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What You Need to Know About Solar Charge Controllers

Creation date: Jan 23, 2024 12:09am     Last modified date: Jan 23, 2024 12:09am   Last visit date: Feb 23, 2024 7:04pm
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Jan 23, 2024  ( 1 post )  
Wang Meihong (chenluseo)

What Is a Solar Charge Controller?

Charge controllers or charge regulators are basically voltage and/or current regulators that prevent batteries from overcharging. In most 12 volt panels, the voltage and current from the solar panels to the battery is regulated, so if there is no regulation, the batteries will be damaged from overcharging. To fully charge a battery, 14 to 14.5 volts are needed.

Is a Charge Controller Always Necessary?

It isn't always necessary to use a charge controller with small maintenance panels, or trickle charge panels, such as those of 1 to 5 watts. The rough rule of thumb is that you don't need one if the panel draws 2 watts or less for every 50 amp-hours of battery power.

For example, a standard flooded golf car battery is around 210 amp-hours. If you want to keep up a pair of 12 volt batteries for maintenance or storage, a 4.2 watt panel would do the job. A 2- to 2-watt panel can be used when maintaining AGM deep cycle batteries, such as the Concorde Sun Xtender. 5 watt panels are close enough without requiring a controller.

Why 12 Volt Panels are 17 Volts?

The natural question that arises is, "Why not design solar panels to produce precisely 12 volts?" The rationale behind this lies in practical considerations. If panels were tailored to output only 12 volts, they would effectively generate power solely under optimal conditions—when cool, with perfect sunlight, and in full sun. However, such conditions are not consistently guaranteed in most locations.

To account for variations such as low sun angles, heavy haze, cloud cover, or high temperatures, solar panels are designed to deliver additional voltage. This ensures that even in less-than-ideal scenarios, such as when the sun is low in the sky or atmospheric conditions are less favorable, the panel can still generate some output. It's worth noting that a fully charged "12-volt" battery typically measures around 12.7 volts at rest (approximately 13.6 to 14.4 volts under charging conditions). Hence, the panel must generate at least this amount of voltage even under adverse conditions.

The role of the charge controller comes into play here, regulating the panel's output, which ranges from 16 to 20 volts, to match the immediate requirements of the battery. This voltage adjustment can vary between approximately 10.5 and 14.6 volts, contingent on factors such as the battery's state of charge, battery type, the operational mode of the controller, and ambient temperature. The charge controller ensures that the solar panel's output aligns with the dynamic needs of the battery system.

Utilizing High Voltage (Grid Tie) Panels with Batteries

Consequences of Using a Standard Controller

Standard controllers, excluding Maximum Power Point Tracking (MPPT) types, may function with high voltage panels as long as the charge controller's maximum input voltage is not surpassed. However, a notable drawback is the substantial loss of power—ranging from 20 to 60% of the panel's rated output. Standard charge controllers channel current to the battery until it reaches full charge, usually around 13.6 to 14.4 volts. As a panel has limitations on the number of amps it can output, the reduction in voltage, while maintaining the rated amps, results in diminished power transfer. For instance, a 175-watt panel rated at 23 volts/7.6 amps may only deliver approximately 7.6 amps at 12 volts into the battery. According to Ohm's Law (Watts = Volts x Amps), the 175-watt panel would contribute around 90 watts to the battery.

Optimizing High Voltage Panels with an MPPT Controller

To harness the full power potential of high voltage grid tie solar panels, an MPPT controller is indispensable. MPPT controllers, detailed in the link above, can accommodate high voltage inputs, often up to 150 volts DC (some even higher, up to 600 VDC). This allows the possibility of connecting two or more high voltage panels in series, reducing wire losses and enabling the use of smaller wire gauges. For example, taking the previously mentioned 175-watt panel, linking two in series would yield 46 volts at 7.6 amps into the MPPT controller. The controller then converts this to approximately 29 amps at 12 volts, optimizing power transfer efficiency.

Types of Charger Controllers

Charge controllers are available in a diverse array of sizes, shapes, features, and price points, catering to the specific needs of solar power systems. The spectrum ranges from compact 4.5-amp controls like the Sunguard, to advanced 60 to 80-amp MPPT programmable controllers equipped with computer interfaces. In instances where currents surpass 60 amps, it's common practice to parallelly wire two or more 40 to 80-amp units. However, the most widely used charge controllers in battery-based systems typically fall within the 4 to 60-amp range.

These controllers can be broadly categorized into three types, each serving distinct purposes with some degree of overlap:

1.Simple 1 or 2-Stage Controls

These controls, relying on relays or shunt transistors, manage voltage in one or two steps. Though considered outdated, they still persist in older systems, and budget options available online may feature these simple controls. Their primary advantage lies in their reliability, attributed to their minimal components.

2.3-Stage and/or PWM Controls

Regarded as the industry standard, 3-stage and/or Pulse Width Modulation (PWM) controls have become prevalent. They supersede the older shunt/relay types, which are occasionally encountered in low-cost systems from discounters and mass marketers.

3.Maximum Power Point Tracking (MPPT) Controllers

Representing the pinnacle of controller technology, MPPT controllers, albeit priced higher, boast efficiencies in the impressive range of 94% to 98%. They offer substantial cost savings for larger systems, providing 10 to 30% more power to the battery. 

Equalization Explained

Equalization, as the name suggests, aims to balance or equalize the charge among all cells in a battery or battery bank. Essentially, it involves a period of intentional overcharge, typically in the range of 15 to 15.5 volts. If certain cells within the battery string have lower charges than others, equalization brings them all up to full capacity. In flooded batteries, this process also serves the vital function of agitating the liquid by generating gas bubbles. While this might not have a significant impact in constantly moving environments like RVs or boats, it becomes beneficial when the vehicle is parked for extended periods, preventing uneven charging. In off-grid systems, generators with chargers can also be used for battery equalization.

Understanding PWM

Many charge controllers offer a "PWM" mode, which stands for Pulse Width Modulation. PWM is commonly utilized as a method of float charging. Instead of providing a steady output, the controller sends out a series of short charging pulses to the battery—a rapid "on-off" switch. The controller continuously assesses the battery's state to determine the pulse frequency and duration. In a fully charged battery with no load, the pulses may occur every few seconds with short durations. In a discharged battery, the pulses might be longer and nearly continuous, or the controller may switch to "full-on" mode. While PWM is effective, it can generate interference in radios and TVs due to the sharp pulses it produces.

Load or "Low Voltage Disconnect" Output

Certain controllers feature a "LOAD" or Low Voltage Disconnect (LVD) output, designed for smaller loads like lights and small appliances. This output includes a low voltage disconnect function, preventing excessive battery drain by turning off connected devices. The LOAD output is commonly used for non-critical loads, such as lights. However, it is not suitable for running anything beyond very small inverters. Inverters with high surge currents may damage the controller.
While the LVD function is not necessary for most systems, it finds use in scenarios with smaller loads. The controller's rating determines the capacity, typically ranging from 6 to 60 amps. In RVs, remote systems, and unattended sites like camera, monitor, or cell phone installations, the LOAD or LVD output proves useful for managing modest loads efficiently.
Most systems do not need the LVD function - it can drive only smaller loads. Depending on the rating of the controller, this may be from 6 to 60 amps. You cannot run any but the smallest inverter from the LOAD output. On some controllers, such as the Morningstar SS series, the load output can be used to drive a heavy duty relay for load control, gen start etc. The LOAD or LVD output is most often used in RV & remote systems, such as camera, monitor, and cell phone sites where the load is small and the site is unattended.
We have a complete list of all our charge controllers on our web store, and you can order online.