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I am a happy owner of a Garmin Fenix3 fitness watch and a developer for watch faces as well. I noticed my watch charges and communicates data through 4 pins (probably for the USB layout 0,VCC, Data+, Data-) on it's back side as shown here (http://i219.photobucket.com/albums/cc18/gaijinnv/fenix-3-back_zpsoxagq2pb.jpg) I have been using my watch in numerous occasions underwater (swimming etc) and i was always curious on how these pins (and especially the charging ones) are not short circuiting leading to power failure or "crazy" behavior.

This charger layout might be helpful too https://forums.garmin.com/filedata/fetch?id=1176424&d=1454672325

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There are basically two ways to transmit energy under water:

  1. without any contact, through a dielectric like plastic, using induction with AC currents. I've seen a lot of devices charging like that.

  2. through pins which are encrusted inside plastic (so only the extremity points outside). Of course, you have to worry current will not flow inside the water, for example, if the current is DC, by inserting a diode. In the case of battery charging, nevertheless, I would be surprised that the battery is fed directly through a diode: even a lead acid battery needs a charging unit that sets precisely the charging voltage. So, there is probably a chip that regulates the voltage from the input according to the battery charging specification. To oversimplify, suppose that the battery is charged though a LM317 voltage regulator (of course, it is not in your watch). Then if you set the voltage to 0 at the adjust pin, there will be no current at the output and input pins of the regulator. So, all you have to do is a unit that feels if some voltage is present at the input pin of the regulator, and sets the voltage to some defined value or to 0V at the adjust pin, according to whether there is some voltage or not. A similar process may have been used in your watch.

Notes:

  1. the device that "feels" if there is a voltage at the input pin and adjusts the voltage at the adjust pin may be nothing more than a transistor.

  2. Yes, it may be possible to charge the watch under water, because the water resistance is sufficiently large to load only slightly the charger, or simply because the plugging is sufficiently tight to isolate the contacts from water.

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Those pins are only connected to the battery or whatever when the power chip senses a charging current available - otherwise there is no real connection...

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  • \$\begingroup\$ If no physical connection is realized between the pins and the circuit how can the charging current be "sensed". Also there are cases when i connect my watch to a usb port and though it gets no charge it still transfers data... \$\endgroup\$ – kokobill May 18 '18 at 8:56
  • \$\begingroup\$ So “the pins and the circuit” which circuit - the internal one to the battery or the external charging one... \$\endgroup\$ – Solar Mike May 18 '18 at 8:59
  • \$\begingroup\$ Some usb cables are charge or data only... \$\endgroup\$ – Solar Mike May 18 '18 at 9:04
  • \$\begingroup\$ Are there 2 different circuits totally isolated from each other? If yes, the outer "charging" one would at least use some current in order to "sense" a connected charger etc. Do you know how this isolation is implemented? \$\endgroup\$ – kokobill May 18 '18 at 9:08
  • \$\begingroup\$ @kokobill I don't think that a physical isolation is implemented given the size of the device. You just switch the lines to a high impedance sensing mode - the leakage current is controlled to be small enough not to matter. \$\endgroup\$ – Arsenal May 18 '18 at 9:28
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I would expect there to be (at least) a diode between those pins and the battery, preventing current from flowing out and across the pins.

You won't be able to charge it underwater, that definitely will be a problem.

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