We should use a method for full discharging protection, especially for the discharging protection of lead-acid batteries. For nickel-cadmium batteries, only use discharging protection in a small range. This discharging protection is necessary to ensure satisfactory battery life, prevent individual battery reversal or failure, and ensure that critical loads are always powered. If the system estimates are correct, this discharging protection will not operate very often during normal battery use. The key to ensuring that the battery is used correctly under discharging protection conditions is to accurately measure the state of charge of the battery. Unfortunately, both lead-acid and nickel-cadmium batteries have difficulty determining measurable characteristics in their state of charge.
1. Discharge protection: limit the discharging capacity to C100
The figure below shows the discharge characteristics of a typical lead-acid battery when it is discharged at various load currents. The graph clearly shows that the battery capacity increases as the discharge rate decreases. Initial voltage and final discharge voltage (here, the load must be disconnected)
In most photovoltaic power generation systems, the battery is estimated to run continuously for several days, and its load current is usually 100 hours of current, denoted as I100. In this case, the battery system can be protected by limiting the final discharge voltage to V100.
In those systems where the load current varies greatly (such as an independent photovoltaic power generation system for civil utilities) the discharge capacity must not exceed the C100 ampere-hour capacity. If it exceeds, the battery can be completely discharged, resulting in a significant reduction in battery life. The following figure (a) shows that when the discharge rate is less than I100, it is possible to discharge the entire battery. Figure (b) below shows that this can often be avoided by limiting the discharge capacity C100.
Many batteries used in small photovoltaic systems are fully discharged at their C100 rating. In this state, their electrolyte density is approximately equal to 1.03 kg/L, which is low enough to dissolve the lead and cause permanent damage. Therefore, such batteries must not be discharged to their C100 rating, and must stop discharging when the battery electrolyte density reaches 1.10kg/L.
2. Automatic discharging protection
2.1 Common methods for automatic discharging protection
Automatic discharging protection can be accomplished by one of the following methods:
① The simplest and most common discharging protection method in small photovoltaic power generation systems is to disconnect the load from the battery at a predetermined voltage value, and notify the user of this situation through a light-emitting diode or a buzzer. Some of these devices can provide a small amount of backup power. The main advantage of this discharging protection method is simplicity and low cost.
②Another discharging protection method is to connect to several load outputs under the control of the regulator. With this configuration, the user can continuously use the primary load like lighting, while the non-primary load will be disconnected. Of course, the user must properly determine which loads have priority.
2.2 Discharging protection of basis for reconnecting the load
In systems used for automatic deep discharging protection, the basis for reconnecting the load must be clearly determined to suit its application. There are some general requirements as follows:
① In those places where battery life must be fully considered and the load is not critical, the load can be kept disconnected until the battery voltage rises to a high level under charging regulation. This level should optimize the amount of charge returned to the battery.
② When a remote control device is impossible to access regularly or only when the device is occupied (such as an isolation room) where there is a load requirement, the load should not be reconnected unless the user resets an external switch. This reduces the possibility of battery cycling during unattended periods.
③ In those systems where power to the load is critical, reconnection may occur after the battery has re-stored a small amount of charge. In this case, the indicator should tell the user that the battery is in a low-charge state, so that the load power consumption can be maintained at. minimum value.
The lithium-ion batteries have a BMS protection board inside to ensure the discharging protection of the battery.
Read more: Charging control of solar power generation and energy storage