hot wire glass removal, battery safety?

[steptoe]

OK, picture a coupla fifteen year olds with a car battery and some bike spokes.....same fifteen year olds place spokes across terminals no fuse, shorting out battery are they? Or is spoke acting like a light bulb filament, jst with O2 about to help it burn. Spoke goes red, orange, white burns up drops onto battery case breaking circuit. The process continues. Same 15 year old then at age 17, with no safety training or regard for power of a battery is using a metal tyre lever to do just that, lever the postive battery clamp and in doing so another part of the tyre lever touches the metal earthed body of the car.

Effin, BANG !! and the corner of the plastic casing of the battery blows off, spraying acid in Gordo's face, staining his undies at same moment - he got the rest of the day off work! Some time later Gordo got a job in a bank, and later went on to win medals for his country.... not Darwin Award neither ,so he's no goose!

Why didn't the tyre lever just get hot and change colour ? Why didn't the spokes do the same two years earlier ? Difference between a 3amp radio fuse of an old car and 80A fuse of a Diesels glo plug circuit kind of thing ?

All this is in the back of my mind when a windscreen guy hot wires a windscreen seal to seal it on install.

A suggestion has been made for removing hot sealed or gunked in glass windows of cars - to use a hot wire looped around the seal and connect it to a battery. How is it done safely not to blow the battery to Kingdom Come , or burn the wire loop out too early to soften the gunks grip ?

Any thoughts ??

[Gannon]

A bike spoke has a cross sectional area of about 3mm sq and may have a resistance of 0.12ohms which at 12v allows 100A to flow, dissipating 1200w of power. The thermal mass and surface are of the wire cannot withstand the heat created and melts.

The tyre lever has a csa of maybe 60mm sq so has 1/20th the resistance of the spoke. So at 0.006 ohms 2000A flows and creates 24,000 watts of power, because the lever had 20 times the weight and thermal mass so takes a while to heat up, the battery terminal is now the weaker link and self destructs.

As for the hot wire, the current is likely only 10A because the resistance is much higher and it likely has a current limiting power supply.

[tambox]

Plus the tyre lever may have made the battery boil, producing explosive gasses, a slight movement of the tyre lever could have produced a spark, igniting the gases in the closest cell, the one on the corner.

You can get resistance wire from Jaycar for heat cutting, with a 12v battery/power supply. A power supply makes it easy as you can adjust the current/heat, rather than a battery, where you adjust the wire length. Made a couple for cutting foam.

[Silverbullet]

Nichrome wire is what you're after although I've used stainless steel with good results.

[niterida]

I might have some nichrome wire lying around (will have to check) but not sure if it would thick enough to do the job without breaking - used it to cut polystyrene with a home made cutting table.
Although from memory I don't think it is too expensive or too hard to get.

[Subydoug]

Mig wire also works well. Ive also used light gauge tie wire as well with good results.

Put it this way Steptoe,

volts = current X resistance

Watts of power = voltage X current.

Heat(jules) = current^2 X resistance X time.

If the battery had been capable of supplying the high current to the tyre lever without failing It too would have glowed red and melted, though that being said, 12v practically might not have got it there.

Regards

Doug

[steptoe]

just thinking I gave my old 6A battery charger away would have been a useful tool here methinks