How to select and Size Solar Charge Controller for Off-Grid PV System

Updated: Aug 5

If you are planning to install an off-grid solar system with battery bank, you’ll need a Solar Charge Controller. It is a device which is placed between the Solar Panel and the Battery Bank to control the amount of electric energy produced by Solar panels going into the batteries. The main function is to make sure that the battery is properly charged and protected from overcharging.

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 currently two type of charge controllers commonly used in PV power systems :

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. Its function is to pull down the voltage of the solar array to near that of the battery to ensure that the battery is properly charged. In other words, they lock the solar panel voltage to the battery voltage by dragging the Solar panel Vmp down to the batteries system voltage with no change in the current.

It uses a electronics switch ( MOSFET ) to connect and disconnect the solar panel with the battery. By switching the MOSFET 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 MOSFET 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.

Due to the nature of their work mechanism, PWM charge controllers cannot use the maximum power produced by the PV system and so their efficiency is low .

For example, say you connect a 100 Watt panel with a current(Imp) of 5.75A & voltage (Vmp) of 17.40V directly to a 12 V lead acid battery, the panel voltage would be dragged down near to the voltage of the battery but the current stays the same at 5.75 amps.This happens because Solar Panels behave like a current source, so the current is determined by the available sunlight.

Now the power (P)= Vbat x Imp = 12V x 5.75A = 66.6W. So the Solar panel is now behaving like a 66 watt panel.

This equates to a loss of 100W-66.6W = 34W ( 33.4%).

PWM controllers are more suited for small solar system. It provides a low-cost solution and is normally used when the solar cell temperature is between 45°C and 75°C.

Some good PWM Charge Controller in the market

1. Renogy Wanderer 10 Amp 12V/24V PWM Solar Charge Controller

2. ALL POWER 20A Solar Charger Controller with USB Port Display 12V/24V

3. GHB 20A 12V 24V Solar Charge Controller Auto Switch LCD Intelligent Panel

4. WindyNation P30L LCD 30A PWM Solar Charge Controller with Digital Display

5. EPEVER PWM Solar Charge Controller 12/24/36/48V Dual USB with Temperature Sensor and LCD Display

MPPT Solar Charge Controller:

MPPT charge controller extracts the maximum power from the PV module by forcing the PV module to operate at a voltage close to the maximum power point. It has been designed to adjust its input voltage to utilize the maximum power output of the solar array and then transform this power to supply the varying voltage requirement. The input voltage is varied by using a DC/DC converter.

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

The performance advantage of a MPPT controller is substantial (10% to 40%) when the solar cell temperature is low (below 45°C). They are more efficient than PWM controller. The efficiency of a typical MPPT controller is around 94-99% .

To fully exploit the potential of the MPPT controller, the array voltage should be substantially higher than the battery voltage. The MPPT controller is the best solution for higher power systems.

Some good MPPT Charge Controller in the market

1. Renogy Rover 20 Amp 12V/24V MPPT Charge Controller with LCD Display

2. Victron MPPT Charge Controller with Built-In Bluetooth – 75V/15A

3. EPEVER MPPT Solar Charge Controller 30A 100V with LCD Display

4. Renogy Commander 20 Amp 12V/24V MPPT Solar Charge Controller

5. EPEVER MPPT Solar Charge Controller 40A 150V MT50 Remote Meter + Temperature Sensor

Sizing Of Charge Controller :

Choosing the most suitable charge controller requires two steps:

1. Voltage Selection :

The charge controller voltage shall be matched with the system voltage. The standard configurations are 12, 24, and 48 volts. If you are wiring your batteries for 12 volts you need a charge controller that is rated at 12 volts.

Some controllers are voltage specific, meaning that the voltage cannot be changed or substituted. Other more sophisticated controllers include a voltage auto-detect feature, which allows it to be used with different voltage settings.

2. Current Selection :

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,

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

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

PV Charge Controller – Upper Voltage Limit

Charge controllers have an upper voltage limit. This refers to the maximum amount of voltage they can handle from the solar array. Make sure you know what the upper voltage limit is and that you don’t exceed it or you may end up burning out your solar charge controller.

Sample Calculation

Example

Consider a 100W solar panel is used to charge a 12V battery bank.

1. Voltage Rating:

The voltage rating of the charge is controller shall be equal to the system voltage i.e 12V in this case.

2. Current Rating:

From the specification of 100 Watt Solar Panel,

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 Charge Controller would be recommended.

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

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

PWM Vs MPPT Charge Controller

You can read a nice document on selecting the PWM or MPPT charge controller prepared by victronenergy.

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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