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I have Motorola Moto 360 2st edition smartwatch and watch original battery is dead. I have converted smartwatch into table watch in DIY wooden block and added 18650 3.7v battery instead and now its works fine for 2 to 3 days but then battery dies and I have to remove 18650 and charge with its charge.

Dead Smartwatch original battery photo.

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Can I convert regular phone charger with 5V output to 3.8V output to run my smartwatch continually? If yes what I have to do ?

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  • \$\begingroup\$ Max voltage for 3.7 V lithium batteries is 4.2 V, so your smart watch ought to survive that much. The simplest solution is to put a 4 V linear regulator between the 5V and the smart watch. \$\endgroup\$ – Dampmaskin Feb 21 at 14:49
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I do not know the internal charge and battery management structure of the watch. In general:

  • There will be short current surge which charger should be able to handle

  • The 5 V can damage the parts. You have to have a LDO regulator with good amount of capacitors. Setting the LDO output of the typical voltage of battery

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  • \$\begingroup\$ Can I use DC-DC converter step-down 1.23-30V 1.5A? Specs Input voltage: 4 – 35VDC * Output voltage: 1.23 – 30VDC * Max current: 3A (1.5A continuous) * Maximum efficiency: 90% \$\endgroup\$ – Skull Feb 22 at 9:24
  • \$\begingroup\$ It's a low power device. The DC DC regulator might be plausible, but before that, I propose to monitor the surge current and current consumption needs over all. \$\endgroup\$ – User323693 Feb 22 at 9:33
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2 silicon diodes in series will drop 1.2 - 1.4 volts or so depending on how much current your watch needs.

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The Lo-PO battery is 4.2V when fully charged so simply add a silicon diode (1N4148) in series with it to have 4.3V which will be fine.

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