Wireless charging is highly viable now, the cost has come down to about $3 it should be in everything.
It does have its thermal challenges but they exist now in such corner cases I think so long as we have wired as a fallback we’re fine.
Math:
- most devices now are thermally constrained - their limit of recharging is determined by temperature and battery ageing.
- high end flagship phones have given themselves headroom via heatpumps, they are copper tupes with a thermally transmitting liquid which evaporates near heat sources, and condense further out. This is a lot higher cost than $3.
- Wired portable chargers are around 94% efficient from their cells to the socket, the socket-cable-socket is losing about 0.5%, and the phone which is recharging is about 80% efficiency . These add up to 75% efficiency overall.
- So wired at 18W, you lose 1W in the powerbank, <1W in the cable and 3.6W in the phone.
- Wireless, the cable 0.5% loss becomes nearer to 15% wireless loss. This is due to the fact wireless is inherently less efficient, laws of our universe, photons scatter in all directions, some of them end up in the receiver, some end up causing eddy currents in nearby metal, some just disappear and absorbed in the next surface (wall, etc). So that <1W in the cable becomes 2.7W lost, most as heat in the proximity.
So then focus on that 2.7W lost as heat and convert it into a temperature.
- Say you used wireless for an hour, 2.7W x 1hr = 2.7Wh.
- 2.7Wh is 3600Joules, or 860 gram calories
- so a typical phone is going to have it’s temperature raised about 2C if evenly spread through the phone, but in reality it becomes focused on the metal near the transmitter, an area weighing just a few grams so it can raise temperature locally by 20C and relies on metal to transmit it to the whole phone for where convection and radiation to outside world.
But that doesn’t mean you don’t do wireless. You’d not want to purely rely on wireless to do the fastest recharging as that heat is being asked to be built up quickly, giving less time for convection/radiation (phone feels hotter).
No, a top-up for a few minutes is fine, as the heat is building for a few minutes. Pick up / put down is a perfect use for wireless. Doing a slow recharge overnight is also fine as the phone can simply thermally throttle down the Wattage in response to temperature, plus the phone’s other components (screen, cpu, GPU) are not generating much heat overnight. Between the top-up during the day and a slow full charge overnight, wireless is today comfortably not ageing phones.
Where I think the engineering challenges are fascinating is when you sandwich the portable charger cells between the wireless and the phone, as you’re putting all the losses (30%) in one compact area. It’s like asking a slice of cheese to not melt between two burgers.
Where wireless is dumb is in a fairly narrow, and likely rare situation, expecting to pay a high-end phone and put on a pad and use it all the same time, and expect it to fast recharge. That expensive CPU GPU just is being throttled during the period and your flagship isn’t being flagship in those moments.
Hence why wired still has a role, in the fastest recharging from empty.
