The topic of EV charging has overshadowed many of the inherent benefits of EVs, because consumers do not want to wait to charge an EV vehicle. The convenience of filling a gas tank over charging an EV battery has pushed the industry to target faster and faster charging capabilities, and rightfully so.

Consumers need fast charging when away from home or on a road trip. Commercial vehicles, including trucks and buses, rely on fast charging by necessity because these vehicles operate continuously.

Unfortunately, frequent fast charging can accelerate battery aging, shortening battery pack lifetime by 10% to 20%. High-current charging stresses battery components and can lead to an irreversible condition called lithium plating. Lithium plating is the buildup of lithium on the anode caused by poor lithium diffusion, meaning that the cell is not ready to accept additional electrons. Temperature, charge level, and aging of the thin SEI layer all influence when plating begins. These stresses and the lithium diffusion of the cell are why device manufacturers recommend against charging a battery to 100%.

EIS helps designers manage fast charging in two ways:

• Temperature monitoring keeps environmental and core battery temperatures within an acceptable range during fast charging. Low temperatures restrict diffusion while fast charging can raise battery temperatures, so constant monitoring is essential.
• Diffusion monitoring provides insight into the diffusion region (the tail of the Nyquist plot) in real time. If the impedance becomes too high, for example, the charge current must be lowered.

While monitoring lithium diffusion provides warning signs of battery aging, these monitors let system designers increase charging currents. Instead of relying on static, modeled parameters of lithium diffusion that require extra built-in margin and effectively limit the maximum charge rate, a direct measurement tells the system when to reduce the current. Keeping the impedance low enables even faster charging.

The EIS engine in TI's BQ79826Z-Q1 enables diffusion sensing with ultra-low-frequency detection ≤0.01Hz and reduces measurement time by exciting up to five impedance frequencies simultaneously (Figure 3-3).