Heat Troller

Ok, I have a purpose for blogging now. I'll share my design of heat troller to anyone who's interested in building one from scratch.
The project started as a way to keep my wife's feet warm while skiing. I have thought of this few years back when snowmobiling and making something to keep the rider warm using electrical heat powered by snowmobile's 12v battery. After spending hundreds of dollar buying heating clothes and socks and heat troller, the whole setup is still tie to the snowmobile's 12v battery.
In order for a portable setup, I need to use lithium battery to power the heated clothes and a way to adjust the temperature to prolong the battery life.
The biggest limitation is that average heated socks draw about 4-5amp of current at 12 volt. This will drain a portable battery very fast.
In order to save battery life and also have ability to control the temperature, I need a control to regulate the current. The most efficient way is to turn the power on and off frequently.
Changing voltage with a voltage regulator or reduce the current with resistors will waste a lot of battery power. The most common way of switching power on and off is using a 555 timer chip to open and close the gate on a transistor or MOSFET.
Having a portable 12 volt battery is also not convenient. Each lithium cell is 3.7volt. This will require a 3 cell lithium battery pack.  Ideally one cell is the best. More cells means that there's a higher potential that the cells will be out of balance and shorten the battery pack life. Almost all the cellphones run on one cell.
The draw back is that a lot of power transistor and MOSFET requiring higher voltage to operate better, because most are designed for the automobile industry. Smaller transistors can not tolerate higher current required in the heat troller.
My initial design is based on 2 cell lithium pack which is 7.4 volt. I'll get to 3.7volt design later after I found a low gate voltage MOSFET.




... to be continue...