Though abundant, solar insolation is an unreliable source of energy. It fluctuates as a function of time, and is not available during the nights or in cloudy sky. Therefore, when the PV systems are used for stand-alone applications, a backup source of energy is necessary to compensate for the balance power demand of the load. Batteries are, generally, used as a backup source in such applications. To reduce the cost of the system, the ratings of the batteries are designed optimally.
Battery feeds the load when the PV output power is less than the load demand – and is charged when the PV output power is more than the load demand.
Fig. 1: Basic block diagram of a stand-alone PV system with battery backup and charge controller…
In applications, where batteries are used, it is critical to prevent overcharging or deep discharging of the batteries to preserve their life and to ensure good performance. This is achieved by what is called charge controllers. The block diagram of a stand-alone PV system with battery backup and a charge controller is shown in the above figure.
Commonly used set points
Charge controllers regulate the charging and discharging of battery. A charge controller senses the voltage of the battery [or ‘state of charge’ (SoC)] and decides either to disconnect it from the source (PV array in this case) to prevent it from overcharging or to disconnect the load (from the battery output) to prevent deep discharging.
Such controllers are mainly used where loads are unpredictable and the batteries are optimised or undersized to minimise the initial cost. The charge control algorithm has set points (threshold values) depending upon which it takes decisions. The commonly used set points are briefly described further and are shown in Fig 2.
Fig. 2: Commonly used set points and their behaviour in charge controllers…
- Voltage Regulation (VR) set point: It is the maximum voltage up to which a battery can be charged (without getting overcharged). If this threshold is reached, the controller either disconnects the battery from the source or starts regulating the current delivered to the battery.
- Voltage Regulation Hysteresis (VRH): It is the difference between VR and the voltage at which the controller reconnects the battery to the PV source and starts charging. If VRH is too small, it will result in tighter voltage regulation but the control will be oscillatory and may deteriorate the battery life. At the same time, a large value of VRH may lead to ‘slight’ overcharging of battery during every cycle. So, in practice, there is a trade-off. VRH also determines how effectively the controller can charge the battery.
- Low Voltage Disconnect (LVD): It is the minimum voltage up-to which the battery can be allowed to discharge, without getting deep discharged. It is also defined as the maximum Depth of Discharge (DoD) of the battery. The charge controller disconnects the load from the battery terminals as soon as the battery voltage touches LVD to prevent it from over-discharging.
- Low Voltage Disconnect Hysteresis (LVDH): It is the difference between LVD value and the battery voltage at which the load can be reconnected to the battery terminals. LVDH is not kept too small, or else the load will be switched on and off more frequently, which can adversely affect battery and the load.
Types of charge controllers
There are four types of charge controllers used in the circuits involving batteries. Those are as follows:
- Shunt type charge controller: The basic block diagram of a shunt charge controller is shown in Fig 3. In this type of charge controller, a switch S1 is connected in shunt with the PV panel, which is turned on when the battery voltage reaches its overvoltage limit (VR). The PV array is short-circuited and it no more feeds the battery. The blocking diode prevents short-circuiting of the battery. The blocking diode also prevents the battery to discharge through the PV array during nights and low insolation periods. The switch S2 allows the battery to discharge through the load. When the battery voltage reaches the threshold value (LVD), the switch S2 is turned off to prevent deep discharging of the battery.
Fig. 3: Shunt charge controller…
- Series type charge controller: The basic block diagram of a series charge controller is shown in the Fig 4. In this type of charge controller, the switch is connected in series with the PV panel. This switch is turned off to prevent the battery from getting overcharged. A major drawback of this method is the additional loss in switch S1 which now carries the PV output current charging the battery. Depending on the application and type of battery used, different charge controller configurations are used which either completely disconnect the array from the battery or allow a regulated current to flow through the battery to maintain the battery voltage.
- DC-DC converter type charge controller: The series and shunt type of charge controllers discussed are able to safeguard the battery, but do not result in an efficient and optimum use of PV source. Instead of the series or shunt controllers, if a DC to DC converter is used to interface the battery and load combinations with PV array, it provides a smoother control with efficient and optimum use of the PV source.
A buck, boost or buck-boost type DC to DC converter can be used to regulate the output of the PV array to feed the load. A typical configuration is shown in Fig 5 The DC to DC converter offers the following advantages:
- There are no additional losses due to the conduction of switches such as S1 and S2.
- The regulation of battery charging current and battery voltage is superior.
- The output voltage of the PV array and the battery voltage need not be identical now.
Fig. 4: Series charge controller…
Fig. 5: DC to DC converter type charge controller…
So the PV can be operated at the maximum power point.
- MPPT charge controllers: To charge battery in a more efficient manner, the PV array is operated at a point where the PV output power is maximum. The output power of the PV array changes with the change in voltage across it. To extract maximum power from the PV array, a DC to DC converter is used between the PV array and the battery. The duty cycle of the DC to DC converter is controlled to impose optimum voltage across the PV array which corresponds to maximum power point.
Project prototype
A prototype model which is implemented for charging a 7Ah 12V battery with the help of 40 Watt solar PV panel is shown below:
Conclusion
A solar charge controller for standalone solar photovoltaic using microcontroller AT89C2051 has been successfully designed and implemented in this referred project.
The controller designed can be modified to be better if some of the electrical components are upgraded and improved.
The theory and concept of the photovoltaic solar charge controller designed in this work is based on shunt regulation. When the sufficient panel voltage is present it is used to charge the battery.
One of the benefits of this work is that the load can be turned on in day and night, if terminal voltage of the battery is sufficiently present.
The power available from the solar panel can be used for charging the battery and at the same time the load can also be turned on.
Due to its charging methodology this controller is good for only small scale standalone PV systems.
For large scale systems, MPPT or PWM technology can be applied, which uses DC-DC converter for operation.
Both MPPT and PWM adjust charging rates depending on the battery’s charge level to allow charging closer to the battery’s maximum capacity as well as monitor battery temperature to prevent overheating of batteries.
A PWM controller is less expensive than an MPPT, so it is a more economical choice for a small system. An MPPT controller is much less efficient in low power applications. For higher power rating system MPPT controllers are the best choice.
The charge controller designed in this project has features like, it prevents overcharging of battery which can reduce battery performance or lifespan, and may pose a safety risk. It also prevents complete draining (‘deep discharging’) of battery by performing controlled charging.
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