modulation (PWM) Controller


A PWM controller (Figure 5.11) rapidly connects and disconnects the panels to the batteries as a way to charge the batteries more efficiently than a voltage regulator. While PWM controllers are more expensive than basic charge controllers, they lose less power to waste than a basic charge controller or voltage regulator. These will almost always include LVDs and integrated displays as well.

Figure 5. 11 - solar charge controllers
Figure 5.11
Very simple PWM controller (left) with no LVD nor display takes up to 30 V and 4.5 A in from panels and outputs 14.1 V with built in lightning protection. More advanced PWM controller (right) with LVD and display takes up to 100 V and 30 A in from panels and outputs charging for a 48 V battery system with a settable float charge of 55.2 V.

Maximum Power Point Tracking (MPPT) Controller


A MPPT controller (Figure 5.12) uses internal converters to charge the batteries at the required voltage while receiving power from the solar panels closest to their maximum power point seen in Figure 4.8. While MPPT controllers are more expensive than PWM charge controllers, they are even more efficient than a PWM controller. These will also almost always include an integrated display.

A significant advantage of a MPPT controller is that it allows the panel voltage to be substantially higher than the battery voltage. This may be needed due to the available panels, and it can also be advantageous because higher panel voltage will allow you to use smaller (and therefore less expensive) wire diameter from the panels.

Another advantage is that MPPT charge controllers do not need to be oversized, and in fact they can be undersized if the situation calls for it. For example, a 70 A MPPT can be connected to a 72 A solar array. The extra 2 A will be lost, reducing the system power, but nothing will break as it would with other charge controllers.

Figure 5. 12 - solar charge controllers
Figure 5.12
MPPT controller (vertical black rectangle in top center) takes up to 150 V and 80 A from panels and outputs charging for 12, 24, 36, 48, or 60 V battery systems.

Charge Controller Comparisons


Charge controllers are available for specific voltage systems, are constrained by a maximum current, and may have advanced features such as advanced battery charging modes. As usual, needs and budget will need to be balanced to select the best charge controller. For high voltage panels, such as those that come built for grid inter-tie, a MPPT is usually mandatory. Table 5.1 shows various aspects to consider and compare when selecting a type of charge controller.

Charge Controller TypeCostEfficiencyEase of Use/RepairEase of System Matching
No controllerLowestHighHighestLow
Voltage regulatorLowLowHighMedium
Basic charge controller with LVDMiddleMiddle-LowLowMedium
PWM controllerMiddle-HighHighLowMedium
MPPT controllerHighestHighestLowHigh
Table 5.1
Comparisons of charge controllers

Displays


Many charge controllers come with a display (Figure 5.11 right). Displays can typically show voltage, current, power and keep track of energy (note that most displays only track approximate energy and can drift as the battery ages). In addition, a display can be added as a separate component (Figure 5.13). Displays help you track and maintain the health of a system as well as keeping track of its effectiveness.

Figure 5. 13 - solar charge controllers
Figure 5.13

Small DC display for voltage, current, power, and energy over time installed at Six Rivers Charter Highschool.

To catch the sun cover - solar charge controllers

Copyleft © 2021 by Lonny Grafman and Joshua Pearce.
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. Published in the United States by Humboldt State University Press.

Get a free PDF of To Catch the Sun at:  https://tocatchthesun.com/ 

Image credit: Kindel Media

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