The attractiveness of today’s consumer mobile devices is largely defined by the capabilities and limitations of the compact chemical factory known as the battery. Consumers want thinner and lighter devices with more battery life, faster charging, and longer battery lifespan. These demands push the predominately lithium-ion technology to its limits—and in some cases, to the point of compromising safety (figure 1).
Figure 1: As mobile device manufacturers squeeze more energy and charge time out of the battery, safety margins decline without active mitigation measures. Gravimetric energy density, expressed as Watt-hours per liter, defines the device’s size, weight, and use time per charge. Charging speed impacts safety and battery lifespan, as most cell damage in mobile devices occurs during charging.
According to the U.S. Consumer Product Safety Commission, major manufacturer recalls of laptops, smartphones, portable battery packs, and battery-powered speakers are at risk of fire due to lithium-ion cells. With the increased industry awareness of battery safety, it is useful to review some of the safety mechanisms that are currently being deployed in mobile devices and also look at additional methods that can be used to make mobile device batteries safer in the future.
When discussing the safety of systems, from aircraft to mobile devices, it is useful to picture overall safety as a series of layers, which contribute to making the system safer. Safety is relative; systems can be made safer but are rarely absolutely safe. This is especially true for cost-constrained consumer devices. These layers may comprise design or material choices, tests or mechanisms to address specific known failure mechanisms or in-use active safety monitors to detect and avoid problems before they occur.
It Starts With the Battery
Battery vendors devote significant energy to delivering safe cells by focusing on cell chemistry, materials, cell design, manufacturing, and process control. It should be noted that these efforts have been largely successful. Catastrophic failure rates are extremely low, generally measured in parts-per-million. However, with billions of consumer devices shipping, it is inevitable that a sizable number of failures will occur, including catastrophic failures. Since the late 1990s, numerous industry bodies and international standards organizations have worked hard to improve all aspects of lithium-ion battery implementation, including cell design, manufacturing, cell stress testing, transportation, system implementation, protection circuits, and best practice use guidelines (figure 2).
Figure 2: Overview of international safety standards for portable battery applications. Numerous international organizations have worked for decades to make lithium-ion battery deployment safer. While the scope and focus of individual documents varies, they broadly cover a) design for safety and manufacturing, b) safe operating limits (current, voltage, temperature) and associated protection circuits, c) verification test procedures and d) safe transport of cells and batteries.
Is It Safe?
To get more specific, let’s take a look at some of the safety mechanisms in a typical single-cell mobile power path from the wall socket to the battery (figure 3). The first line of defense is the AC adapter, which typically isolates the secondary output from the AC input and provides output voltage, current regulation, and protection—even for extreme line conditions. The power management IC (PMIC) on the mobile device is responsible for regulating the voltage and current to the battery and generally includes charging and fuel gauge functionality. The PMIC provides an additional level of safety, including implementation of the charging profile over temperature (JEITA guidelines typically reduce charging current and/or voltage at cold/hot temperature extremes). The last line of defense is the protection circuit module, or PCM, which provides under/over- current/voltage and short-circuit protection.
Figure 3: Typical single-cell mobile device power path. Redundant electrical and thermal protection is typically provided at the adapter, in the device, and in the battery pack.
Given an extremely deep set of standards and multiple layers of electrical protection, why do cell failures still occur? And given that failures are still happening, what additional steps can be taken to make mobile devices safer? Let’s step back and consider the aircraft example. Aircrafts have become infinitely safer over time due to a two-pronged approach of integrating better sensors with real-time control over all aircraft sub-systems. Both aspects are critical: better “eyes” to sense the critical control parameters, coupled with more “brains” to process the feedback with real-time closed-loop control.
Now let’s apply these concepts to batteries. Today, batteries are charged much as they were a century ago with “dumb” open-loop CCCV (constant-current, constant voltage), or step-charging. Without timely feedback, cells can evolve towards trouble with no mechanism to detect or avoid problems. The issues that are occurring today are not visible when looking only at current, voltage or temperature. If they were, one of the above redundant protection mechanisms would have been able to stop them.
The time is ripe for the mobile industry to take a different approach to battery safety. Standardization efforts and redundant electrical protection are necessary but not sufficient. The aircraft industry continues to implement and improve dozens of real-time control processes with better algorithms, coupled with more accurate sensors and diagnostics. It is time for the mobile industry to take a similar approach with batteries. Processing power is abundantly available, and for all intents, free. Recent innovations in understanding chemical health and cell degradation in real-time have also provided an enhanced view inside the battery.
A deeper look into battery health will likely provide predictive clues as to when batteries will begin to misbehave, providing an opportunity to stop charging dangerous cells. Battery vendors are not capable of producing zero-failure cells, which creates the need for intelligence on the device to monitor and avoid a potentially catastrophic (though rare) occurrence. The combination of more intelligent charging algorithms, coupled with better health prediction, will add a significant additional level of safety to mobile devices in the future.
Filed Under: M2M (machine to machine)
