Fully charging hearing aids from power banks

Started 24Oct2022. This note is in group Tech notes and sub-group E-FUN (Electronics fun).

If your hearing aid charger, on an off-grid travel, won’t charge longer than a minute from your power bank, here’s what to try out. Or perhaps, here’s what I did when that need arose in the family. You need some fluency with the soldering iron etc. (like have resistors floating around), if not – find someone who has.

The problem is that some power banks will disconnect their output power when the current drawn falls below some level. That level I have measured to be between 50 and 100 mA for different power banks. The reason for this is I guess, not to trickle or idle charge your phone more than necessary, but most importantly, not to drain the power bank. Watch out for a button to press to get the thing started. It might indicate that there also is a current limit.

So, what I suggest here is to introduce a bleeding resistor into the cable. For this power bank inserting a resistance that draws 125 mA seemed low enough. 90 mA was too low, since the power bank decided to cut off some times with that value.

This is only rocket science if nobody has suggested this to you before.. Well, not even then. It’s plainly just obvious.

However, this does mean that if you don’t need it any more, your power bank will also “bleed”! So: unplug the cable when it’s not needed! (But if you have a 20 000 mAh power bank, it will survive for 20000 mAh / 125 mA = 160 hours – or let us say 100 hours – so you shouldn’t need to watch over and unplug it the first second after your hearing aid signals that it’s fully charged.)

Fig.1 – Power bank and charger and meter

Here’s what I did. I opened the charging cable, which was an USB-A to USB micro-B cable. (In the future it might be USB-C in the one end. I assume that micro-B will stay with us for a long time). I introduced two resistors in series as 18 Ω +  22 Ω = 40 Ω (Ohm). I call this for a resistor in singular after this. This is often written as 18 + 22 = 40, to differentiate from kOhm etc., which would be 18k+22k=40k. The reason I used two is to avoid them becoming too hot. As you can see from the Ohm’s law calculations:

I=V/R   : 5 V / 40 Ohm   = 0.125 A     = 125 mA
W=V*I   : 5 V * 0.125 A  = 0.625 Watt  = 625 mW
W=R*I*I : 18 Ohm * 0.125 mA * 0.125 mA = 281 mW
W=R*I*I : 22 Ohm * 0.125 mA * 0.125 mA = 343 mW

..is that the 22 Ohm needs to dissipate 343 milliwatt. (Voltage is in Volt, current in Ampere and resistance in Ω or Ohm). So I used resistors that could withstand this (like 400 mW or 500 mW). I soldered on a little board and used heat-shrink tubing around the resistors to avoid any internal short circuits, and also around the whole piece for protection. I can feel it gets a little warmer than my fingers, but that’s all. It is however still definitively not a good idea to keep it under you pillow – give it proper air cooling!

Fig.1 does not show the hearing aid charger. But it does show my new USB meter which tells that the cable itself now draws 0.10A. At 5.09 V it should have shown 5.09 V / 0.040 kΩ = 127.25 mA or 0.13 A, but fair enough. My 40 Ω was pretty accurate. (UNI-T USB Tester, UT658DUAL. Disclaimer)

Why does it work? The hearing aid charger gets its needed constant 5V from the power bank at all times, even when the charger’s own power consumption goes to almost zero (*). The power bank continues to feed 5V into the cable’s internal resistor, which draws 125 mA. This is so much that the power bank won’t ever give up on it. Even if the hearing aid charger might draw only 5 mA, the sum of 125 + 5 mA will of course be above the power bank’s cut-off limit, since 125 mA is so.
(*) Say proportional to milliamps consumption, since power here is milliamps times the constant 5V.

A better idea would have been to produce a cable for this, one that contained some electronics that cut off the resistor when the user pressed a button, and then remembered the current drawn by the other consumer, and used it the next time it was powered.

Norwegian: hvordan få en lader til høreapparater til å kunne blie ladet fra en powerbank som slutter å levere strøm når nivået går under en viss terskel.

Ac src meters with meter-shunts speaker with load-shunt rev2Aside:  This figure shows the parallel resistor RL. – which is what we would call our resistor as well. I actually drew this picture for Wikipedia. I drew it two times, after I first had photographed an old analogue milliamp-meter’s face and used that photo in the figure. I wasn’t allowed to use it, because I could not find the proper owner of that face. Fair enough. So I made thsi second drawing instead, from scratch. I am Aclassifier a Wikipedia Commons (here).

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