Charge controller

A charge controller is the connection point between the PV source and energy storage. It may also have the capability to power small DC loads.

The charge controller in an off-grid PV system serves as the connection point between the PV source and the energy storage system. Every type of energy storage has specific charging and discharging preferences that must be taken into account to ensure that a long life (See the article on lead acid batteries for specific details). The charge controller works to manage the incoming power from the PV source to maximize charging when the batteries can accept it and to reduce it when batteries begin nearing full. Overcharging a battery will cause the chemicals and materials in battery to breakdown and generate significant amounts of heat, which will lead to reduced battery life or permanent damage to the battery. Chronic undercharging of a battery is more common and occurs when a battery is not allowed to return to a full state of charge on a regular basis, which will lead to a build up of sulfation on the lead plates inside of the battery which over time will reduce battery life. A charge controller often cannot protect an energy storage system from over-discharge due to the operation of loads, an off-grid system should always include a low voltage disconnect that is integrated into the charge controller, inverter or is a seperate piece of equipment.

There are a variety of different charge controller designs on the market that vary greatly in voltage/current capacity, performance, functionality and cost. Investing in a quality charge controller will ensure the longevity of the other components in an off-grid PV system.

Charging phases

The different charging stages of a charge controller with current (I), voltage (V) and state of charge (battery icon).

All battery chargers for lead acid batteries, not just charge controllers for PV systems, follow the same basic three stage charging pattern: bulk, absorption and float. A charge controller moves through these different stages based upon programmed voltage set points and the ambient battery temperature or ambient temperature. Smaller capacity and lower cost charge controllers may not offer the capability to program the voltage set points and will rely on values set by the manufacturer. If the charge controller does enable the programming of the voltage set points, the user manual for that specific battery should be consulted as the voltage set points vary based upon manufacturer and battery type (FLA, AGM, GEL).

Bulk phase

When a battery is between 0-80% state of charge, the charge controller will send the full current of the PV source to the battery bank to bring the voltage of the system up. The charge controller will continue supplying full current in bulk mode until a certain voltage is reached, which is typically around 14.6-14.8V for a lead acid battery.

Absoprtion phase

As the battery becomes more full at around 80% of capacity, the charge controller switches to absorption mode at which point it attempts to hold the battery bank at the maximum voltage achieved during the bulk charging phase (~14.4-14.8V for a 12V lead acid battery) using the minimum amount of current necessary to do so. As the battery becomes more full, the amount of current required to hold it at the fixed voltage decreases. The charge controller will continue in this mode until either a set amount of time has passed or the amount of current required to hold the battery bank at a fixed voltages decreases to a programmed minimum. This typically occurs at around 95% of battery capacity.

Float phase

A battery nearing complete charge can no longer accept as much current, so the charge controller moves into the float phase, which means that it tries to hold the battery bank at a lower voltage (~13.2-13.8V for a 12V lead acid battery) using the minimum amount of current necessary. The slow gradually charge can bring the batteries up to a 100% state of charge.

Equalization charge

An equalization charge is not a standard charging phase, it is a planned over-charge of the batteries that can help to reduce the long-term deterioration of batteries due to a build up of sulfation on the lead plates inside. The voltage of the battery bank may be increased as high as ~16.2V for a specified period of time. Not all charge controllers have this capability. Only flooded lead acid batteries can undergo an equalization charge. The user must either schedule or activate an equalization charge and it should only be done a day with abundant sun as the over-charge will require more energy than normal.

Charge controller types

A comparison between the performance of a PWM charge controller and an MPPT charge controller on a cool day.

There are two main types of charge controllers used in off-grid PV installations: pulse width modulation (PWM) and maximum power point tracking (MPPT). Both types of charge controllers continue to be popular as each as distinct advantages depending upon the application.

Pulse width modulation (PWM)

The simpler of the two options, a PWM charge controller measures the voltage of the battery and the temperature (ambient or battery temperature) to estimate the state of charge of the battery and regulate charging. This type of charge controller can only limit the amount of current that is supply to the battery bank and not the voltage of the battery bank or PV source. The PV source may be capable of operating at a higher voltage and supplying more power, but this type of charge controller does not offer this functionality.

As the charge controller cannot regulate the voltage of PV source, the modules/array must be designed to work with the voltage of the battery bank. This means that the PV source will have to operate at a relatively low voltage. There are limited module configurations that will work properly with a PWM charge controller:

• A 36 cell module is referred to as a 12 volt nominal module and will be able to supply an appropriate voltage to a 12V battery bank. These modules can be put in parallel to supply more power for a 12V battery bank or can be connected together in series (2 per series string for 24V battery bank and 4 per series string for a 48V battery bank).
• A 72 cell module is referred to as a 24V nominal module and will be able to supply an appropriate voltage to a 24V battery bank. These modules can be put in parallel to supply more power for a 24V battery bank or can be connected in series (2 per series string for a 48V battery bank).

Ratings

• Voltage: 12V, 24V, 48V
• Current: 6A-60A

PWM or MPPT

• Cannot regulate voltage therefore can only be used with 36 cell (in parallel for 12 volt systems, two in series for 24 volt systems, or 4 in series for 48 volt systems) or 72 cell modules (in parallel for 24V systems and two in series for 48 volt systems). • Significantly cheaper than MPPT charge controllers – can be less than half the price for the same size charge controller. • Lower performance than MPPT charge controllers in cold climates.