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Consumers and suppliers have increased their interest in this convenient charging technology, called inductive charging, abandoning fiddling with plugs and wires in favour of just putting the phone straight on a charging base.
The standardization of charging stations and the integration of inductive charging coils in many new smartphones has resulted to quickly growing technology adoption. In 2017, 15 car designs announced the incorporation of consoles in cars for inductively charging consumer electronic equipment, such as smartphones— and on a much bigger scale, many are considering it for charging electric car batteries.
ISSUES WITH WIRELESS CHARGING
Inductive charging allows a power source to transmit energy across an air gap without using connecting wire, but one of the primary problems with this charging mode is the quantity of unwanted and possibly harmful heat it can produce.
There are several sources of thermal generation connected with any inductive charging system — in both the charger and the charging unit. The fact that the unit and the charging base are in close physical touch makes the extra heating worse. Simple thermal convection and convection can transfer any heat produced from one machine to another.
In a smartphone, the power receiving coil is close to the phone’s back cover (which is generally electrically non-conductive), and packaging limitations require proximity to the phone’s battery and power electronics, with restricted possibilities to dissipate heat produced on the phone or shield the phone from the heat produced by the charger.
It has been well documented that batteries age faster when stored at high temperatures and that exposure to greater temperatures can thus substantially affect the batteries ‘ state of health (SoH) over their helpful lifetime.
The rule of thumb (or more technically the Arrhenius equation) is that the reaction rate doubles with each 10 ° C (18 ° F) increase in temperature for most chemical reactions. In a battery, the responses that can happen include the rapid development rate of passivating movies (a thin inert layer that makes the underlying surface unreactive) on the electrodes of the cell. This happens through cell redox reactions, which irreversibly boost the cell’s internal resistance, eventually resulting in performance degradation and failure. A lithium-ion battery residing above 30 ° C (86 ° F) is typically regarded to be exposing the battery to the danger of shorter helpful lives at high temperatures.
Battery manufacturers ‘ guidelines also specify that the upper operating temperature range of their products should not exceed 50−60 ° C (122−140 ° F) to prevent the gas generation and disastrous failure.
These facts resulted in the scientists to conduct experiments comparing temperature increases in ordinary wire battery charging with inductive charging. However, the scientists were even more interested in inductive charging when the customer misaligns the phone on the loading basis. To compensate for bad phone and charger alignment, inductive charging systems typically boost transmission power and/or adjust their working frequency, resulting in further efficiency losses and increased heat generation.
This misalignment can be a very prevalent event as the real position of the receiving antenna on the phone is not always intuitive or evident to the customer using the phone. Therefore, the study team also tested telephone charging with transmitter and receiver coils being deliberately misaligned.
COMPARING CHARGING METHODS
The researchers tested all three charging methods (wire, aligned inductive, and misaligned inductive) over time with concurrent charging and thermal imaging to generate temperature maps to help quantify heating impacts.
In the case of phones charged with conventional mains power, the maximum average temperature reached within 3 hours of charging did not exceed 27 ° C (80.6 ° F).
In contrast, for the phone charged by aligned inductive charging, the temperature peaked at 30.5 ° C (86.9 ° F) but gradually decreased for the latter half of the charging period. This is comparable to the highest average temperature observed during the misaligned inductive charge.
In the case of misaligned inductive charging, the peak temperature was of similar magnitude (30.5 ° C (86.9 ° F)), but this temperature was reached earlier and continued at this stage for much longer (125 minutes versus 55 minutes for correctly aligned loading).
Regardless of the charging mode, the right side of the phone showed a higher temperature gain rate than other phone fields and stayed larger throughout the charging cycle. A mobile CT scan showed that this hotspot is where the motherboard is situated.
Also noteworthy was the fact that in the test where the phone was misaligned (11 watts) the maximum input power to the charging base was more significant than the well-aligned phone I watts). This is due to the charging scheme that increases the transmitter power under misalignment to preserve the device’s target input power.
The maximum average temperature of the charging base while charging under misalignment reached 35.3 ° C (95.54 ° F), two degrees higher than the temperature researchers detected when the phone was aligned, reaching 33 ° C (91.4 ° F). This is symptomatic of system efficiency deterioration, with extra heat generation attributable to electronic power losses and eddy currents.
The scientists notice that future approaches to inductive charging design can decrease these transfer losses and thus decrease heating through the use of ultra-thin coils, higher frequencies and optimized drive electronics to provide compact and more effective chargers and receivers that can be incorporated with minimal change into mobile devices or batteries.
In conclusion, the study team discovered that while inductive charging is convenient, it is probable to lead to a decrease in the battery life of the mobile phone. For many consumers, this degradation may be an acceptable cost for the comfort of charging, but cable charging is still suggested for those who want to eke out the most extended life from their phone.
Source: University of Warwick
Original Study DOI: 10.1021/acsenergylett.9b00663
Reproduced in part under the Attribution 4.0 International license from Futurity.org.
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