Do cases slow down wireless charging?

Over the past few years I've had a lot of questions regarding whether phone cases affect wireless charging. Time to do some testing.
Written by Adrian Kingsley-Hughes, Senior Contributing Editor

Wireless charging is becoming all the rage, and I can understand why. There's a convenience to dropping a phone onto a charging pad that you just don't get from scrabbling under the bed or desk for the cable and then fumbling it into the port. While it's not a fast or particularly efficient way to charge a device, we live in a time when expediency rules.

But the growing popularity of this technology has resulted in a few curious readers wondering if a phone case has any effect on charging.

Time to do some testing.

Some of my test equipment.

Some of my test equipment.

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For this test, I assembled a pile of test gear, from USB metersUSB meters that can run from a wireless chargerpower meters, charging pads, and a selection of smartphones and smartphone cases, and I headed over to the PC Doc HQ to see what I could discover.

Many days later…

Before I dig into the results, a few pointers. None of the test gear I've used is "lab grade," so accuracy might be a little varied. That said, I have tested the gear against loads, both fixed resistor loads and electronic loads, and I'm comfortable that any errors are small enough not to matter here.

I also used a video camera to time how long it took the smartphones to charge a fixed amount (say from 25% to 75%), which gives me another metric to use.

As far as phone cases when, I dipped into my stash and used a selection from companies such as RhinoShield, UAG, and OtterBox. I also used some cheap no-name silicone cases that claimed to be OK for wireless charging. I didn't use any really thick cases, or any with metal in them, as this would skew my results.

So, what's the bottom line? Do cases make a difference?

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It makes a difference, but one so small that unless you are looking for it, you won't notice it. How much of a difference am I talking about? On average, the difference was no more than four minutes to charge a phone's battery by 50%, and usually, it was about half that.

What I did notice was that the wireless charging pad drew more current from the wall adapter when charging a smartphone in a case, which indicates that there is some additional load of a few tens of milliamps, and suggests that the case does add extra load, but that the charging pad and device can negotiate a higher power draw to compensate.

I did find one particular combination of device and charging case that caused charging problems: The AirPods Pro case inside a Spigen Rugged Armor case. I could get it to work, but I had to get the placement on the charging pad spot on.

I also found another scenario where wireless charging and smartphone cases don't mix, and that is in hot weather (or when the room is hot). However, this is down to the case causing the smartphone to overheat, and the battery's thermal regulation circuits to reduce, or sometimes completely cut off, the charging to drop the temperature. 

If I was in a rush to charge up, and wireless charging was my only option, this is about the only scenario where I'd bother to whip the case off my phone.

One thing that I didn't try -- which I'd like to in the future -- is whether cheap no-name wireless charging pads are any worse than the more expensive, branded pads. I'd also like to try out multi-coil pads alongside their single-coil siblings.

But for now, that's enough testing, and now you know that unless it's hot, keeping your smartphone in its case while it's charging on a wireless charger isn't making much of a difference.

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