Rights to return faulty car...long story

shaun_from_Africa

EdGasket wrote: »

haven't you heard the saying:

"It's the volts that jolts but the mils (milliamps) that kills"? A car battery can supply hundreds of amps.

A single lithium AA battery when shorted out can easily supply 4 or 5 amps for a short time and in theory, this amount of current is more than enough to kill.
However, even though it's current that does the damage, you still need enough voltage before that current can do its work.

The generally accepted safe level for DC is 50 volts although it is possible to feel less than this on dry skin.
I work with aircraft DC systems (which are normally 28v) and occasionally get a slight tickle when working on live circuits and although a fully charged aircraft Ni-Cad battery can easily deliver 1500 amps if needed, I've never heard of anyone getting killed or injured as a direct result of an electric shock (not counting people who have been burnt due to shorting out wiring with a watch, jewellery or tools).

securityguy

"Technically if a car battery current breaches your skin resistance (through wire penetration and/or skin cuts) it could provide a shock to your heart that could kill."

It's not just the skin resistance at the point of contact that keeps you safe, it's the resistance of the rest of your body and crucially of the skin at the point at which your body goes to ground.

A car battery delivers 12V from a very low source impedance which has massive current capability. So a dead short from a car battery to ground could easily carry 500A.

However, your body isn't a dead short. Suppset you grabbed hold of the positive terminal of a car battery with a wet hand with some cuts. The path to earth is now via you, and the current will be V/R, where V is 12V and R is the resistance between the point the 12V is applied (your low-resistance hand) and ground (presumably out through your feet). Even if you assume a resistance of 0 ohms at your hand, the resistance at your foot will still be reasonable (probably of the order of a kiloohm), even ignoring the fact that your shoes are unlikely to be low-resistance either.

Grabbing hold of both of the terminals of a car battery with wet hands is a bad idea, as it's a low resistance path across your heart. But anything short (ho ho) of that is pretty safe.

Joe_Horner

EdGasket wrote: »

haven't you heard the saying:

"It's the volts that jolts but the mils (milliamps) that kills"? A car battery can supply hundreds of amps.

Technically if a car battery current breaches your skin resistance (through wire penetration and/or skin cuts) it could provide a shock to your heart that could kill. So I would not say "Not at all, not ever, not in a million years." but rather "extremely unlikely, not in a thousand years."

Yes, a car battery can supply hundreds of amps, but only through a low enough resistance. Ohms law tells us that for 100 amps to flow with a 12v potential difference the resistance must be 12/100 = 0.12 ohms.

Even without skin resistance (for example, by sticking needle electrodes into yourself) the internal resistance of the human body is of the order of 500 ohms. Obviously that varies person to person and also depends on the distance between the electrodes. But it's a good ballpark.

A 500 ohm resistance with a 12V potential difference applied will carry a current of 12/500 = 0.024 amps or 24mA. That's regardless of the supply's current sourcing abilities into a dead short.

Rule of thumb figures say that you'll feel around 5mA of DC current, so you'd certainly be aware of it if you stuck two needles in your hands then touched them to a car battery but it wouldn't do you any harm.

Fibrillation of your heart from DC requires around 300mA+ unless it's got a direct feed into your heart muscle so it's barely 1/10th of the level needed to stop your heart.

The total heating power (across the whole path bewteen contacts) would be .024 *12 = 0.288 Watts. So you ain't going to cook any tissue either.

All that's based on deliberately piercing your skin and connecting directly to the soggy stuff underneath - which I don't recommend, btw, but yes I have done during a similar discussion with a VM once upon a long ago*. It was a curious tingle but not unpleasant.

Grabbing the terminals normally with your hands (even when wet) is very unlikely to even be felt and, if it is, will never, not ever, at all, "electrocute" you :beer:




* I've also accidentally connected about 90V AC between my left temple and my hand. That made my eyes shake at mains frequency, which was really strange :rotfl: