Charge controller programming

Example of a programmable charge controller with an LCD screen and buttons.

The simplest charge controllers rely upon factory settings and do not permit any programming. A charge controller is vital to ensuring the longevity or cycle life of batteries, therefore a charge controller that cannot be programmed must be carefully selected to ensure that the factory settings are appropriate for the type, voltage, and size of energy storage that will be used with the system. Charge controllers for all but the smallest systems offer varying degrees of programming from basic settings (system voltage, battery type) to complex modifications to how the charge controller regulates charging. To properly program a charge controller, it is necessary to have the manuals for the charge controller, battery type, and any communications systems on hand.

Basic settings

Charge controllers that allow programming will still often come with pre-programmed settings for different battery types and charging voltages, but it is recommended that all settings are examined and adjusted to the specifications of the specific system.

• Nominal voltage: The nominal voltage of the battery bank. Typically 12V, 24V or 48V.
• Battery type: Adjusts charging parameters for the particular type of battery including temperature correction.
• Energy storage capacity: The total energy storage capacity of the system. This will be in Amp-hours (Ah) for lead acid batteries.
• Low voltage disconnect: If the charge controller has lighting control, the charge controller can often be set to automatically disconnect lighting loads if the battery bank voltage reaches a certain minimum value in order to protect it from deep discharges that can greatly reduce cycle life. Typically set at around 20% state of charge (SOC). It may be also possible to set the value at which the lighting is allowed to reconnect to give the battery bank sufficient time to recharge - a higher value than 20% SOC is recommended.
• Charging set points: The voltages at which different charging phases start and stop vary depending on the battery type and manufacturer. Manufacturers of batteries will provide specific charging specifications in the product manual for bulk, absord, float, and equilization charging. These values will often be given as a range, it is recommended to program the charge controller to the upper end of the range for each of these values.

Advanced settings

Maximum charge rate

The maximum amount of charging current that a type of battery can handle varies based upon its size, type and manufacturer. Specifications for the maxmimum charge rate can be found in the manual for the battery and will typically be given as a percentage of the C-rate. Flooded lead acid (FLA) batteries and Gel batteries generally have a maximum charge rate between 10%-20% of the C/20 rate.. Absorption glass mat (AGM) batteries are often able to accept higher charge currents, sometimes as high as 35% of their C/20 rate..
It is important that the maximum charge rate is set taking into account all parallel strings in the battery bank.

Example 1: A 48V system has 2 parallel strings of batteries rated at 205Ah @ C/20 rate. The recommended charge rate for this type of battery is 12% of the C/20 rate. What is the maximum charge rate for this battery bank?

Maximum charge rate = C/20 rate × parallel strings × manufacturer maximum C/20 rate percentage

Maximum charge rate = 205Ah x 2 parallel strings x .12 (12%) = 49.2A

Timed absorption phase

The absorption phase of a charge controller can be set to hold the battery bank at a steady voltage for a set duration of time. A sufficiently long absorption phase is desirable as it ensures that the battery receives a full charge under substantial current, which is helpful to ensuring a long cycle life. The appropriate duration of an absorption charge depends upon the battery type, the size of the battery bank, and the maximum available charge rate for the system. A manufacturer may give a recommendation in the manual for the particular battery, but often times other sources must be consulted.

Rolls Battery recommends the following approaches for flooded lead acid (FLA) batteries:

Absorption phase time = 0.42 × C ÷ I

• C = 20 hr rated capacity (total AH capacity of battery bank)
• I = Maximum available charging current provided by the PV source. This value can be calculated for different charge controller types as follows:

PWM charge controller maximum available charge current = parallel strings × ISC

MPPT charge controller maximum available charge current = Total PV source power rating ÷ nominal system voltage

This number may be limited by the programmed maximum charge rate for the charge controller or by the maximum output current of the charge controller, in which case that value should be used.

Example 1: A battery bank has 2 strings of 6 Volt 6 CS 25P model batteries. These batteries are 853Ah each. The PV source has a maximum available charging current that is 10% of C/20 or 170Ah (2 parallel strings x 853Ah × .10 (10%) = 170Ah).

Absorption phase time = 0.42 × (853Ah x 2 parallel strings) ÷ 170Ah

Absoprtion phase time = 4.2 hours

Rolls Battery recommends the following approaches for valve-regulated lead acid (VRLA) batteries:

Absorption phase time = 0.38 × C ÷ I

• C = 20 hr rated capacity (total AH capacity of battery bank)
• I = Maximum available charging current provided by the PV source. This value can be calculated for different charge controller types as follows:

PWM charge controller maximum available charge current = parallel strings × ISC

MPPT charge controller maximum available charge current = Total PV source power rating ÷ nominal system voltage

This number may be limited by the programmed maximum charge rate for the charge controller or by the maximum output current of the charge controller, in which case that value should be used.

Example 1: A battery bank has 2 strings of 6 Volt S6-460 AGM batteries. These batteries are 460Ah each. The PV source has a maximum available charging current that is 15% of C/20 or 120Ah (2 parallel strings x 460Ah × .13 (13%) = 120Ah).

Absorption phase time = 0.38 × (460Ah x 2 parallel strings) ÷ 120Ah

Absorption phase time = 2.9 hours

Finish current absorption phase

The absorption phase of a charge controller can be set to hold the battery bank at a steady voltage using the minimum amount of current necessary to do so until a set minimum current is reached. The appropriate minimum current depends upon the battery type, the size of the battery bank, and the maximum available charge rate for the system. A manufacturer may give a recommendation in the manual for the particular battery, but often times other sources must be consulted.

Trojan Battery recommends the following approaches for flooded lead acid (FLA) batteries:

Absorption finish current = 1-3% of C/20 rate

Example 1: A battery bank has 2 strings of 12 Volt SPRE 12 225 model batteries. These batteries are 225Ah each.

Absorption finish current = .01 × (2 × 225Ah)

Absorption finish current = 4.5A

Trojan Battery recommends the following approaches for flooded lead acid (FLA) batteries:

Absorption finish current = .5% of C/20 rate

Example 1: A battery bank has 2 strings of 12 Volt SAGM 12 205 model batteries. These batteries are 205Ah each.

Absorption finish current = .005 × (2 × 205Ah)

Absorption finish current = 2.05A