Insight into the inside of the hearing aid

Hearing Aids
#121

The plot sickens. :stuck_out_tongue_winking_eye:

If you think the Phonak Marvel rechargeable battery replacement is difficult, then wait until you get a glimpse at the (soon to be ubiquitous) inductive/wireless charging models.

First, let’s look at the basic features of inductive/wireless charging. You need two coils. The primary coil in the charger induces a current in the secondary coil in the device being charged.

Second, let’s look at the coil in a sample hearing aid (the Signia Pure Charge&Go AX). The coil sits on top of this hearing aid model (circled in red). Extending outwards from the coil (I am guessing) is a metal/conductive magnet that bends around each side of the hearing aid.

How the heck does one remove the battery from that cocooned spot and replace it with a new battery?

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#122

A bonus of the inductive charging is that it should be possible to build our own chargers.

I’ve had a quick go with my P90 Life aids and a standard wireless charger and they definitely sense the charging current but seem to turn on rather than actually charge. There seems to be some sort of handshake going on.

The nearest I’ve got was adding 2% charge via my Pixel 6 Pro’s inbuilt wireless charger to the HAs.
I haven’t spent much time on it but getting any charge in at all gives me some hope that it’s possible without too much difficulty.

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#123

Probably just pull/pry it off I’d think,it looks like it clips into place? but at least it’s not glued/molded into the housing, all that matting (glue) on the Phonak is one thing, but the whole enclosed case is a PITA, I’m really looking forward to the end result on this Phonak DIY project.

#124

I guess, it’s probably more a question of the correct frequency of the magnetic field.

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#125

Curious about frequency with magnetic field.
I thought this was DC.

#126

An alternating field is required to transfer energy inductively.
The coil of the receiver (the HA) is tuned for a certain frequency at which the transmission works optimally.
The receiver circuit then requires an additional rectifier circuit and the charge control.

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#127

Thanks, lots of parts!

#128

I see that these are still available for phones that don’t have built-in wireless charging. Cheap and thin, they contain a rectifier as well I guess. New Qi Wireless Charging Receiver Type-C Charger Adapter For Android Phones | eBay

#129

The specs are actually in the manual: the charger is rated for less than 45 dBuV/m at 135kHz.

The charger I tried it with supported 135kHz, the lights on the charger did come on but it turned on the hearing aids which deactivated charging. I don’t have a working oscilloscope at the moment or I would have investigated further.

The Life induction charger itself is bulky, quite heavy and is awkward to carry around so some custom options would be great.

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#130

Did you say that Oticon has replaceable battery?..
Might be a good move eventually…
Adios Phonak!

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#131

Yeah could well be, but there’s the other manufacturers that we don’t know about yet (as in, is it possible to DIY the rechargeable battery) but Oticon are on to it for sure by allowing the user to replace.

#132

Interesting!
So 135kHz seems to be kind of a standard.
The charger also seems to detect a small enough load as a HA charging current, which would be much smaller than what is needed to charge a phone.
The question is: why did the HA switch ON? Maybe the magnetic field induced a large enough current in internal lines? Such as the lines to the input detecting the toggle switch?
It may be a little farfetched…
When it deactivated charging: do the lights on the charger go off?

#133

Well Yes, we do know. I already checked. It’s easy to determine. Just look at the hearing aid to see if it has a battery door.

Phonak, ReSound, Signia, Starkey, and Widex rechargeable HAs are all sealed and without a battery door for access to the rechargeable battery. I will save evidence of this for another Topic.

#134

My HAs are set to come on automatically when removed from the charger. I’m assuming that was the behaviour that was being triggered.

I haven’t spent a huge amount of time playing around but it might be as simple as getting the positioning correct.

The Phonak chargers aren’t dumb as I have had to reset mine a few times after it stopped working which is why I assumed there might be something more going on here.

I got very excited when I thought that I could just put my aids on the back of my phone for 5 mins and get a quick boost but sadly I only managed to get 2% into one aid and couldn’t get it to work reliably.

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#135

I would be very careful about using non compatible wireless chargers. It’s usually not as simple as sending rf wave of a fixed frequency across and usually there is a handshake that happens before the actual charging process.

On the other hand the charger with metal contacts is very simple. I’m 99% sure that if you send 5v to the little metal contacts the HA will charge correctly.

#136

You’ve misunderstood what I was saying, I’m well aware of which are rechargeable and sealed and which are disposable, what I was talking about was we’re not sure which can easily be swapped out DIY, so far we know that Phonak is a PITA, but for the other manufacturers we don’t know.

Please don’t bother, we don’t need to be educated by you as what can or cannot, I’d leave that to those that actually do the hard yards.

#137

I think that it is actually vice-versa:
If the battery management and charging circuitry is in the charger, which is what it looks like with the Marvels that we have taken apart, then putting 5V on the pins is way too much.

Edit: I just checked with a voltmeter and the Marvel charger does supply 5V directly to the pins

The cells are only 23mAh and to charge at 1/3C as they do in the charger (3 hours) then you would need to regulate the current to under 8mA at 3.7V and then reduce that as the cell reaches full charge. 5V would wreck the cell pretty quickly and maybe even start smoke coming out.

With induction charging it makes sense for the battery management to be in the aid itself which is probably one of the reasons why they are bigger then the standard models. This means that it takes a higher or lower voltage from the induction coil and uses a buck converter or similar to then charge the cell.

If you are using the pins yourself to charge a Marvel then I really wouldn’t go over 20mA and 4.2V.

#138

What I meant (if you insist that I spell-it-out) is that I maintain a PDF document (on this specific subject) and it has pictures showing each of the manufacturer’s hearing aids with sealed cases. But it’s not appropriate to post a link to such a document in these open forums. That’s what I meant by “evidence for another Topic”.

Oh btw, that PDF Document (and related thread dated Apr-2022) precedes this Sept-2022 topic by 6 months. So you’re welcome. What? Oh wait; I thought I heard someone say “Thank you” for bringing this rechargeable hearing aid issue to our attention.

imo; If you think that you are just going to pry off that miniaturized wireless charging coil (along with it’s PCB connectivity) and then later bend it back on, then you have a rude awakening coming.

Apologies for the disruption. Carry on.

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#139

The charging pins are not directly connected to the battery but to the BMS

#140

I missed that in the photos and when I took mine apart they looked like they actually touched the cell casing, that isn’t the case though.
The charger supplies them with 5V, and they only have a small drop across them otherwise.

I just think it’s an odd design choice when space is at a premium: using a diode for short protection and leaving the charging circuitry in the charger would be optimal for space. It’s quite a common design choice.

My original knowledge of LiPos comes from designing and building multicopters where space isn’t normally much of a problem but weight is, so every gram really counts. That’s mainly why I assumed they’d leave that circuitry somewhere else but obviously here I’m wrong.

Maybe if the short protection failed then it would be possible for the contacts to be shorted by sweat or something else potentially causing burns behind the ear.

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