Researchers from Zagreb University, AVL-AST company and South Kazakhstan University named after M. Auezov developed an improved charging system for lithium-titanate batteries allowing for significant reduction of charging time without compromising reliability and resources of the batteries. There is no need to replace the equipment to implement this — it is enough to update the software of the existing charging device.
The lithium-titanate batteries represent a specialized category of lithium-ion batteries, where anode is made of lithium-titanate (Li₄Ti₅O₁₂). These batteries feature high re-charging speed, stable performance under low temperatures and record-long service life — they can survive through over 20,000 charge and discharge cycles. Thanks to these parameters they are actively used in electric vehicles including e-buses and trains, in marine vessels, in aviation, as well as in energy accumulation systems, especially at solar power plants and hybrid power plants. At the same time, very accurate control of voltage and current during recharging is required to make their service life longer and to assure their safe operation.
As a rule, the batteries are charges using the standard scheme: at first, using the direct current, and upon achieving the pre-set threshold, the charger switches to the constant voltage mode, when the current is gradually reduced. This method is called CCCV (Constant Current/Constant Voltage in English). It is easy and widely used, but it does not take into account the real condition of the battery, due to which the charging may take longer than really required, especially if the battery was not completely discharged.
To resolve this problem, the scientists proposed to improve the classic charging scheme by introducing adaptive feedback. The system they developed was called CCCV-OCV, it used the data about the current condition of the battery, in particular, the open-circuit voltage (OCV) directly linked to the сpower level. In the real-time mode the system monitors this voltage and based on its value regulates the charging rate providing for faster and safe recharging.
The technology is based on the mathematical simulation of the battery and on special adaptive algorithm evaluating the inner parameters of the battery based on the real-time values of current and voltage. To improve the accuracy, the test signal was added to the charging circuit, and the stability / reliability of the system (even in the environment of disturbances and defects) is assured with the help of Lyapunov criterion — a mathematical method guaranteeing the stability of algorithm within a broad range of conditions.
The efficiency of the new system was tested both during the computer simulation, and at the pilot unit. The scientists compared the traditional and the adaptive approaches given different initial power levels and different values of the charging current.
The results proved that the developed system is capable to reduce the charging time by 10–25% on average, especially in the situations when the battery is not completely discharged. The final power level is not different from the one achieved during the standard charging process — the difference is less than 0.25%. On top of that, the level of thermal losses remained unchanged confirming the safety of the proposed method.
One of the key advantages of the new technology is its compatibility with the existing chargers: to switch to the improved charging method it is enough just to update the software without replacing the equipment.
