Dervy's guide to changing a battery :-)

I do try to inject some witty or sarcastic comments* in the site just to see if anybody is still reading it! :)

*at my own discretion, a remark that may be taken as witty or sarcastic may not be intended to, and vice-versa. :)

If you actually did that to a car battery (obviously you'd have to "extend" the terminals with some wire), what would happen ? I'm assuming it would totally vapourise your tongue ?

Why? It's only 12v. Go find a PP3 9v battery and stick your tongue on its terminals - it tingles somewhat. 12v would just tingle somewhat more.

No - all you'd get is a tingle - unless you let the wires touch, when you'd get a burn.

It's the current which kills, and current is determined by the resistance of the device its passing through. And even wet skin has a high enough resistance not to pass much current at 12 volts. Up the voltage, though, and the current goes up too, the resistance being a constant. Which is why mains at 230 volts can kill.

Many of my colleagues test radio mic PP3s (9 volts) by putting them on their tongue - they can tell by the level of tingle how much life they've left. Personally, I prefer a bacon sarnie.

It isn't the voltage, it is the current.

Howmuch current can a fully charged car battery dump as part of the shorted circuit before it catches fire .

Not much. You'd get a 'tingle' about 80% stronger than that from a standard 9v battery, so it's not very nice, but you wouldn't vapourise anything.

Try it - you might like it :)

Probably in the order of a kA. But you're not short-circuiting it, you're putting it across your tongue.

How long dose the sarnie last in a radio mic?

You will be shorting it, but with a lot of resistance through you body, unless you have a brace fitted and touch it with both wires.....

Ouch.

Which isn't "shorting it"! Shorting means applying a nominal zero impedance across a power supply.

Who remembers that Bond film with Jaws and the table lamp?

It depends on the resistance of your tongue and the internal resistance of the battery. The internal resistance is like a resistor in series with whatever you connect to a battery. If you are applying a resistance R across the terminals, and the internal resistance is r, then the current is calculated as V/(R+r).

The difference with car batteries is that they have *very* low internal resistances compared to say 9V or AA batteries. You can see from the formula that if your R is much larger than r (ie your hand or your tongue) then the internal resistance doesn't really make much difference. However, if R is very small (ie a spanner!) then it is the internal resistance that makes the difference between nothing happening and your spanner melting.

This is why when your draw very high currents from the battery (ie starter motor) the voltage across the terminals drops significantly.

My tougue is not long enough to make contact with both terminals of a car or bike battery. I'd have to be stupid to hold one terminal while licking the other. Any tingle would be more likely due to traces of acid on or near the terminal.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Yes, well, I wouldn't advise liking the battery itself, or any other parts in the engine bay. As Nom said, you would need 'to "extend" the terminals with some wire'. Holding one terminal and making electrical contact to the other with your mouth would have no tangible effect.

The battery's about the only thing I do like in my engine bay, got that anchor of a Triumph 1500 lump next to it...

;)