An Introduction to Fast Charging and Pulse Charging

This story is contributed by Dhevathi Rajan Rajagopalan Kannan

• Fast charging is critical for the adoption of electric vehicles (EV’s), but higher current charging typically comes at the expense of battery life.
• Multistage constant current (MCC), pulse charging, boost charging, and variable current profiles (VCP) are among the fast charging methods used to reduce charging time without impacting battery life.
• Pulse charging uses high current pulses separated by short relaxation periods in an effort to minimize degradation.
• The literature suggests that it may be possible to reduce charging time by 5–20%, but commercial pulse chargers for EV’s require new hardware and are not yet available.

Introduction

In the past few years, lithium-ion batteries (LIBs) have made significant improvements in design, performance, and lifetime. As a result, applications of rechargeable lithium-ion batteries have expanded from electronic devices, including phones, laptops, and digital cameras, to electric vehicles (EV’s) and grid storage applications. Each time a battery undergoes a charge and discharge cycle, however, it loses a little capacity, eventually reaching a point at which it can no longer store the amount of energy needed for the application and needs to be replaced.

Battery management strategies are used to minimize this degradation and its effect on the consumer. One of the most common strategies is to limit the voltage or state-of-charge (SOC) of the battery. For example, many commercial lithium-ion batteries can be cycled from 2.5V to 4.2V or 0% to 100% SOC. During regular operation, however, the battery management system may keep the SOC between 5% and 95%, preventing the battery from operating at the extremes of its charge and discharge limits and allowing the battery to maintain its desired level of capacity for many more cycles.

Fast Charging Lithium-Ion Batteries

The charging strategy itself is critical for extending battery life. While faster charging is almost always desired, the general rule is that the faster the battery is being charged, the more quickly it degrades. As a result, many different strategies, including pulsed charging, are being developed and implemented to mitigate degradation from fast charging.

Existing charging methods

Lithium-ion batteries are typically charged using the constant current-constant voltage (CC-CV) method, usually a half hour to two hours (C/2 to 2C) in the CC phase plus another half hour to one hour in the CV phase to achieve full charge, depending on the battery chemistry and design. (A 1C discharge means that the current applied will charge an empty battery completely in 1 hour whereas a 2C rate will charge the battery in 30 minutes.)

Existing fast charging methods

Faster charging can be achieved simply by increasing the current, but repeated charging at higher currents tends to degrade battery life and performance. While the amount of degradation caused by fast charging can vary with battery chemistry, the most commonly used chemistries, lithium cobalt oxide (LCO) for electronic devices and nickel manganese cobalt oxide (NMC) and nickel cobalt aluminum oxide (NCA), are susceptible to capacity loss over time when charged continuously at higher currents.

Common fast charging strategies are described below:

Constant current — constant voltage (CC-CV) is by far the most common charging method. The battery is charged at a constant current (CC) up to a voltage cutoff, followed by a constant voltage (CV) hold until the current decays to near zero. The CV phase makes it possible to access more of the battery’s capacity without exceeding the voltage limit, but, since the current decreases exponentially during the CV phase, this extends the charging time significantly.

Constant power — constant voltage (CP-CV) is similar to CC-CV except that constant power is used instead of constant current to charge to a specific voltage limit before switching to constant voltage. In one study, CP-CV charging resulted in better capacity retention when the cell was charged at 1C, possibly due to the lower current at high SOC, while CC-CV had better retention at 0.5C.

Multistage constant current (MCC) protocols seek to mitigate degradation from fast charging by limiting the higher C-rate charging to a reduced SOC window. Typical protocols consist of multiple CC stages followed by a CV stage, with higher currents at lower SOC’s. For example, a C-rate of 1.5C to 3C might be applied for only a few minutes at low SOC’s before the current is reduced to less than 1C for the remainder of the charge. This approach seeks to reduce the heat generation and avoid conditions that enable lithium plating and other modes of degradation. But even this reduction in charging time may not be fully realized. Although charging at a higher current allows the cell to reach the maximum charge voltage faster, it typically also causes the CV step to take more time. One study showed that the MCC-CV approach results in higher capacity loss compared to CC-CV and CP-CV with the same average C rate [1].

Pulse charging protocols periodically interrupt the charge current with short rest periods or discharge pulses. Pulse charging is believed to inhibit dendrite growth and can be implemented with or without the CV phase. Some studies have shown that pulse charging without CV reduces the charging time while maintaining higher capacities, due to higher active material utilization [3], [6], [7]. Other studies, however, suggest that there is no significant difference in the capacity fade between different pulse charging protocols and CC-CV. Pulse charging will be discussed in greater detail in later sections of this article.

Boost charging protocols supply high average current at the beginning of charge, followed by a CC-CV at lower currents. The initial boost charge stage typically consists of a high current CC or a CV in which the cell is immediately brought to the maximum voltage or a combination of the two in a CC-CV. The overall profile may resemble CV-CC-CV or CC-CV-CC-CV. The results from boost charging compared to CC-CV methods are not conclusive. Some boost protocols are able to reduce charging time in exchange for higher capacity fade.

Variable current profiles (VCP) with different complexities have also been proposed for fast charging. One example of this, the universal voltage protocol (UVP) was derived by optimizing a set of CC-CV curves and is designed to reduce both charging time and energy losses due to heating. In this method, the current increases rapidly at low SOC’s and then drops quickly after that. As the cell ages, however, the current profiles need to be adjusted to take into account the change in internal resistance of the cell. Although this method was able to reduce the charging time, the effects of long term aging cycling still need further investigation.

