Sat. Dec 3rd, 2022

With the rapid growth in demand for courier services, electric motorcycles have become increasingly popular due to their advantages of larger battery capacity than electric bicycles and electric scooters. The greater the capacity, the longer the travel time, which helps save time and enables delivery over longer distances.

With the rapid growth in demand for courier services, electric motorcycles have become increasingly popular due to their advantages of larger battery capacity than electric bicycles and electric scooters. The greater the capacity, the longer the travel time, which helps save time and enables delivery over longer distances.

Electric motorcycle battery packs have multiple voltage platforms, the most common of which is 60V, which requires 16S or 17S Li-ion cells in a battery pack.

Achieving longer runtimes requires addressing three design challenges:

・ High-precision battery voltage sampling to improve battery capacity calculation accuracy.

・Battery voltage balance.

・ Low system current consumption, especially in standby mode.

The low current consumption 16S-17S battery pack reference design can help solve the design challenges mentioned above. It uses the BQ76940 battery monitor for battery pack low 15 string cell voltage sampling monitoring and a dual channel general purpose operational amplifier LM2904B to monitor the high two string cell voltage. Greater cell balancing current is achieved through external metal-oxide-semiconductor field-effect transistors (MOSFETs). The block diagram of the battery pack reference design is shown in Figure 1.

Fast and Furious: More Durable Electric Motorcycle 16S-17S Lithium-Ion Battery Packs

Figure 1: 16S-17S battery pack block diagram

High-precision battery voltage sampling

The BQ76940 directly monitors the low 15-series battery, so the voltage accuracy of the low 15-series battery is directly determined by the BQ76940. Typical voltage sampling accuracy from 3.2 V to 4.6 V at 25°C is ±15 mV. If necessary, its voltage sampling accuracy can be further improved by additional calibration. The discrete circuit shown in Figure 2 determines the accuracy of the two upper cells.

Fast and Furious: More Durable Electric Motorcycle 16S-17S Lithium-Ion Battery Packs

Figure 2: Discrete circuit diagram of the two upper cells

Take 17 strings of batteries as an example. When Q25 operates in linear mode, one channel of the LM2904B operates as a negative feedback circuit together with the P-channel MOSFET Q25, R89 and R96. The negative-phase input voltage of the amplifier is equal to the positive-phase input voltage, which is the voltage of the 16-string battery. Therefore, the current generated by the voltage of the 17th string of single-cell batteries applied to both ends of R89 flows through Q25 and R96 and returns to the reference ground. The sampling of the 16th string of single-cell batteries is similar to this.

By measuring the ADC_16 and ADC_17 voltages using an analog-to-digital converter (ADC), the 16th and 17th string single-cell battery voltages can be monitored. Taking into account the tolerances of R89, R96, R87, R94 and the ADC reference, a two-point calibration is required for higher accuracy. Figure 3 shows the process of a two-point calibration.

Fast and Furious: More Durable Electric Motorcycle 16S-17S Lithium-Ion Battery Packs

Figure 3: Two-point calibration process

I tested the calibrated string 16 and 17 battery voltage accuracy in the lab; the results are shown in Figure 4. Accuracy is ±2mV.

Fast and Furious: More Durable Electric Motorcycle 16S-17S Lithium-Ion Battery Packs

Figure 4: 16- and 17-string battery voltage accuracy (at 25°C)

Cell balancing

Since the 16th and 17th strings are monitored by discrete circuits and the lower 15 cells are monitored by the BQ76940, the effect on cell balancing must be considered.

Figure 5 shows the main current paths. Red represents the power supply path of the general-purpose operational amplifier, green represents the voltage sampling path of the 17th string of batteries, and gray represents the sense path of the 16th string of batteries. The general-purpose op amp’s supply current is supplied by the entire battery pack and flows back to ground reference, so it discharges the entire battery pack and does not cause imbalance. The voltage sampling path of the 17th string of batteries also flows back to the reference ground from the entire battery pack, so it will not cause imbalance. However, the voltage sampling path of the 16th string of batteries flows back to the reference ground from the lower 16th string of batteries, which will cause a voltage imbalance between the 17th and lower 16th strings of batteries. This imbalance occurs only when the voltage of the 16th string is detected.

To reduce the effect of unbalance, you can turn off Q21 when the 16th string of batteries is not being detected, and consider the current of the control circuit of Q21 when calculating the effect of unbalance.

Based on the analysis here, and assuming a voltage sampling period of 250ms, the unbalanced current for this reference design should be less than 0.1 µA.

Fast and Furious: More Durable Electric Motorcycle 16S-17S Lithium-Ion Battery Packs

Figure 5: Discrete circuit current path diagram

Low system standby consumption

In a previously written article “Pedal Power Solutions: Better Durability 13S, 48V Li-Ion Battery Packs for Electric Bikes and Electric Motorcycles, I explained how to use the LM5164 and system-level design to reduce system-level in standby mode Current consumption. Now, I would like to briefly discuss how to reduce the current consumption of discrete circuits in standby mode. Neither charging nor discharging is performed in standby mode. Battery voltage sensing acts as a protection, and the frequency can usually be reduced by increasing the idle time. To reduce power consumption in standby mode, you can turn off the circuit without the need to sense the voltage.

The solution in Figure 2 uses a P-channel MOSFET Q20 to switch the power supply to the LM2904B and is controlled by a microcontroller. To reduce the current further, I added Q22 and Q21 to cut off the battery voltage sensing line, thus saving even more energy. Assuming a voltage sensing period of 250 ms and an idle time of 250 ms, the average current consumption in standby will be fairly low. Typical current in the solution shown in Figure 2 is less than 1 µA.

in conclusion

Overall, this reference design provides a cost-competitive battery pack solution covering up to 17S cells, ideal for electric motorcycles. The design achieves longer runtimes by:

・ Improve the battery voltage sampling accuracy.

・ Reduced current consumption in standby mode.

・ Eliminate unbalanced effects.

This design is also suitable for telecom backup battery packs that require a 16S/48-V Li-Ion Phosphate battery pack.

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