Dane.Kouttron

[9.29.13] Motorola S305 Headset Repair  [quick modification]

What? I aquired a used pair of Motorola S305 bluetooth headphones and unfortunatley they would not charge. They are very lightweight and great for running along with a bluetooth media player, but without any coloumb storage, they arent terribly useful. The following documents the issue and a workaround to get your tunes pumping again.

This is a walk through of a workaround for the S305 'blinking red light of doom' issue.

Initial Debug

Power-up

Conclusion

Image Directory

Initial Debug

Details	Image / Media	Image / Media
Blinking Red LED of doom The headphone model is a Motorola S305 [link]. The problem started with plugging in a micro-usb cable (shown) and recieving a blinking red light on the only light on the thing. I went and checked what the internet had to say.  The responses mainly hovered around 'no idea, get a replacement' [link] Finally in the manual [link], I found "Red flashing indicates a low battery on your headphones". So I left the fellow to charge for 5 hours...
AND NOTHING HAPPENED After five hrs of miscelaneous science, i checked on the headphones again. The same red blink, and no power-on / pairing was to be had. Out came the screwdriver... The heaphones are interesting, the foam earpiece snaps off to reveal the inner workings.
After removing the screws... The layout is interesting. To distribute mass, a tiny lipo is used on the left ear side, and the bluetooth reciever is installed on the right. The left speaker is floated on top of the battery, as only 2 wires are used on the return path to the headset. This is a bit wonky  as to charge the battery, current is flowing through the speaker. I think this cost-cutting topology might have saved a wire, but resulted in some undesired issues.
A low voltage lipo I metered the lipo cell, and it was hovering around 2.6v. This is below 0% state of charge for a lipo, simply applying current and bringing it back to life was the solution, but there are some gotchas. For different cell chemistries, there are 'thou shalt not recover' undervoltage thresholds. For lipos, below ~1v lithium starts to dissociate into the electrolyte and forms fairly conductive dendrites. The more dendrites the quicker the internal discharge. If the cell had reached 0v, chances are there was a small conductive bridge connecting the anode and cathode. Recharging while this bridge is in place results in localized thermal dissapation and generally horrific results.
SLOWWWW charge To prevent any thermal events, I recharged the cell at ~5mA (i had to use an external ammeter to get it that low). Given thatit is a ~120maH cell that would take forever. I left it recharging for 6 hours, on a ceramic plate and observed the cell back at a happy 3.6v. This indicated that, there was a fairly low chance anything internal was shorted. If i had observed the voltage stay the same after that period of time, chances are the current applied was not charging the cell, and some form of damage had occured to the cell, or the connected headphones.

Power-up

Wait, was that it?
Continuing the charge, more waiting and micro-usb I waited for the cell to reach 3.8v and then disconnected the external supply. Using a voltmeter I measured the cell voltage to two digits (3.82v) and let the setup sit for 2 hours. The purpose of this test was to verify that, by itself, the cell maintained its internal state of charge and did not have a high internal self discharge rate. After 2 hours I checked and the cell hadnt budged much (3.80v). Note that a ~ 2mv differential isnt much and chances are it was an artifact of the harbor-freight multimeter I was using. With everything still flopping all over the lab bench, I connected a micro usb charger. The headphones blinked red ~3 times, and jumped to solid red (it was charging from USB).
Repackaging everything Attempt to put all the srews back the same way the headphones were taken apart
Retrospective: Designing an Undervoltage Lockout. Its dangerous to recharge a secondary lithium cell if it has gone below its dissociation potential. Fortunatley that wasnt the case for this pair of headphones. Its possible the designers knew of this issue and chose an artificially high threshold, or, they were simply unable to detect the cell voltage directlry and did so via some other means, given the two wire cable going from the left ear (cell + headphone). This issue could also arise as, during charge, the headphone is passing charge current, and if the cell potential is too low, the speaker could be dissapating more power than it is capable of.  I'd imagine this issue (blinking red led, no charge) only occurs after leaving the headphones at a low state of charge for lengthy periods of time.

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Concluding Remarks:

Do not charge a secondary lithium cell if it has gone below its dissociation potential. Do not solder near the cell, its a very thermally sensitive component. The cell in this set of headphonesl had not fallen below 2v and was recoverable. If your headphone cell is anywhere near 2v, please do not preform this procedure and instead attempt to return the item. A permanant workaround would be to keep the headphones charged after use / periodically.

If you have questions or comments, ask below or send over an email.

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Dane.Kouttron
Rensselaer Polytechnic Institute 
Electrical & Electrical Power
631.978.1650