@LuckyGuy what do you think about simply using a wire to short a mobile battery instead of using the packs?
Looks like +-10ft of 32 AWG enameled copper magnet wire shorting a 3.7V Lithium Ion battery pack might do the trick?
On ebay “NEW 2 X Battery + AC WALL charger for Sprint HTC EVO 4G / Droid incredible 6300” is $8 delivered to the states.
Specifications:
1. Battery Type: Lithium Ion
2. Battery Capacity: 1900 mAh
3. Voltage: 3.7V
I understand Lithium Ion is good for 500 charge cycles so you should get a cost of something approaching $0.01/hot-hands-equivalent.
Other considerations:
It doesn’t say if these batteries have a low-voltage disconnect.
I do not know how much additional heat the batteries will create as they discharge.
I think you could simply (permanently) affix the wire to the battery with aluminum shielding tape but there may be a better option.
There are many ways to connect the wire to the contacts. I am leaning towards solder with a tiny disconnect switch somewhere along the length of the wire.