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PWM Solar Charge Controller
Nov 05, 2018

 A charge controller which goes between the solar panels and the battery bank is with the function of  preventing the solar panels from overcharging the batteries. The algorithm, or control strategy, of a charge controller determines the efficiency of battery charging and solar panel utilization, which ultimately influences the ability of the system to meet the load demands, and life of  the battery.

 off grid solar system

PWM stands for Pulse Width Modulation (PWM), it is the most effective means to achieve constant voltage battery charging by switching the solar system controller’s power devices. When in PWM regulation, the current from the solar array decreases in responding to the battery’s condition and recharging needs.


PWM solar chargers employ technology like other modern high quality battery chargers. When a battery voltage reaches the regulation set-point, the PWM algorithm slowly reduces the charging current to avoid heating and gassing of the battery, yet the charging continues to return the maximum amount of energy to the battery in the shortest time. The result is a higher charging efficiency, rapid recharging, and a normal battery at full capacity.


Three Stage of PWM Charging


1. Bulk Charge

Bulk StageThe primary purpose of a battery charger is to recharge a battery. This first stage is typically where the highest voltage and amperage the charger is rated for will actually be used. The level of charge that can be applied without overheating the battery is known as the battery's natural absorption rate. For a typical 12 volt AGM battery, the charging voltage going into a battery will reach 14.6-14.8 volts, while flooded batteries can be even higher. For the gel battery, the voltage should be no more than 14.2-14.3 volts. If the charger is a 10 amp charger, and if the battery resistance allows for it, the charger will put out a full 10 amps. This stage will recharge batteries that are severely drained. There is no risk of overcharging in this stage because the battery hasn't even reached full yet.


2. Absorption Charge

Absorption StageSmart chargers will detect voltage and resistance from the battery prior to charging. After reading the battery the charger determines which stage to properly charge at. Once the battery has reached 80%* state of charge, the charger will enter the absorption stage. At this point most chargers will maintain a steady voltage, while the amperage declines. The lower current going into the battery safely brings up the charge on the battery without overheating it. This stage takes more time. For instance, the last remaining 20% of the battery takes much longer when compared to the first 20% during the bulk stage. The current continuously declines until the battery almost reaches full capacity.


3. Float Charge

Float StageSome chargers enter float mode as early as 85% state of charge but others begin closer to 95%. Either way, the float stage brings the battery all the way through and maintains the 100% state of charge. The voltage will taper down and maintain at a steady 13.2-13.4 volts, which is the maximum voltage a 12 volt battery can hold. The current will also decrease to a point where it's considered a trickle. That's where the term "trickle charger" comes from. It's essentially the float stage where there is charge going into the battery at all times, but only at a safe rate to ensure a full state of charge and nothing more. Most smart chargers do not turn off at this point, yet it is completely safe to leave a battery in float mode for months to even years at a time.

 3 stage-charge controller

Features of a PWM Charge Controller

1. Ability to recover lost battery capacity and desulfate a battery. 

2.  Dramatically increase the charge acceptance of the battery. 

3. Equalize drifting battery cells. 

4. Reduce battery heating and gassing. 

5. Automatically adjust for battery aging. 

6  Self-regulate for voltage drops and temperature effects in solar systems


Major Functions Performed by Solar Charge Controllers


Besides the primary function of any charge controller is to control the amount of charge entering and exiting the battery, solar charge controller perform several other useful functions:

1. Block reverse current

This function facilitates a unidirectional flow of current from the solar panel to the battery, and blocks the reverse flow during the night.

2. Under voltage protection

Under voltage occurs when the batteries have lost 80% of their charge. It is recommended to take the battery out from the circuit and connect it back only during charging.

3. Prevent Battery Overcharge

The charge controller stops the charging of the batteries once these are sufficiently charged.

4. Configure Control Set Points

Various set points could be edited and re-programmed using the charge controllers. This helps in fine tuning of your battery charging and discharging cycles to ensure the most efficient performance and longer life.

5. Displays and Metering

Some commonly monitored parameters include: Voltage level, Charged percentage, Current Discharge time at fill load, etc.

6. Troubleshooting and Events History

Some charge controllers have an built-in memory to save events and alarms with a date and time stamp. This events and alarms history helps for quick troubleshooting.


Programmable Parameters


There are four key parameters that can be programmed in charge controllers.

1. Regulation Set-Point

This is the maximum set-point voltage. Any charge controller will protect the battery to reach a voltage in excess of this Voltage. At this point, it will discontinue any further battery charging.


2. Regulation Hysteresis Set-Point

This is the difference between the Regulation Set-Point Voltage and Voltage when full current is reapplied, also called Regulation Hysteresis Voltage Span. This set point should be as high as possible to prevent switching disruptions and harmonics.


3. Low Voltage Disconnect Set-Point

This is the minimum set-point voltage. Any controller will not allow the battery to reach a voltage lower than this Voltage. At this point, it will disconnect the load to prevent battery under discharge.


4. Low Voltage Disconnect Hysteresis Set-Point

This is the difference between the Low Voltage Disconnect Set-Point and Voltage at which the load will be reconnected, also called Low Voltage Disconnect Hysteresis Voltage Span. This set point should be as high as possible to prevent frequent disruptions to the connected load.


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