Using a fuel gauge chip to achieve fast and intelligent charging of dual-series lithium batteries
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The current fast charging solutions are widely used in the mobile phone market. The mainstream solutions include QC2.0/3.0 based on the processor manufacturer Qualcomm platform, the upcoming 4.0 standard, MTK's Pump Express standard, and flash charging of domestic mobile phone manufacturers. The QC fast charging solution uses high-voltage input to increase power, and can use low-current cables while increasing input power. Flash charging technology requires customized high-current cables to cooperate. The common point is that they are applied to single-cell lithium battery products and cannot be applied to fast charging occasions of dual-cell series lithium batteries. The typical full charge voltage of dual-cell series lithium batteries reaches 8.4-8.8V, which is used in terminal products such as walkie-talkies and POS machines. TI has a wide range of charging solutions for multi-cell batteries. BQ24725A is a charging controller that supports high current and SMBus communication. BQ24725A supports an output voltage of up to 19.2V, a maximum charging current of more than 8A, and an output voltage accuracy control of 0.5%.
It is actually very simple to charge lithium batteries using integrated charging management chip solutions such as BQ4725A. The lithium battery is directly connected to the BAT pin of the charging control chip BQ24725A, and the Charge Voltage, Charge Current, and Input Current Limit of the protection function of the charging control chip are configured to enable the charging function. Charge Voltage (VCHG) is the maximum full charge voltage of the battery, and Charge Current (ICHG) is the maximum charging current allowed by the battery. The typical charging characteristics of lithium batteries are shown in the figure below, which are divided into two stages: CC (Constant current) and CV (Constant Voltage). In the CC stage, the battery is continuously charged at the maximum charging current. At this time, the amount of electricity charged into the battery per unit time is constant Q=ICHG*T. As more and more electricity is charged into the battery, the battery voltage will rise. When the voltage rises to the maximum full charge voltage, the charging chip will control the output voltage to be a constant maximum full charge voltage. At this time, the current begins to gradually decrease, and the rate of increase of the charged electricity slowly slows down, but the total charged electricity is still increasing. Until the current decreases to the cut-off current point, the charging chip stops charging and the lithium battery reaches a fully charged state.
Based on this management process, a two-way radio battery (full charge voltage 8.8V) was charged using the charging chip BQ24725A, and VCHG, ICHG and VCELL (voltage on the cell side) were recorded during the charging process. The following figure shows the trend of the various parameters corresponding to the charging time (minutes) based on the horizontal axis. It can be seen that in less than five minutes, VCHG reached its maximum value, and ICHG began to decrease from the peak current of 4A, experiencing a switch from the CC interval to the CV interval. The 4A charging duration of the CC stage at the maximum charging rate is very short. It can be expected that if this battery is to be fully charged, the CV interval will take a long time.
By analyzing the charging curve, it is found that there is a voltage difference between the output voltage VCHG of the charging management chip and the voltage VCELL of the battery cell, which causes VCELL to reach 8.0V when VCHG reaches the maximum voltage of 8.8V. This voltage difference may be caused by the impedance from the charging end to the battery end, such as the charger PCB and the battery protection board impedance. In addition, pluggable batteries are commonly used in the walkie-talkie industry. When the battery is separated and charged, the contact between the contact and the charging seat spring will also cause impedance uncertainty.
By introducing a fuel gauge chip and coordinating the control of the charging chip, the voltage difference from VCHG to VCELL can be compensated. In addition to providing power functions, the fuel gauge chip also collects the battery cell voltage parameters. By accessing the cell voltage VCELL sampling results in the fuel gauge chip in real time, it is decided whether to increase the VCHG of the charging chip to maintain the CC charging state of the battery based on whether the VCELL voltage reaches the maximum full charge voltage. Until VCELL reaches the maximum full charge voltage of 8.8V and enters the CV range, stop increasing VCHG, and lower VCHG based on the continued increase trend of VCELL, maintain VCELL at the maximum voltage of 8.8V to protect the battery. By reading the VCELL voltage and coordinating the changing VCHG, a fast and intelligent charging management process can be achieved.
In conjunction with the fuel gauge chip BQ28Z610, the second charging test of the dual-cell 8.8V walkie-talkie battery was conducted on the BQ24725A evaluation board. The recorded curve is shown in the figure below. The continuous charging time at 4A in the CC stage reached 41 minutes, which is much longer than the ordinary charging solution in the previous test.
Based on the comparison of the test results of the two charging schemes, the fast and intelligent charging of the dual-cell lithium battery is realized by using the BQ28z610 fuel gauge chip and the dual-cell battery charging control chip BQ24725A. The BQ28z610 can also provide the real-time voltage, temperature, current and other parameters of the battery cell to the MCU that manages the charging control. The MCU adjusts the charging voltage and current of the BQ24725A according to the real-time parameters to achieve real-time and safe closed-loop adjustment, which can not only ensure the safe charging of the battery cell, but also maximize the constant current charging time to achieve fast charging.
Author: Harson Zhang Analog Field Application Engineer
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