Selecting Solar Charge Controller

If you are planning to install an off-grid solar system with battery bank, you’ll need a Solar Charge Controller. Charge controllers act as a gateway to your battery, and prevent the battery from being over/under charged by the solar panel.

A solar charge controller regulates the voltage and current coming from the solar panel . As the input voltage from the solar panel rises, the charge controller regulates the charge to the batteries preventing any over charging and disconnect the load when battery is discharged.

Types of solar charge controllers

There are two types of charge controllers popular in market :

1. Pulse Width Modulation (PWM) controllers

2. Maximum Power Point Tracking (MPPT) controllers.

PWM Solar Charge Controller:

PWM stands for Pulse Width Modulation, which stands for the method it use to regulate charge. It uses a transistor to connect and disconnect the solar with the battery.

By switching the transistor at high frequency with various pulse widths, a constant voltage can be maintained. The PWM controller self-adjusts by varying the widths (lengths) and frequency of the pulses sent to the battery.

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

The PWM charge controller regulates the charge by opening the solar panel during regulation (at high frequency), if you were to measure the array voltage during this time, the Solar panel voltage can be anywhere between battery voltage and open circuit voltage depending on the regulator’s charging stage.

PWM controllers are cheaper and more suited for small DIY solar systems with a couple of low voltage solar panels.

MPPT Solar Charge Controller:

The Maximum Power Point Tracking (MPPT) charge controller, the Solar panel voltage vary from the battery voltage. By varying the solar panel input, the charge controller can find the point at which the solar panel produces the maximum power.The solar panel voltage is varied by a DC/DC voltage converter that can take excess Solar panel voltage and convert it into extra current at a lower voltage without losing power.

MPPT controllers do this via an adaptive algorithm that follows the maximum power point of the Solar panel and then adjusts the incoming voltage to maintain the most efficient amount of power for the system.

MPPT controller more efficient than PWM controller. The efficiency of a typical MPPT controller is around 94-99% .

Rating Of Charge Controller :

To select the proper Charge Controller, you have to know the maximum output current of the solar panel and Battery Voltage. The maximum possible current that a Solar panel can generate is the “short circuit current,” indicated as Isc in the panel’s label or specs sheet.

Now select a charge controller with,

1. Current Rating more than Short Circuit Current (Isc) with consideration of a safety factor ( 1.25 x 125 = 1.56 )

2. Voltage Rating same as the battery voltage.

Safety Factor :

1. We use a standard factor to account for all Solar panel output-boosting circumstances like a sunny day with very clear snow pack. (additional light reflected off the snow). That factor is 1.25 or 125%

2. A second de-rating factor may be required for systems in continuous operation, additional protection must be included, according to the National Electric Code (NEC), to allow for heat and equipment stress. This factor is also 1.25 or 125%.

So the overall safety factor is 1.25 x 1.25 = 1.56

Sample Calculation

For a typical 100 Watt Solar Panel, the Short Circuit Current (Isc) is 6.32 A.

Charge controller rating = Isc x Safety Factor = 6.32 x 1.56 = 9.85A

In this case, 9.85 Amp or higher rated (10A) Charge Controller would be recommended.

The battery voltage selected is 12V.

So, the solar charge controller rating is selected as 10 Amps /12 Volt

Wiring Of Solar Charge Controller:

PWM or MPPT ?

When you are finding which type of solar charge controller to purchase, you need to know about their functionality and features but it’s also helpful to see a straightforward comparison of your options. For easier selection, look the below comparison table for PWM and MPPT Solar Charge Controllers.

Pros and Cons between PWM vs MPPT
Pros and Cons between PWM vs MPPT

PWM Vs MPPT Charge Controller

I hope, now you have some confidence on selecting the Solar Charge Controller for your DIY Off- Grid Solar System. If you like this article, please share it. Thank you !

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