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Thursday, 27 June 2019

Wireless charging can reduce cell phone battery life

The three modes tested: (a) charging plugged into the network; (b) aligned inductive loading and (c) misaligned inductive loading. [Image: Loveridge et al. - 10.1021 / acsenergylett.9b00663]

Inductive charging

The way you recharge your phone - from the standard charger, plugged in, or the inductive, wireless charging - can change the life expectancy of your battery.

This is the conclusion of Melanie Loveridge and colleagues at the University of Warwick in the UK who compared three modes of charging the cell phone, two of which involved charging wirelessly.

Inductive charging allows a power source to transmit electricity through an air gap , without the use of wiring.

The inclusion of inductive charging coils in several newer models of mobile phones has led to the rapid increase in adoption of the technology. In 2017, automakers announced the inclusion of consoles within 15 models to inductively charge consumer electronics devices, including cellphones - and on a much larger scale, several companies are considering charging electric vehicle batteries in the same way.

The problem is that this charging mode generates a lot of unwanted heat, which harms the battery, decreasing its life.

There are several sources of heat generation associated with any inductive charging system - both in the charger and in the apparatus being charged. This additional heating is aggravated by the fact that the apparatus and the charging base are in physical contact, which means that any heat generated in one of them is transferred to the other by simple conduction and thermal convection.

On cell phones, the coil that receives power is attached to the back cover of the phone, next to the battery and everything else, which limits the possibility of dissipating the heat generated inside the phone or to protect it from the heat coming from the outside.

The life of a battery is closely related to the temperature at which it operates. The higher the temperature, the smaller the number of cycles in which it can be charged and used. 

Batteries and temperature

The batteries of lithium ions are chemical devices, and a rule of thumb - or, more technically, the equation Arrhenuis - establishes that for most chemical reactions, the reaction rate doubles for every 10 ° C increase in temperature.

In a battery, undesired reactions that may occur include the accelerated growth rate of passive films (a fine inert coating rendering the underlying surface non-reactive) on the electrodes of the cell. This occurs through redox reactions, which irreversibly increase the cell's internal resistance, resulting in degradation in performance and, ultimately, failure.


An additional problem encountered by researchers occurs when the coil of the device being charged is not perfectly aligned with the coil of the charger - the results are even worse, with greater heat generation.

Although manufacturers warn of catastrophic failures - explosions, for example - at operating temperatures above 50 or 60 ° C, a lithium-ion battery with a temperature above 30 ° C is typically considered at elevated temperature, exposing the battery to the risk of a shorter life expectancy, say researchers.

So, although the team has not established how much battery life your cell phone will lose in each case - which would require long-term observations and a large number of handsets to establish an average - the message is quite clear: Cell phone heats up with inductive charging, and battery and high temperatures do not.

Alignment between the machine and charger coils is essential for greater wireless charging efficiency. [Image: Loveridge et al. - 10.1021 / acsenergylett.9b00663]

Charging and reducing battery life

In the case of the telephone charged with the charger plugged into the conventional mains, the maximum average temperature reached within 3 hours of charging did not exceed 27 ° C, starting from an ambient temperature of 25 ° C.

In contrast, with the phone being charged by aligned inductive charging, the temperature peaked at 30.5 ° C, which was gradually reduced during the second half of the charging period.

In the case of misaligned inductive loading, the peak temperature was of similar magnitude (30.5 ° C), but this temperature was reached earlier and persisted for much longer at this level (125 minutes, versus 55 minutes for correctly aligned loading) .

The maximum average temperature of the charging base during charging under misalignment reached 35.3 ° C, two degrees above the temperature detected when the phone was aligned, which reached 33 ° C. This signals the deterioration in system efficiency with additional generation of heat attributable to energy losses and parasitic currents.

Also noteworthy was the fact that the maximum input power at the charging base was higher in the test where the phone was misaligned (11W) than with the phone well aligned (9.5 W).

The team's conclusion is that the inductive charging, while convenient, will likely lead to a reduction in the battery life of the mobile phone. For many users, this degradation may be an acceptable price for convenience, but for those who wish to take advantage of the longer phone life, cable charging is still recommended.



Bibliography: 

Temperature Considerations for Charging Li-Ion Batteries: Mains versus Inductive Charging Modes for Portable Electronic Devices
Mel J. Loveridge, Chaou C. Tan, M. Faduma Maddar Guillaume Remy, Mike Abbott, Shaun Dixon, Richard McMahon Ollie Curnick, Mark Ellis, Mike Lain, Anup Barai, Mark Amor-Segan, Rohit Bhagat, Dave Greenwood
 ACS Energy Letters
 Vol .: 4, 5, 1086-1091
 DOI: 10.1021 / acsenergylett.9b00663

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