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Qi /Qi2 overheating

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I'm developing a product which has body-worn wireless sensors. It's very convenient to use wireless charging for these as it's possible to encapsulate them in a sealing package with no need for connectors (the sensor design is such that it needs to be airtight, not just showerproof).

Previously, I was using Qi receivers intended as aftermarket upgrades for phones, and low cost wireless power transmitters. These worked pretty well - I only need a small battery so they charge quickly. The only irritating factor was that the rx/tx distance needed to be short and the devices well aligned, or they didn't charge.

Recently, Qi2 chargers have become very widely and cheaply available, and they tend to be the default. I have not yet moved away from the aftermarket receivers although I probably will do as they're quite big and smaller options are becoming readily
available. But my typical setup at the moment is a Qi2 transmitter and a Qi receiver having no magsafe-style alignment magnet

I am seeing problems with the sensors overheating whilst on charge, with the transmitter current consumption going up to 2A even though the battery current is only 100mA. The inefficiencies of the wireless coupling are such that I'd expect the transmitter to take no more than 250mA.

I believe the problem is that the Qi2 transmitters are capable of putting out more power to compensate for suboptimal coupling, but when they do this they get hot. This then heats up the sensor. I haven't yet proved that the sensor doesn't generate heat or overcharges the battery but  I don't think so. The mechanism seems to be that the transmitter grills the sensor.

It's also possible that the reason for the poor coupling is that some other element of the system is absorbing power from the magnetic coupling. Quite early wireless charging designs used smart power control to avoid just building an inductive heater but compensating for poor coupling that is really due to an adjacent 'shorted turn' may be an issue.

Trying to find out how to stop this lipo-frying effect, I find many people complaining of their phones overheating due to wireless charging. It's apparently a growing problem and may point to a screwup by Qi or common implementations that get it badly wrong. These seem to particularly involve built-in chargers from large automotive manufacturers. I can imagine it becoming a very visible problem when it causes a car fire.

Is anybody else struggling with this problem ? I'd like to find app notes, recommendations etc. Perhaps a thermal cutout that stops the receiver maintaining transmission if it gets too hot. Maybe modern receivers have this built in whilst older Qi receivers don't. If so, the backwards compatibility has a rather major flaw.


If you put just one of your Lipo batteries on top of the Qi2 transmitter with nothing else attached to the battery, does it heat up when the Qi2 charger is on i.e. does it get hotter than the charger?

Can you disassemble the device and then place it on the charger? Try to figure out where the heat is coming from.

--- Quote from: artag on June 15, 2024, 10:54:55 am ---These seem to particularly involve built-in chargers from large automotive manufacturers. I can imagine it becoming a very visible problem when it causes a car fire.

--- End quote ---
If wireless EV charging ever becomes a thing, there definitely would be sensors to detect abnormally high temperatures, that's exactly what I have seen on a proof of concept demo unit. But the relatively poor efficiency means it's unlikely to be used while parked, a robotic arm would be just as "wireless" as far as the user is concerned, have the efficiency of regular wired chargers, and be cheaper to implement. Using while driving would be a much better use case for wireless charging, but I don't see that becoming practical in the foreseeable future. Tracking the vehicles would be a bit of a challenge to implement, especially with the need to keep the cost down to a practical level. Energized lines or rails similar to what trains use would be far more likely to be practical.

I can’t get too specific but I have some domain experience.

On the general heating problem: yes, there are devices out there which cut back the power flow to limit temperatures

Some basic questions:
- do you have any permanent magnets near the PTx or PRx coils? They tend to make ferrites saturate, which can cause excessive current flows in the coils
- are you using a certified Qi transmitter? There are allegedly an off-off-brand units out there which don’t behave right, don’t implement safety features etc.
- how much control do you have over the design of your receiver?

Some more advanced questions:
- is your receiver getting up to and regulating at the desired bus voltage?
- does the problem disappear (or get better) if you carefully align the receiver coil for best coupling?
- does your product have any ferrous material (eg steel) in it that isn’t related to the power receiver?

Thanks for the input. I have a bunch of tests I know I need to do but was looking for initial directions from people who may have investigated already.

I have complete control over the receiver design but am trying to use off-the-shelf coils+controllers at the moment as they're cheaper when in low volumes. I am, however, moving from the 'vinyl-sandwich with USB' type to 'coil+pcb', and these seem to use newer designs. At least, they have an awful lot less passives. Maybe I should add some pics to illustrate that better. It's the older ones I'm seeing the problem with - but I haven't tried the new ones yet.

I want to be able to use off-the-shelf transmitters, partly to avoid another custom manufactured accessory but also because I expect the end users to use any convenient charger even though I will supply a tested one.

I have a selection of transmitters. Most are generic unbranded but of designs comon on ebay/aliexpress. A couple are branded but from phone manufacturers (both Samsung, I think). I have seen the problem on at least two types. I also have a presumed genuine apple transmitter (packaging clearly marked apple and bought from a very large supermarket chain with only a small discount). Fakes are possible but the only more certain source would be an actual apple store.

There are no Rx magnets at present but I plan to use an apple-like magnet ring to help with alignment. The first problem I saw was indeed with a no-name magnet-ring transmitter but I'm pretty sure the next was with a non-magnet type (though it was dual-pad). This adds to my suspicion that the Tx is increasing power to compensate for poor coupling without limiting the temperature rise. It's possible they rely on the receiver to disable itself which is probable on phones but not with the generic adapters. There is some steel but not large : a bmp280 pressure sensor and the shielding can over an nrf52840 bluetooth module. They might be nickel plated or tinned copper, I haven't checked for that.

NiHaoMike : Although the reports are from people with Teslas and other, I should make clear that this is nothing to do with EVs. It's a Qi phone charging pad fitted to the car by the manufacturer. Presumably a bought-in item but still capable of doing reputational damage if they set fire to a phone in a car.


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