Researchers Show Inductive Charging Risks Battery Life Reduction

July 15, 2019 by Scott McMahan

Researchers at WMG at the University of Warwick have demonstrated that use of inductive charging, risks depleting the life of mobile phones using typical lithium-ion batteries. The graphics above illustrate three modes of charging, based on (a) AC mains charging (cable The three modes of charging, based on (a) ac mains charging (cable charging) and inductive charging when coils are (b) aligned and (c) misaligned.charging) and inductive charging when coils are (b) aligned and (c) misaligned. Panels i and ii show a realistic view of the snapshot of the thermal maps of the phones undergoing the various charging methods after 50 min of charging.

Consumers and manufacturers have quickly begun adopting this convenient charging technology, abandoning the hassle of fiddling with plugs and cables. It lets users just set the phone directly on a charging base. Inductive charging bases for smart devices are now included in numerous vehicle models. The use of inductive charging is even being considered for charging of EVs. So a tremendous amount has been invested in the technology.

Inductive charging lets a power source transmit energy across an air gap, without using a connecting wire.

One primary issue with this charging mode is the potentially damaging unwanted heat that is generated, according to the researchers. Both the charger itself and the device being charged can be sources of heat generation with inductive charging. This additional heating is worsened by the close physical contact of the device being charged. Therefore, simple thermal conduction and convection can transfer any heat generated in one device to the other.

In a smartphone which allows inductive charging, the power receiving coil is close to the phone's back cover (which is usually electrically non-conductive).

Packaging constraints require the placement of the phone's battery and power electronics in close proximity. This means that typical inductive charging designs offers few opportunities for dissipating the heat produced in the phone, or shielding the phone from heat generated by the charger.

Batteries are known to age more quickly when stored at elevated temperatures, and exposure to higher temperatures can significantly influence the state-of-health (SoH) of batteries over their useful lifetime.

The rule of thumb known as the Arrhenuis equation is that for most chemical reactions, with each 10°C rise in temperature, the reaction rate doubles. In a battery, the reactions which can occur include the hastened growth rate of passivating films (a thin inert coating that makes the surface underneath unreactive) on the cell's electrodes.

In this way, cell redox reactions irreversibly increase the internal resistance of the cell, eventually resulting in performance degradation and failure. A lithium-ion battery in an environment with an ambient temperature above 30°C is typically considered to be at elevated temperature, the researchers assert. So, exposing the battery to temperatures above 30°C risks a shortened useful life of lithium-ion batteries.

Battery makers also specify that the upper operational temperature range of their products should not surpass the 50°C to 60°C to avoid gas generation and catastrophic failure. These facts led WMG researchers to compare the temperature rises in conventional battery charging by wire with inductive charging.

However, the researchers were even more interested in inductive charging when a phone is misaligned on the charging base. To compensate for poor phone and charger alignment, inductive charging systems typically increase the transmitter power and/or adjust the operating frequency. This typical compensation provokes further efficiency losses and increases heat generation. Such misalignment can be a very common occurrence as the actual position of the phone's receiving antenna is not always obvious or intuitive.

The WMG research team, therefore, also experimented with phone charging that deliberately misaligned the transmitter and receiver coils.

Their tests included all three charging methods (wire, aligned inductive and misaligned inductive). They used continuous thermal imaging during charging over time to generate temperature maps to help quantify the heating effects. The results of the experiments were published in the journal ACS Energy Letters in an paper titled, "Temperature Considerations for Charging Li-Ion Batteries: Inductive versus Mains Charging Modes for Portable Electronic Devices."

When the phone was charged using conventional mains power, the maximum average temperature reached within 3 hours of charging did not exceed 27°C.

However, when the phone was charged via aligned inductive charging, the temperature peaked at 30.5°C, but it gradually decline for the latter half of the charging period. This is similar to the maximum average temperature they observed during the misaligned inductive charging.

With the misaligned inductive charging, the peak temperature was of similar magnitude (30.5°C), but this temperature was attained sooner and continued for much longer at this level (125 minutes compared to 55 minutes for properly aligned charging).

Also noteworthy was that the maximum input power to the charging base was higher in the test where the phone was misaligned (11W) than it was in the well-aligned phone (9.5W). This difference is due to the charging system increasing the transmitter power under misalignment to maintain the target input power to the device.

While charging under misalignment, the maximum average temperature of the charging base reached 35.3°C, two degrees higher than the temperature observed when the phone was aligned, which achieved 33°C. This increase in temperature is a symptom of the deterioration in system efficiency, with the added heat generation the result of power electronics losses and eddy currents.

The researchers do note that future inductive charging design approaches can diminish these transfer losses, and thereby reduce heating including the use of ultrathin coils, higher frequencies, and optimizing drive electronics. Such designs enable chargers and receivers that are compact and more efficient, and they can be integrated into mobile devices or batteries with minimal change.

The team found that inductive charging, while convenient, risks a reduction in the life of the mobile phone battery. For a lot of users, this degradation may be an acceptable price for the convenience provided. However, the researchers recommend cable charging for those wishing to ensure the longest possible life from their phone battery.

They concluded that higher power density devices that are capable of inductive charging will require better magnetic shielding and heat dissipation.

Reference Materials

Loveridge, M. J., Tan, C. C., Maddar, F. M., Remy, G., et al. "Temperature Considerations for Charging Li-Ion Batteries: Inductive versus Mains Charging Modes for Portable Electronic Devices." ACS Energy Letters, 2019, 4, 5, pgs 1086-1091.