This Instructables show how to use a metal sweet box to make a battery hand warmer.

Step 1: Preparation

Metal Sweet Box

Any metal box that can fit a battery holder in it.

Heating Wire

I have tried a few different resistant value heating wire, this time I am using around 5 meters of 0.3 Ohm/m heating wire.

Thermal Switch

KSD9700 45° threshold, means switch off power if over 45°. You may select another threshold depends on your requirement.

Battery Holder

A battery holder, with built-in power switch preferred, number and size of battery depends on the selected metal box. More battery should be warmer; larger battery should be last longer.

Other Material

A card board for the temporary support of winding heating wire, an A5 size post card should be good enough.

Masking Tape for warping the heating wire, wider is better.

Step 2: Measurement

Measure the inner size of the Metal box for winding heating wire.

Step 3: Temporary Support Case for Winding Heating Wire

According to the measured inner size, prepare a card board support case that can fit in the metal box.

Step 4: Heating Wire Inner Support

Warp the card board support case with masking tape, sticky side face outside.

Step 5: Winding Heating Wire

Wind heating wire on the masking tape sticky face. The length of the heating wire tube should a little bit shorter than the metal box.

Step 6: Heating Wire Outer Support

Cut both side of inner support masking tape into a few part, flip the masking tape on the heating wire. Warp the heating wire with masking tape for further support.

If you think the inner and outer support is strong enough, try to remove the temporary support case.

Step 7: Install Heating Wire

Try to squeeze the heating wire tube a little bit and put it into the metal box. Restore the heating wire tube in the metal case. The tube should just fit in the metal box inner wall.

Step 8: Patch Battery Holder

My battery holder is a little bit larger than the metal box. But it can fit in if:

remove the cover and cut out the protrusion part for the cover
cut the power switch protrusion part
stick a masking tape and expose a tip on the upper part make it easier take out the battery holder from the metal box

Also we need to patch one side of battery holder for installing the thermal switch between the battery and the heating wire.

Step 9: Soldering Work

battery holder positive wire -> heating wire -> thermal switch -> battery holder negative wire

Note:

If you are using a battery holder without buit-in power switch, add a power switch between battery holder positive wire and heating wire.

Step 10: Turn on the Power

Put on the battery, switch on, put the battery holder in the metal box and close the cover. The metal case should become warm in a few minutes (depends on using what battery).

Step 11: Testing

I am using 2 eneloop AA battery, it can heat up to around 42° after power on 5 minutes. As same as other hand warmer, it is not recommend expose the hand warmer in the air. In normal use, it should put in the cloth pocket. I use a pair of socks warp the hand warmer to simulate put in cloth pocket, it can reach around 45° (thermal switch threshold) in 4 minutes.

During the test, the room temperature is still around 28° and the hand warmer can last over 2 hours. I will make some further test when the winter really comes.