Car battery question - Page 1
Forum section: electronics / beginners

author: Jimbz
author_rank: Contributor
timestamp: September 15, 2013, 05:54:14 am
content: Small question regarding car batteries and the chassis ?As you know, car batteries are 12V and the negative terminal is connected to the chassis of the car. All the electronics in the car is connected to the car chassis for an electrical loop ?My question is: if you were to get yourself an inverter, to boost your voltage to 230/110 mains (I think they sell ones that connect to sigaret lighters), and say you short the inverter, or even regulate the current to the lethal amperage zone, or you accidentaly connect the positive terminal of the inverter to anywhere on the chassis, does this mean your car is essentialy a moving metal that kills people via electricity by touching the car ? Or is there somekinda safety mechanism to limit the current ?Or have I gotten something wrong ?

author: IanB
author_rank: Super Contributor
timestamp: September 15, 2013, 06:44:46 am
content: It needs a circuit to be completed for electricity to flow. The output of the inverter has two terminals, and you can get an electric shock if you connect yourself between those two terminals. Therefore even if you connect one terminal to the body of the car, there is no circuit unless you also touch the other terminal.If you deliberately or accidentally connect both inverter terminals to the body of the car then you will make a short circuit across the inverter output and it will shut down.Therefore the danger of an electric shock is minimal. You would have to somehow touch both output wires of the inverter at the same time.

author: Jimbz
author_rank: Contributor
timestamp: September 15, 2013, 07:42:40 am
content: Quote from: IanB on September 15, 2013, 06:44:46 ambut the circuit is complete, the chassis is the neutral and the inverter positive is live

author: peter.mitchell
author_rank: Super Contributor
timestamp: September 15, 2013, 08:19:09 am
content: Quote from: Jimbz on September 15, 2013, 07:42:40 amThat assumes the inverter output is not isolated, which it may not be, but if it wasn't, you'd have a short circuit and the inverter would shutdown (gracefully or not is another matter)

author: G7PSK
author_rank: Super Contributor
timestamp: September 15, 2013, 09:23:37 am
content: If you are worried that you may electrocute yourself by standing on the ground and touching the car! you wont.To do that one of the inverter out puts would have to be connected to ground, the one that is not connected to the car body.All the car inverter units that I have seen have the output direct from a transformer neither leg of which is connected to the cars return/ chassis.To get an electric shock the current needs to flow through you, this is why linesmen can work live/hot they are insulated from the ground and so can handle live wires without getting killed.Even if one of the output terminals of an inverter was connected to the car chassis and some how you came into contact with the other terminal it is unlikely that you would get a shock as most of the inside of a modern car is covered in plastic and sound insulation which is normally a good electrical insulation as well, Oh and I forgot to mention that most cars sit on four insulators.

author: David_AVD
author_rank: Super Contributor
timestamp: September 15, 2013, 11:54:42 am
content: The best thing for the OP would be to draw out the circuit of the proposed dangerous situation. Show the voltages you expect between the points of concern. Hopefully seeing it drawn out will help you more.

author: Jimbz
author_rank: Contributor
timestamp: September 15, 2013, 04:02:22 pm
content: Quote from: David_AVD on September 15, 2013, 11:54:42 amNaw, no need, i understood my mistakeSo, to get shocked you need to stick your fingers between the positive terminal and neutral/chassis.Shorting the inverter wont work, as magic sparks will destroy the inverter.Adding a resistor that allows lethal currents between the positive inverter (not isolated) terminal and the chassis will allow a shock, but only if the chassis is grounded (for example by a snake who came from a swim that is conneted between the chassi earth ground) and you are grounded,I think thats the principle here ye ?

author: c4757p
author_rank: Super Contributor
timestamp: September 15, 2013, 04:18:08 pm
content: Basically, but you've still got one point wrong. The real principle here is thatyouare a resistor. The current through a resistor is determined by the voltageacrossit, therefore, you will have to put a voltage across yourself to cause a current. That requires two connections with a voltage difference between them.Quote from: Jimbz on September 15, 2013, 04:02:22 pmShorting the inverter won't work - not because it will be damaged, but just because it won't have a chance to put a voltageacrossyou. Even if connecting it to your car's chassis would raise the chassis to a high voltage relative to something, it wouldn't be relative toyoubecause no other voltage is connected to you.QuoteA resistor between the inverter and the chassis will only draw power from the inverter, not energize the chassis. Remember that any load you connect to the inverter could be considered a resistor in this very same position. If the chassis is grounded, then its voltage relative to ground is zero - if you are also grounded and you touch it, you have zero volts across you and therefore no current flows through you.There will be a small voltage difference along the ground path through the chassis due to its internal resistance, but this resistance will be very low, and so the voltage difference will be very low as well.