Batteries 
Batteries
There are a number of factors to consider when selecting a battery for any application. They include power consumption, energy density, capacity, lifetime, shape, weight, and cost. For the glove heaters the major parameters were that the battery be flat and relatively flexible and that is provide enough power to use the gloves for at least one day (6-8 hours) before having to charge them. Rechargable batteries were selected because heating elements tend to consume a lot of power and disposable batteries would therefore have to be replaces every day.
The next step in selecting a battery was to estimate the power consumption of the glove. This does not depend on what type of heating element we use because most heating elements are practically 100% efficient. However this does depend on the type of fabric we are going to use as an insulator.
We did a very rough calculation with Professor Ceder assuming there was no insulation on the glove (this is probably an overestimate):
To look at how long a battery will last we need to look at the number of Watt*hours produced. This is just the battery's capacity times the voltage. Then you can figure out how long the battery will last given your power consumption. For instance since our gloves needed 1 Watt, a battery with a capacity of 2 Amp*hours and 3 volts will give us 6 Watt*hours so it would run our gloves for 6 hours before it ran out.
A good resource to get specific information about batteries already on the market is the Handbook of Batteries by David Linden (McGraw-Hill, second edition, 1995). You can find the information you need to calculate how long a given battery will last here.
The power of watch batteries is much too low for this type of application because generating heat requires a lot of power. The best candidate for our glove heaters is a lithium polymer battery.
Lithium polymer batteries are rechargeable, flat, and relatively flexible. They can be manufactured in a wide variety of shapes and sizes. Because they are usually a higher voltage than alkaline batteries (3.6V compared to 1.5V), they can usually produce more Watt*hours, which is another benefit for the gloves. The energy density of lithium polymer is 200 Watt*hours/kg-three times that of nickel-cadmium batteries. Lithium polymer is also very lightweight, and the solid (or gelled) electrolyte eliminates the need for a metal casing. In addition, the batteries are very safe to use because there is much less danger of leakage. Lithium polymer batteries are constructed of a number of thin layers:
While the layers are often then rolled together as pictured here, the battery can also be kept flat as will be the case for our gloves. One potential problem is that they may be relatively difficult for us to obtain because they do not yet have a widespread market. Here are some companies that make lithium polymer batteries:
Valence Technology Inc.
Lithium Power
UltraLife Batteries
We have contacted Lithium Power and are waiting to hear back form them about the possibility of obtaining a prototype. The ideal specifications of our battery are
Voltage = 3.6V
Capacity = at least 1Amp*hour
Dimensions = at most 5cm by 4cm by 0.5cm.
References:
http://www.3m.com/front/lith/
"Batteries." NASA. http://spacescience.nasa.gov/osstech/battery.htm
"Batteries in a Portable World." http://www.buchmann.ca/
Buchman, Isidor. "The Lithium-Polymer Battery: Substance or Hype?" http://www.powerpulse.net/powerpulse/archive/aa_080601a1.stm
Essex, David. "Solar Power, Batteries Included." Technology Review, August 17, 2001. http://www.technologyreview.com/articles/essex081701.asp
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