CONSTRUCTION OF BATTERY CHARGE CONTROL FOR PHOTOVOLTAIC SYSTEMS
Background of the study
A charge controller is an essential part of any alternative energy system. In its simplest form, a charge controller's job is to make sure the power (such as a solar panel) 'plays nice' with the load (such as a battery). The simplest implementation of this is a single diode placed in between a solar panel and battery. This ensures that the battery does not discharge into solar panel at night. A more sophisticated implementation will be adding the ability for the charge controller to disconnect the solar panel when the batteries are fully charged in order to prevent over-charging damage to the batteries (James and Dunlop, 1991).
The current version of the open source free charge controller is a converter for charging batteries, A bulk converter steps down voltage from a higher voltage level to a lower voltage level. In this case, it would step voltage down from the 18volts of a solar panel to the 12volts of a battery (Harrington and Dunlop, 1992). Since the converter is software controlled, it can be programmed to charge any battery chemistry, change it drive frequency to achieve maximum conversion efficiency as well as implement MPPT to allow a solar panel to deliver maximum power, all without any changes to hardware (Robert and Isaac, 2007). A charge controller also called charge regulator or battery regulator limits the rate at which electric current is added to or drawn from electric batteries. It prevents over charging and may prevent over voltage, which can reduce battery performance or life span, or may pose a safety risk. It may also prevent completely draining (deep discharging) a battery, or perform controlled discharges, depending on the battery technology, to protect battery life.
The term charge controller or charge regulator may refer to either a stand-alone device, or control circuitry integrated within a battery pack, battery-power device, or recharger (Dunlop, 1991; Harrington and Dunlop, 1992) Basically, there are four types of charge controllers. These are namely :-
Series charge controller or series regulator Shunt charge controller or shunt regulator Pulse width modulated charge controller (MPPT) Maximum power point tracker (MPPT)
A series charger controller disables further current flow into batteries when they are full. A shunt charge controller diverts excess electricity to an auxiliary or shunt load as electric heater, when batteries are full (Harrington and Dunlop, 1992).
Pulse width modulated (PWM) and maximum power point tracker technologies adjust charging rate depending on the battery voltage level to allow charging closer to its maximum capacity. Charge controller may also monitor battery temperature to prevent over-heating. Some charge controller systems also display and transmit data to remote displays and data logging to track electric flow over time (Sanjit, 1980; James and Dunlop, 1991; Robert and Isaac, 2007) The primary function of a charge controller in a stand-alone PV system is to maintain the battery at highest possible state of charge while protecting it from over charge by the array of solar panels and from over discharge by the loads, Although some PV system can be effectively designed without the use of a charge control, any system that has unpredictable loads, user intervention, optimized or undersized battery storage (to minimize initial cost) typically requires a battery charge controller (James and Dunlop, 1991). The algorithm or control strategy of a battery charge controller determines the effectiveness of battery charging and PV array utilization, and most importantly the ability of the system to meet the load demands. Additional features such as temperature compensation, alarms, meters, remote voltage sense leads and special algorithm law enhance the ability of a charge controller to maintain the health and extend the lifespan of battery, as well as providing an indication of operational status to the system caretaker (James and Dunlop, 1991; Harrington and Dunlop, 1992).
AIM AND OBJECTIVES The project is aimed at the following:-
Preventing battery overheating to limit the energy supplied to the battery by the PV array when the battery becomes fully charged. Preventing battery undercharge to disconnect the battery from electrical loads when the battery reaches a low state of charge. Providing load control functions to automatically connect and disconnect an electrical load at specified time, for example operating a lighting load from sunset to sunrise. Designing a control algorithm for charge controller to determine which particular algorithm will be suitable and efficient for charge regulation (Stevens, 1999, Robert and Isaac 2007) Knowing how to design, select and match guidelines for battery application and charge control requirements in PV systems.