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Wireless charging metrics arguments heat up

Posted: 06 Aug 2015     Print Version  Bookmark and Share

Keywords:WPC  Qi  wireless charging  A4WP  PMA 

Wireless charging may have a standards battle to struggle with, but the Wireless Power Consortium (WPC) believes there's also a major a measurement problem. So WPC camp up with a study to find out the best method for measuring energy efficiency in wireless charging.

"Efficiency studies on existing products are hard to find and for some standards in development, non-existent," the study abstract stated. "Making matters worse, studies and marketing materials that tout efficiency numbers use different measurement techniques and measure different sub-circuits."

The wireless charging market is currently led by WPC's Qi standard, a coil-based system with products on the market that WPC describes as neither inductive nor resonant. The Association for Wireless Power (A4WP) and Power Matters Alliance (PMA) back a loosely-coupled coil architecture called resonant charging, but they run over two different frequencies, neither of which currently have widely available products. Other wireless charging startups use novel approaches with RF or lasers.

These competing standards have "resulted in pent-up consumer demand," said Colorado State University professor and study co-author Cameron Moore. Consumers have failed to adopted products because they don't want to buy into a technology that may be overtaken or displaced, he said.

In their study, WPC and CSU assessed the efficiency of two wireless charging architectures: a 100kHz to 200kHz Qi system and a 6.78MHz resonant system on a 2,100mA hour battery charging application. Researchers measured load-efficiency for each wireless power system, power transfer efficiency versus output current, as well as total energy loss for device and system. The study has been submitted to the IEEE-PELS group for review.

Wireless charging

Efficiency measurements are taken at either end of this block diagram. Source: WPC

Wireless charging efficiency should be measured by the amount of power into the transmitter and the amount of power into the battery, but models for this don't exist, said John Perzow, WPC VP of marketing. Researchers found it takes 26,000 joules and 150 minutes for a lithium ion battery to charge 90 per cent, then calculated the battery "cost" of energy input into the wireless transmitter using the measured load-efficiency characteristics.

Still, competing standards bodies and power experts are sceptical about the need for an efficiency measurement standard, or if an unbiased one is possible. Several analysts said global interoperability is is a timelier, important issue.

"There's always a good case to be made for a single, unified, standard measurement technique," said IHS analyst David Green. "But it's an ideological view, and rarely happens in practice. The best you normally get is just a handful of similar measurement techniques that are used across whole regions or applications."

Measuring efficiency is not a priority in the wireless charging industry because there are such a wide variety of devices, Power Matters Association (PMA) president Ron Reznick said. Devices that use resonant or loosely-coupled coil systems will have less efficiency because they require more power to charge over distances.

"The bottom line point is it would be extraordinarily complicated and costly to measure efficiency using the same parameters for every device," noted Reznick. "[Efficiency is not the most important factor right now because [standards developers are] already taking it into consideration. It's also not something they're ignoring."

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