13S, 48V lithium-ion battery pack for electric bicycles and electric motorcycles

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13S, 48V lithium-ion battery pack for electric bicycles and electric motorcycles [Copy link]

As electric bicycles and electric motorcycles become more popular, consumers are demanding longer battery life. Extending the battery life of a battery pack allows the vehicle to travel further without frequent charging.

There are two ways to increase the range of lithium-ion (Li-ion) battery packs: increase the total battery capacity or improve energy efficiency. Increasing the total battery capacity means using more or better performing battery cells, which significantly increases the overall cost of the battery pack. Improving energy efficiency, on the other hand, gives designers more usable energy without increasing capacity. There are two ways to improve energy efficiency: improve state of charge accuracy and/or reduce current consumption.

To get longer runtime, you need to extract as much energy as possible from the battery pack; but if over-discharge occurs, the battery will be permanently damaged. To avoid over-discharging the battery, accurate information about the battery capacity or state of charge is critical. There are three ways to accurately measure the state of charge:

Battery voltage measurement.
Coulomb counting.
TI Impedance Track technology.

Battery voltage measurement is the simplest method, but it also has low accuracy in overload conditions. Coulomb counting measures and integrates current over time. However, achieving better SOC accuracy requires regular full-run-idle learning cycles, and SOC accuracy will be affected by self-discharge and standby current. Cold and aged batteries will also reduce SOC accuracy. Impedance Track technology directly measures the effects of discharge rate, temperature, age, and other factors by learning the battery impedance. Therefore, the Impedance Track method can give you better SOC measurement accuracy even with aged batteries and cold temperatures.

Our Accurate Measurement and 50μA Standby Current, 13S, 48V Li-Ion Battery Pack Reference Design uses the BQ34Z100-G1 , an Impedance Track fuel gauge for Li-Ion, Lead Acid, Nickel Metal Hydride, and Nickel Cadmium batteries, and works independently of the battery series battery configuration. This design supports external voltage conversion circuits. The circuit automatically controls to reduce system power consumption and provide users with longer operating time on each charge without worrying about possible damage caused by over-discharge. Due to the low current consumption, the entire system has very limited impact on the measurement results. Therefore, we read the data directly from the BQ34Z100-G1 through BQStudio at a constant discharge current at room temperature . Figure 1 shows the discharge state of charge test results.

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Figure 1: Discharge state of charge test results at constant discharge current

The second way to improve energy efficiency is to reduce current consumption. The precise measurement reference design introduces an optimized bias supply solution, as shown in Figure 2.

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Figure 2: Entire System Bias Power Graph
This design utilizes our new LM5164 as an auxiliary power source. The 100 V LM5164 is a wide-input, low quiescent current step-down DC-DC converter that protects the system from potential transients from the nominal 48 V battery and powers the 3.3 V microcontroller (MCU) and the BQ34Z100-G1 . The inputs to the LM5164 are controlled by two signals: REGOUT from the BQ76940 and SYS from the MSP430 MCU. A high on either of these signals turns on Q1 and enables the inputs to the LM5164 —thus enabling the MCU power. When the board is fresh from the factory and the battery management board is powered up for the first time, it is in factory mode. The entire system is unpowered, except for the BQ76940 , achieving a low 5-μA factory mode current consumption. Pressing button S1 sets REGOUT high and turns on system power. When the MCU powers up, it sets SYS high. No matter BQ76940 is in shutdown mode or normal mode, the whole system has stable power supply.

You need to turn on the MCU power to implement all e-bike battery pack functions in standby mode, including charger connection/removal and load connection/removal. Q1 should be powered. To reduce the standby mode current consumption, the BQ76940 is set to shutdown mode through I2C command. Therefore SYS is high, keeping Q1 powered. The LM5164 is set to a low switching frequency to reduce switching losses, and the MSP430 MCU is in low power mode. All charger connection/removal and load connection/removal detection are implemented by firmware. The standby current consumption is typically 50μA, as shown in Figure 3. Figure 4 shows the factory mode current consumption of the main board.

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Figure 3: Standby mode current consumption

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Figure 4: Current consumption in factory mode

in conclusion

In summary, the reference design achieves accurate state-of-charge measurement (via BQ34Z100-G1 ) and reduces standby and shipping mode current consumption (via optimized bias supply solutions). Together, these two solutions improve the energy efficiency of the e-bike battery pack, providing users with longer usage time.