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I want to make an Arduino wristwatch that can be wirelessly charged by inductive charging. The two 3.7v Lithium Ion button cells are hooked up in series (to make around 7.4v) which powers my Arduino Nano (requiring at least 5v input).

I found out how to transfer 12v through a coil to another coil which receives the voltage and converts it to 5v. The first coil will be inside a "dock" I made for the wristwatch. The second coil should be inside the wristwatch itself. This means that when the wristwatch is placed on the dock, I receive a steady 5v @ 500mA.

What would I have to do in order to get the two 3.7v Li-Ion button cells (in series) to be charged by the 5v received from the wireless induction module?

If you think 5v is not enough, how should I alter the coil to increase the voltage wirelessly transferred? How much voltage is needed for this? Any circuits for a better wireless charging module?

Is there any other way of powering the Arduino with a small rechargeable battery that can be recharged by wireless inductive charging?

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Yes, you can charge your batteries with the 5 volts.

1) You would need a boost converter to boost the 5 volts up to the voltage needed to charge the batteries, which would eventually be 8.4 volts.

2) You will need a constant current/constant voltage power charger so that the batteries are charged correctly,

3) Lithium batteries in series need to balanced if they are charged in series. This means that you need to add a battery management chip to your charger that will balance the voltage of the batteries, and make sure that the batteries are not over charged or under charged - very important. Lithium ion will catch on fire or worse if not managed properly.

4) It might be easier to have a boost converter and ONE lithium ion battery and just boost the voltage of that battery to what your watch needs. It would make the charging of the battery easier. Of course you would need to find a battery of sufficient capacity.

Good luck

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    \$\begingroup\$ Also, if you go along with the approach suggested in 4) check this out: sparkfun.com/products/11231. Even if you don't use it directly, their freely available schematics are a good start to designing this kind of power circuitry \$\endgroup\$
    – Zuofu
    May 27 '14 at 0:41
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Why are you only getting 5V from your inductive transfer system?

If the receive coil is resonant with hi-Q the voltage unloaded can be very high and the loaded voltage would be set by load behaviour. For a 2 x LiIon battery in series (=2S) load the voltage will settle to around 2 x Vbattery.

IF the 5V RX voltage is caused by clamping or regulation change or remove it.

If the RX coil is not resonant, resonate it.

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