Why Does Wireless Charging Seem Slow?

Why is wireless charging slower than charging via a cable?

Wireless charging is around 70% efficient, meaning 30% of the power is lost during the wireless transmission process. The majority of the lost power is converted to heat which is why your phone can feel warm or even hot when wirelessly charging.

What kind of charge rates should I expect from a wireless charger?

There are many variables which can impact the charge rate of your phone such as battery percentage, applications, having the screen on/off and ambient temperature all have an impact.

Different phone manufacturers also implement different approaches to manage the charge rate in the aim to protect the phone's battery.

Best Case Scenario

Almost flat battery (Less than 5%)
No Apps running, especially ones which use the GPS
Screen off
Low ambient temperature (~20 degrees C) in shade

We've seen charge rates of 1% increase every 2 minutes or a 30% increase in battery charge every hour.

Worst Case Scenario

The battery is almost full (Above 80%)
Running lots of Apps, one using GPS (maps etc)
Screen on
Hot day (30 degrees C) and in direct sunlight

You may only see a 1% charge increase every 10 mins. 6% battery level increase in 1 hour.

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Why does performance drop off so much in worse case scenarios?

There are many factors which impact the charge rate:

When the battery is almost full (over 80-90%) the phone slows the charge rate to protect the battery as it reaches 100% capacity.
When you're running lots of apps, especially apps using GPS, with your screen on, your phone is consuming a lot more battery power so the wireless charger is working hard to keep the battery topped up, let alone increase the charge level.
Heat is not good for batteries. Phone manufacturers know this so they monitor and control the temperature of the battery. If the battery gets really hot your phone can reduce processor speed or even shut itself down. Wireless charging generates heat, and if the battery heats up above a threshold temperate (usually around 30-40 degrees C) the phone will slow the wireless charging rate in an attempt to protect the phone's battery from overheating. If your phone is being charged outside on a hot day (above 25 degrees C) the battery will overheat much quicker so it will appear to charge much slower. Also, most phones have their battery positioned directly above the wireless charging coil, which is where the heat is concentrated during charging!

That said, wireless charging is a great technology and can be very convenient in the right applications, but unfortunately for the foreseeable future it will never be as fast as cable charging.