What is battery charge/discharge, and why is it important?
Electronic gadgets have become so commonplace, that we almost take them for granted. And because our electronic gadgets are typically powered by rechargeable batteries, battery recharging has also become commonplace. Let's take a look at that charging process in more detail, and how the Ensurge microbattery improves upon conventional lithium-ion (Li-ion) and lithium polymer (Li-poly) battery performance.

Charging and Charge Rates
A typical Li-ion and Li-poly battery goes through a cycle of charging and discharging, over and over again, during the course of its lifetime. To maximize the battery's potential lifespan, both charging and discharging must be managed very carefully. The Li-ion and Li-poly charging process requires more than merely plugging in the battery. A typical Li-ion battery will use a specifically designed battery charging IC that combines the two different charging methods:

• Constant Current (CC) charging, which maintains a constant current over time during a charging event.
• Constant Voltage (CV) charging, which maintains a constant voltage over time during a charging event.

This combination is known as "CC-CV", and it is the dominant method when charging Li-ion and Li-poly batteries. More specifically, CC mode is initially used to steadily increase the battery's voltage. When the voltage reaches a certain threshold, typically the battery's voltage rating, the charging system then switches over to CV mode. This holds the charging voltage steady while the amperage (amount of potential current) is reduced. When both the battery's voltage and amperage ratings are reached, charging is considered complete and the charging cycle ends. This combined process helps ensure that Li-ion and Li-poly batteries are charged safely. Fire risk, overcharging risk and dendrite formation are all minimized with this approach. Unfortunately, it is also relatively slow, with a total charging time often greater than two hours from depleted to fully charged for microbatteries with less than 100 mAh capacity.

Figure 1: A sample CC-CV charging method curve. Image courtesy of Igensol.

Li-ion and Li-poly batteries must also be discharged carefully and hence usually have battery protection battery ICs to ensure it is safe and does not overheat. Most Li-ion and Li-poly batteries are limited to a discharge rate of 1C to 2C of discharge current which impacts its ability to support wireless transmissions or other high current load events thus forcing design engineers to oversize the battery. In other words, design engineers not only need to size the battery to the correct size for their product but also the current draw requirements of the circuits.

How Ensurge Microbatteries are Better
The Ensurge solid-state microbatteries have a different internal architecture which changes the way they are charged and discharged. Let's look at how that is and how that improves battery performance.

The Ensurge microbattery has what is called "anodeless architecture". It is also solid-state, which means instead of a liquid electrolyte, it has a solid electrolyte. These two features mean that the Ensurge battery is manufactured, charged and discharged very differently than a Li-ion battery.

To start with, the Ensurge microbattery, as manufactured, does not have a pre-assembled anode. It does, however, contain a reservoir of lithium. When the cathode is initially connected to a power source for charging, the electric current carries a small amount of lithium from one section to another within the battery. That lithium then forms a thin, uniform deposit opposite the cathode. That layer functionally becomes the anode, but only after the first charge is complete. This offers several safety and manufacturing advantages:

• The Ensurge microbattery is compatible with more sophisticated manufacturing processes, particularly Surface Mount Technology (SMT), simple clean-room environments and more tolerant manufacturing temperatures.
• Charging speed is also much faster than Li-ion in general. Ensurge microbatteries do not need to rely on the two-phase CC-CV charging protocol described above. Instead, the Ensurge battery can charge via Constant Voltage (CV) alone, and without the need for complicated battery charge control.
• Ensurge microbatteries can also be charged faster than Li-ion batteries, achieving an 80% charge capacity in less than 20 minutes see Figure 2 below.

Figure 2: Charging curve of Ensurge's solid-state unit cell microbattery at a constant voltage (CV) of 4.2 V.

Pulse Discharge Rates
One additional advantage for Ensurge microbatteries is the pulse discharge ability. Bluetooth® and Wi-Fi® technology both rely upon pulse discharge performance, which Li-ion microbatteries have difficulty providing. According to Arvind Kamath, Vice President of Technology and Engineering, "the longer and more frequently wearables and hearables communicate wirelessly, the more battery power they consume. They need high current pulses during wireless transmission that results in oversizing the Li-ion battery." Specifically, Li-ion batteries typically can only produce pulse current up to twice their rated current. If a wearable or hearable product needs better performance, the manufacturer must rely on higher-capacity batteries than would otherwise be needed.

The Ensurge battery avoids that problem through its higher pulsing capability. According to Arvind, the Ensurge battery "can deliver greater than 5C pulse current discharge rates (5 times the rated battery capacity) for several seconds. This supports both Bluetooth® and Wi-Fi® communication (see Figure 3:) chipsets."

(a)	(b)

Figure 3: Figure 3a. shows pulse discharge cycles of 6.4C for the solid-state lithium microbattery, from 4.2 V to 3.0 V. Figure 3b. illustrates the 6.4C discharge cycle across a battery capacity window of 0.55C to 0.65C. Tests were conducted using Ensurge packaged microbattery samples.

In summary, Ensurge architecture offers several charge and discharge advantages over conventional Li-ion batteries, offering manufacturers numerous advantages and performance enhancements to empower their products with a richer feature set and capabilities. For more details about Ensurge microbattery performance, Arvind Kamath presented Ensurge test results at the Techblick Solid-State Battery Event Feb 15 16 2023 - YouTube.