1. The goal of controlling the device
The battery is the “heart” of an independent power supply system and must be prevented from being overcharged (overcharging can lead to plate corrosion, gassing and moisture loss) and discharging below the cut-off voltage, which can lead to permanent damage to the battery and capacity loss. Damage from overcharging can be manifested in the form of substantial moisture loss, plate swelling, and loss of active material. Active species such as precipitates can deposit on the bottom of the cell, which can eventually lead to a short circuit at the bottom of the plates. A controller (also called a regulator) is an essential part of any battery-based electrical system, and it protects the battery from overcharging and the damage caused by overcharging.
2. Charge-discharge cycles
A chemical reaction occurs during battery charging, converting lead sulfate on the positive plate into lead peroxide, releasing sulfate and increasing the specific gravity of the electrolyte. The typical specific gravity of a fully charged battery is 1225~1250kg/m², depending on the design of the battery. The rate at which a battery can be charged is limited by the rate of chemical reactions. When the battery is nearly fully charged, it is impossible to quickly recharge the battery even if there is still discharge material remaining on the plates. Therefore, in order to achieve a higher state of charge without losing too much moisture, it is necessary to reduce the charging current. Current exceeding the minimum charge rate only splits the water in the electrolyte into hydrogen and oxygen, causing the cell to begin gassing. At the same time, the terminal voltage begins to rise rapidly.
A chemical reaction occurs during battery discharge, converting lead peroxide on the negative plate into lead sulfate. The participation of sulfuric acid in this reaction reduces the specific gravity of the battery. The rate at which the battery discharges is bound by the rate of chemical reactions, an effect that can be seen, for example, in the extreme case of trying to start a car with a battery in a low voltage state. When the chemical reaction cannot keep up with the delivered current, the terminal voltage drops and the engine starting power will drop. Slow start can be heard as the available power drops. If a given time allows the chemical reaction to catch up to the desired rate, the battery may regain enough surface charge to attempt a restart.
3. Overcharge protection
In order to prevent the battery from being overcharged, the charge controller senses that the battery is fully charged and stops or reduces the charging current. Any controller must be able to take into account the inherent characteristics of the battery, regardless of how it is charged. The connection method of the charging controller is shown in Figure 1.
In Figure 1, when the controller trips, the auxiliary load uses the excess power, for example, to pump water into the sump.
4. types of controllers
Controllers can be broadly classified into linear controllers and switching controllers. Switch controllers can be of either parallel or series type.
Series controllers can employ voltage-controlled switches or high-speed switches.
5. Linear controllers and switch controllers
The linear controller continuously adjusts the charge into the battery at any time to maintain the optimum voltage, and changes the current into the battery by connecting a variable resistance element in series with the battery. This enables the controller to provide all or a portion of the power to the battery. Its main disadvantage is that it requires very high power transistors to control the current.
The switch controller is a switching device that removes the charging power source when the battery voltage indicates full charge. Switching power supply controllers are the most common controllers used in photovoltaic systems.
6. Parallel controller and series controller
As mentioned above, the switch controller can be a parallel controller or a series controller.
In a parallel controller circuit, the power supply operates at full power, excess power can be stored or transferred to a dummy load, and the excess power can also be used to heat water or run other systems. Since the parallel controller is installed in parallel with the power supply and battery, its main advantage is “zero insertion loss”. In wind and hydro power systems, they also provide loads to prevent generators from overspeeding, but this is not very common.
A series controller (usually installed as a simple switch controller) is connected between the power supply and the battery.
When charging is not required, the power supply can be easily disconnected, and the series controller switches power to a backup system, such as a pumping system or an auxiliary battery pack.
7. Over-discharge protection of the controller
When the battery is discharged below the cut-off voltage, there will be a permanent loss of capacity. Also, if the supply voltage is lower than the operating voltage range of the device, damage to the electrical equipment may occur. To prevent overdischarge, the controller simply disconnects the load when the battery voltage drops too low. Some controllers will turn on the generator if a backup generator is configured. In many independent supply systems, switching can be done by an inverter with a low voltage disconnect feature.
8. Controller temperature compensation
Temperature compensation will adjust the charging voltage according to changes in the temperature of the battery electrolyte. When the battery temperature drops, a higher voltage is required to complete charging; when the battery temperature rises, a lower voltage can be completed. Temperature compensation is a very important feature that helps extend battery life.
Some controllers double as a monitoring device for the performance of system components, with the following functions:
(1) Voltage detection. To avoid voltage reading errors caused by voltage drops in the power cables, separate voltage detection leads are provided that connect directly to the battery terminals.
(2) Electric meter. Provides digital or analog display of PV array and load current.
(3) Voltmeter. A voltmeter (or LED display) shows the state of charge of the battery by indicating the battery terminal voltage.
(4) An hour watch. Battery voltage is not the best indicator of battery state of charge. When the battery reaches the cut-off voltage, its voltage remains relatively constant. Another way to monitor a battery is to use an amp-hour meter that acts as a “gas gauge” for the battery. If the amp-hour meter is set to zero when the battery is fully charged, the battery’s charge loss will be indicated as a negative number on the amp-hour meter, but the amp-hour meter will return to zero when it is fully recharged. This means that the capacity of this battery when fully charged must be known, which is a good indication of the battery’s state of charge. Keep in mind that batteries don’t charge 100% efficiently, so the ampere-hour meter is just a state-of-charge indicator.
An hour meter can be included with some inverters and controllers or purchased separately.
9. Controller installation
From a circuit point of view, the controller can be installed near the battery or in a location where it is easy to monitor.
When installing the controller remotely, it is necessary to use a module with individual battery voltage sensing capability, considering the voltage drop that may occur on the cable.
Power electronic equipment needs good ventilation, but the controller is parallel type, when the current flows directly from the battery, it will generate a lot of heat, this heat must be dissipated, so the parallel type controller must be installed in sufficient ventilation to avoid Dissipate excess heat. For safety reasons, electronic equipment cannot be installed on the battery pack, nor can it be installed in any location where it is possible to ignite the battery pack and generate hydrogen gas.
10. Controller selection
Many factors must be considered when selecting a controller, and the controller must match the voltage of the system. For example, a 12V system should use a 12V controller, and a 24V controller should be used in a 24V system, and this basic requirement cannot be ignored.
In PV systems, the controller must be able to handle the maximum array current, so the array short-circuit current should be used to determine this number conservatively. The following special factors are worth noting:
(1) In snowfall areas, the reflection of snow increases the array current.
(2) Some thin-film batteries are much more powerful than their rated power when initially installed. It should be ensured that the controller can handle this initial power.
AS4509.2 recommends that the controller should be able to withstand 125% of the short-circuit current of the array when the radiation intensity exceeds 1000W/m². For example, some controllers manufactured by Plasmatronics have a current limit of 125% of the short-circuit current of the array, but many other manufacturers The controller uses a relay, and overcurrent may cause the controller to fail (burn out the relay).