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So I am working on a design for battery-heated wrist warmers. My mother has Reynauld's and arthritis, and as winter approaches it gets much worse.

My basic design is that I will take a form-fitting sleeve (made from a shirt or tube sock), weave nichrome wire through the sleeve, wrap thinsulate or space blanket around that, and then fix a nice-looking knitted wrist warmer over it. At the very least I think it would be a good start even without electric heating.

For the electric heating, I am thinking of using a 1000mA LiPo battery, a button switch, and a USB charger.

Ideally, the charger will charge the battery while also providing heat. My specific question is: can the charger I linked to provide that? In a more general sense, I would appreciate any suggestions you might have!

Thanks, and happy holidays!

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LiPo batteries require complex charging patterns to charge the battery quickly and properly. The IC used in that package goes through a preconditioning, fast charge, and then constant voltage mode before it considers charging complete.

Drawing power from the LiPo while charging will interfere with this charging mechanism. It may work, but it will abuse the LiPo and probably affect its life. It may overheat.

Normally the charging circuit will power the system directly while also charging the battery. This can be implemented connecting Vcharger -> diode D1 -> Vin, and Vbatt -> diode D2 -> Vin. When charging Vcharger > Vbatt, so D2 will be reversed biased and the battery effectively disconnected. When there is no charger, D1 prevents the battery from energizing the charging circuit which is obviously not a good idea. If you are concerned about the voltage loss due to the diodes, the concept can be extended such that the diodes control the gates of power MOSFETs instead of directly passing the current. Circuit showing diode connections allowing charger to directly power load when connected

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TI and other IC manufacturers offer ICs that can handle both powring the system and charging the battery at the same time (after all, how can your cell phone stay on while being charged if it will affect the battery). Look at the Battery Selection Tool

It's important to note that you will need enough current going through the charger IC for both battery charging and powering the system.

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It can be considered a serious safety concern to be using a LiPo type battery when it is being charged.

You should also seriously consider that your warmer should include some safety circuitry to prevent uncontrolled current flow from the battery into the heater. You need to consider that wires can fray and short when used in an application where there is constant flexing going on. The safety circuitry could also do well to detect temperature and ensure that conditions do not exist that could burn the wearers wrists.

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  • \$\begingroup\$ I agree! I won't be letting anyone wear it if I can't be sure of its safety. I'll be making sure my charger doesn't use the battery to power the heater while it's charging it. As for the wire, I'll be using insulated nichrome wire so it won't short. \$\endgroup\$ – Nick Strupat Dec 18 '12 at 20:13
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    \$\begingroup\$ @NickStrupat - but you will still have wires going from the battery to the cuffs. And consider that joining copper wires to nichrome wire is always going to be a stress point. \$\endgroup\$ – Michael Karas Dec 18 '12 at 22:36

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