Figure 1: Schematic representation of common types of fast charging protocols. a) Constant current — constant voltage (CC-CV), b) Constant power — constant voltage (CP-CV), c) Multistage constant current — constant voltage (MCC-CV), d) Pulse charging, e) Boost charging with a CC-CV-CC-CV scheme, f) Variable current profile (VCP) [1]

Pulse Charging

Pulse charging, one of several charging methods for reducing charging time while maintaining cycle life, consists of repeated high current pulses separated by low current or short relaxation periods, as shown in Figure 2. A longer CV phase is still needed to access the capacity at the top of charge, although one study has suggested that an initial CC phase can also be used to capture some of this capacity [1], [5].

Figure 2: (a) Illustration of a pulse-CV profile for charging lithium-ion batteries

Figure 2: (b) Schematic representation of current pulse profile used in pulse charging where Ip refers to the peak pulse current, Iavg the equivalent constant current, Δt the pulse width, and T the period.

Common parameters in pulse charging include the frequency, pulse width, and relaxation, and some protocols even pulse negative currents during relaxation. The pulse frequency typically ranges from 200 mHz to 13 kHz with a duty cycle between 25% and 75%. The average current IAvg is the product of the peak current Ipk and the duty cycle D:

where duty cycle D is the ratio of pulse width Δt to the pulse period T. Because of this duty cycle, however, the peak pulse current needs to be much higher in order to maintain the same average current over charge. Figure 3 shows a simplified pulse charging circuit and pulse waveform.

Figure 3: (a) Pulse charging micromodel; and (b) pulse waveform [3]

Effects of pulse charging on lithium-ion batteries

Pulse charging, when implemented properly, can offer some advantages over conventional CC-CV charging. Most studies show that pulse charging can reduce the charging time by 5–20% with no effect or only a slight reduction in cycle life [5], [8]. Some of the literature suggests that pulse charging is more beneficial for chemistries that are capable of higher C-rate charging, such as LFP and LTO. Although these cell chemistries can already charge at higher C-rates, one study claims that pulse charging has the added benefit of increasing round-trip efficiency because of the ability to utilize more active lithium inventory compared to CC charging [3]. Comparatively, higher energy density chemistries, such as lithium cobalt oxide (LCO), used mostly in electronics, nickel manganese cobalt oxide (NMC), and nickel cobalt aluminum oxide (NCA), tend to incur greater degradation with pulse charging.

Is pulse charging feasible for EV’s?

The short answer is no, at least not yet. Practically speaking, more advanced charging circuitry is needed for both the higher peak currents and the pulse waveform, which remain to be optimized. In addition, these higher peak currents can heat the cells rapidly compared to conventional CC-CV, because resistive losses are proportional to the square of the current, and these elevated temperatures may have a negative effect on the lifetime of the battery.

While pulse charging is being actively studied, the technique has not yet reached commercialization, and promising directions, such as the use of negative currents, are still under investigation. Given that fast charging is critical for the adoption of EV’s, there remains plenty of interest in the development of pulse charging and other advanced charging techniques.

References

[1] A. Tomaszewska, Z. Chu, X. Feng, S. O’Kane, X. Liu, J. Chen, C. Ji, E. Endler, R. Li, L. Liu, Y. Li, S. Zheng, S. Vetterlein, M. Gao, J. Du, M. Parkes, M. Ouyang, M. Marinescu, G. Offer, B. Wu, Lithium-Ion Battery Fast Charging: A Review, eTransportation 1 (2019) 100011. doi:10.1016/j.etran.2019.100011.

[2] S. U. Jeon, J. -W. Park, B. -K. Kang and H. -J. Lee, Study on Battery Charging Strategy of Electric Vehicles Considering Battery Capacity, IEEE Access 9 (2021) 89757–89767. doi:10.1109/ACCESS.2021.3090763.

[3] J.M. Amanor-Boadu, A. Guiseppi-Elie, E. Sánchez-Sinencio, The impact of pulse charging parameters on the life cycle of lithium-ion polymer batteries, Energies 11 (2018) 1–15. doi:10.3390/en11082162.

[4] F. Savoye, P. Venet, M. Millet, J. Groot, Impact of periodic current pulses on Li-ion battery performance, IEEE Trans. Ind. Electron. 59 (2012) 3481–3488. doi:10.1109/TIE.2011.2172172.

[5] D. Rajagopalan Kannan, M.H. Weatherspoon, The effect of pulse charging on commercial lithium nickel manganese cobalt oxide (NMC) cathode lithium-ion batteries, J. Power Sources. 479 (2020) 229085. doi:10.1016/j.jpowsour.2020.229085.

[6] C.-Y. Lin, S.-C. Yen, The Application of Pulse Charge for Secondary Lithium Battery, ECS Trans. 11 (2008) 55–62. doi:10.1149/1.2938907.

[7] J. Li, E. Murphy, J. Winnick, P.A. Kohl, Studies on the cycle life of commercial lithium ion batteries during rapid charge-discharge cycling, J. Power Sources 102 (2001) 294–301. doi:10.1016/S0378–7753(01)00821–7.

[8] D. Rajagopalan Kannan, M.H. Weatherspoon, The Effect of Pulse Charging on Commercial Lithium Cobalt Oxide (LCO) Battery Characteristics, Int. J. Electrochem. Sci., 16 (2021) 210453. doi:10.20964/2021.04.30.

Dhevathi Rajan Rajagopalan Kannan holds a PhD from FAMU-FSU and is currently Manager of the Battery Cell Testing team at Romeo Power, based in Southern California. Romeo Power produces battery packs for heavy-duty commercial electric vehicle fleets.

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