Charge controller programming

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

The simplest of 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. With charge controllers that permit programming, 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.
• Energy storage capacity: The total energy storage capacity of the system. This will be in Amp-hours (Ah) for lead acid batteries.
• 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 of around 13% of the C/20 rate. Absorption glass mat (AGM) batteries are often able to accept higher charge currents, sometimes as high as 20% 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

Duration of absorption phase

The absorption phase of a charge controller holds 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, although many do not.

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 = ISC ÷ 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

6. Cutout voltage = о If there will be any lighting loads – 20% state of charge is the minimum value, although in some situations it may be

advisable to raise this number (or 11.60 volts for a 12V system).