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After many years, I'm getting back into some simple wiring. I'm trying to make a self-sustained wifi tracker. This tracker will have a 3.3v Button cell lipo battery as a battery backup and I want to be able to charge this battery backup any time it's plugged into the main battery. The main battery can be up to 42v, and needs to be stepped down to 3.3v to charge the button cell without damage and also to run the ESP8266 wifi module.

In the diagram below, I'd like to be able to shutdown the pololu step-down converter when the button cell lipo is charged, but I don't know what this is supposed to look like. Possibly an N-channel mosfet?

Here's the diagram: enter image description here

I'm hoping this circuit isn't completely wrong in the first place, but if it's not - how do I use the SHDN (shutdown) pin on the Pololu to not allow any more voltage to come through? I feel like I need a switch behind the button cell to turn off when the button cell is 3.3v or more?

Edit: The reason I want the button cell in there is incase the main battery falls off or is disconnected in any way. Also, this will be "always on", so when it IS hooked to a battery, it will charge the 3.3v button and "turn back on" if it has previously died.

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  • \$\begingroup\$ Button cell lipo? \$\endgroup\$ – Bruce Abbott Sep 15 '17 at 16:56
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    \$\begingroup\$ What I believe you want is a battery charger with powerpath management. You would step down to 5V ish and let the charger take care of the cell \$\endgroup\$ – sstobbe Sep 15 '17 at 18:12
  • \$\begingroup\$ @BruceAbbott, I just realized the batteries I have are LImn, not lipo :( So now I'm just dead in the water \$\endgroup\$ – ntgCleaner Sep 15 '17 at 18:31
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Most coin cells have very low current specifications, both for charging and discharging. For discharging, this means you have to switch off wifi or your supply voltage will drop dramatically (basically to zero). For charging this means you can only charge very slowly.

If you want to "fast" charge your coin cell, ie with more than 10mA, you severely degrade the number of cycles. I once did some measurement with some Maxell LiMnO2 cells (IIRC CR2032) and didn't get more than 10-20 cycles out of them.

I suggest you use something else than a coin cell.

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  • \$\begingroup\$ Hmm, that's going to be tough, I'm trying to get extremely small (20mm x 20mm -ish) and lightweight. I wonder if I can use the 3.3v from the pololu to charge the coin cell and also run the wifi at the same time, then if the pololu has no current, just run the wifi on the coin cell? \$\endgroup\$ – ntgCleaner Sep 15 '17 at 16:56
  • \$\begingroup\$ The coin cell will not be able to deliver enough current to run wifi from it. The resistance of the coin cell is just too high. \$\endgroup\$ – Attila Kinali Sep 15 '17 at 22:46
  • \$\begingroup\$ This answer is a very valid point but does not adress the question. IMHO it should be a comment. \$\endgroup\$ – KubaFYI Jan 9 at 15:54

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