Wireless Charging: What causes this?

[lopri]

I bought a Nokia wireless charger ($25, AT&T) and it has been working great for my Nexus 5. Then I charged (or so I thought) my Nexus 4 yesterday and found this.

Check out the "Charging" row. It looked like the charger was trying to maintain the battery level of the phone when it was put on (50~55%), instead of filling it up. The troubleshooting was simple yet delicate in that I had to find the right position on the charger. I got curious what mechanism is at work in this situation.

 

[JeffMD]

Induction chargers aren't the most effective when it comes to raw power input. Just like if you used a low powered charger,if the phone is doing something that exceeds the power draw then it won't be able to make forward momentum on the battery.

 

[paperwastage]

if you had to reposition, this means the induction coil in charger cannot deliver power properly to the device.

my best guess is that the charger is behaving like the turbo mechanism in CPUs. in "turbo" mode, the charger pushes max power and can deliver part of it to the device.

after a while, device overheats or goes past the limits and has to throttle. in "throttle" mode, it doesn't deliver any power to the device. after a while, it can go back to "turbo" mode

 

[Belegost]

The Qi standard uses unidirectional communication from the charging device to the charger, this communication is done by providing loading on the receiving coil which changes the current draw. The charger will then respond by adjusting the output behavior.

One part of this is that when the current falls below some limit the charger assumes the device is fully charged, and will stop providing charge full-time. It will occasionally send a burst which serves to monitor the state of the device (and determine if charging needs to restart) and provide a small amount of charge that should maintain the battery level given the drain from the idle device.

It seems like the positioning caused the current transferred to be lower than expected, so the charger thought the phone was charged too early.

 

[lopri]

@Belegost: If I am understanding correctly - phones reduce currents as they near the capacity, and the charger misunderstands that signal, which was caused by a sub-optimal position the phone was placed on a charger, as a Stop-sign, as if the phone was fully charged?

@Jeff: I don't think the plus/minus nature explains why the charger stops intermittently.
@paperwastage: I did not notice a consistent pattern of phone getting warmer while being charged wirelessly. No more so than wires. Although there were times I felt the phone got hotter than expected, that did not coincident with this odd charging behavior.

Though I am certainly curious when/why the phone gets warmer while being charged - without a recognizable pattern. Sometimes it seems to get warmer but other times it doesn't, but I have no idea why it isn't always one way or the other.