Why RF wireless charging might be a good fit for Electronic Shelf Labels

Aug. 2018

The other day, I read about a company making electronic shelf labels (sometimes also referred to as digital price tags) that is prototyping the use of RF-based charging to deliver energy to these labels.

This is a good news for the young long-range wireless power industry.

The problem being solved is real. A typical US store might have 50,000 such electronic labels. If they last 5 years on a battery, this means that on average 10,000 labels have their battery run out every year. If you were a maintenance crew that worked 250 days a year, you’d need to replace 40 such batteries or labels every day. That’s a lot of work.

I also think RF charging could be a good fit. Here’s why:

Electronic shelf labels require a tiny bit of energy, and thus are within the range that can be delivered using RF power.

Let’s make a technical back-of-the-envelope calculation: SES-Imagotag is a leading company in the digital price tag space. They publish specifications of their tags on the Web site. For instance, their G-Tag S4 label says that it uses a CR2450 coin Lithium battery and it is rated to last 5 years at 2 updates a day. These labels use epaper so they don’t require energy to sustain the display, just to update it.

A CR2450 battery is a 3V battery with a capacity of about 610 mAh, so total capacity of 1.83 Watt Hour (WH). Dividing this over 5 years and 365 days a year, it comes out that the required power is approx 0.001 WH per day, or 1 milliwatt hour per day. Thus, if you are able to deliver 1 mW for one hour each day, you can sustain this display ‘forever’.

In contrast, a low-end model of the Wi-Charge power transmitter can deliver approx 24 WH per day, so enough energy to serve about 20,000 labels, if there was a good way to get this energy to each label.

Electronic shelf labels are spread throughout the store, so a technology that ‘floods’ the space is helpful.

Putting safety considerations aside, if you think about RF energy like radio waves that flood the space, very small amounts of energy can reach around obstacles. Thus, they may be able to deliver these tiny amounts of energy even to labels that are hidden on a bottom shelf.

In contrast, IR wireless charging works best as a point-to-point mechanism. This is advantageous when it comes to power, range, efficiency and safety, but does present some challenges (which can be solved) when trying to deliver energy for a label that is hidden from view.


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