# Automotive electrical system. How does everything work?

I recently grew a big interest into the electronic world. I also love auto/mechanics, and I’m trying to understand where these two wonderful areas meet.

I’m trying to understand the fundamentals of how an automotive electrical system works, and I’m a bit confused on a number of points. I was hoping by posting here on this wonderful forum, I could get some more insight. This post is long and drawn out. It would be a treat if someone could spend the time to answer all the questions in my thirst for knowledge, but if not -- partial/incomplete answers would be awesome as well.

So I understand so far it all starts with the battery, and the positive and negative terminals and all the wiring in-between.. I’m not so much interested in how the battery generates electricity, there are many resources online for that aspect, and I think I have a pretty good understanding of the basics -- the electrical reactions within the battery itself cause the electrons gathered at the negative terminal to want to attract to the ones at the positive terminal, but the electrolytes prevent this attraction so they have to go through the electrical wire to the other end in order to get the electrons to the other side (i.e. from basic physics, opposites attract).. I’m trying to understand the following, which has been hard to discover in a manner I understand so far.

I’ve read that ‘conventional’ current makes the current ‘flow’ from the Positive terminal of the battery to the Negative, and anything connected to that positive along the way is thus given electricity, i.e. lights or other accessories. I then read that conventional current is more of a science term for actual current in science, but the literal electron flow is from negative to positive, i.e. electrons flow opposite from the ‘current’ direction. After trying to grasp that concept, my main confusion is in a few areas that stem from that original point:

a. If electrons flow from negative to positive, how is it that the negative wire is also ‘ground’ to the chassis and is the ‘return’ path of the electrons? Saying you have a flow of electrons from negative to positive (and powering stuff along the way) and then saying that same path is a return path seems a bit contradictory?

b. If the negative wire is the ‘ground’ wire and the answer to question A above is the ground wire is only utilized when there is a short (i.e. the short ends up as electrical flow in the chassis), then how is it that fuses blow upon short circuit? i.e. how does the ‘short circuit’ path both blow a fuse and ‘exit’ through the chassis ground of the negative terminal cable at the same time?

c. Kind of an extension to question b above, whether it’s a fuse blowing or the electrical energy being released into the chassis, how does the electricity in the system know to ‘deviate’ from its route and take the ground or the fuse route (Whichever one is correct) to be a safe alternative route? I assume there is some kind of device that measures the amps/voltage and if it exceeds the application it was designed for, it forcefully re-routes the electricity? This point is interesting to me because it seems like every single tiny device in the system is grounded, even if it’s just a small light.

d. I understand how the alternator generates electricity itself, but how does its power ‘move’ to the battery? Is it like a negative end on the alternator and a positive end on the battery? (I know the positive cable is connected to the alternator and is recharged by it, but how does this recharge work? Is it similar to the negative terminal flowing electricity into the positive terminal in the battery example?

I know I’ve written an essay of questions here. Thank you to anyone who takes the time on answering some or all of them.

EDIT: Thank you for the previous answers: (07/01/14)

For anyone, I have moved on from the battery/fuses/ground concepts (Thanks for answers) and moved on to reading about Relays. A quick question. I understand that a relay has a small amount of current applied to it, and through the electromagnet inside, switches another circuit that is more powerful. My question is - is that secondary, more powerful circuit within the relay still powered/limited by the 12v/whatever amps DC car battery, like all the other non-relay powered components connected to the 2 terminals?

a. grounded negative chassis is a convention, it can be a grounded positive instead, but every connection has to be reversed. ground is a concept, a point of reference, but not a physics reality.

b. the current passes through the battery positive terminal and the fuse, and so it doesn't matter what the return path is; a negative cable or chassis. the fuse will still blow on fault. the fuse has to be part of the circuit for it to blow.

c. there is no alternative route, the current must pass through the fuse. it may also pass through other things, but this is not important; the fuse will still operate if the current and time allow it to.

d. the alternator has a cable connected to the battery positive, and either a cable connected to battery negative or the chassis is used as the path. it works because the alternator generates a voltage higher than the battery, and the voltage carries power into the battery. during charge, the battery is a load with respect to the alternator.

• Thanks for your answers. I think they really helped me clear up most of my confusion. So as long as every 'load' that is utilizing the sending electricity from the negative cable has a fuse as part of the circuit, the fuse ITSELF knows when there is too much current for that specific load, and blows as a result (Thus, the fuses have Amp ratings). Makes sense to me. – amalik Jun 29 '14 at 17:17
• On point a, If the electricity literally flows from negative to positive, and the negative terminal is connected to the body of the vehicle, that means all the electricity flows from the negative end, to the vehicle body, to the positive wires/fuses and back to the battery? Thus if you disconnect the negative cable off the chassis, nothing electrical in the system will function? If this is correct, I think all that I've been misunderstanding this whole time is the metal of the body is a good path to send the electricity from in terms of safety. – amalik Jun 29 '14 at 17:18
• I think I was confusing the role of the fuse and the role of the metal chassis in the circuit. – amalik Jun 29 '14 at 17:31
• The chassis ground is mostly convenience (you only have to run a positive wire to things and let the bodywork supply the negative, saving on wire). As you say, things don't function if not connected to ground. – pjc50 Jun 29 '14 at 19:23
• Regarding safety and return current it would in case of any unsafe situation be the dissipation of heat along the return path. Ideally the car's body is an excellent conductor such that there is very little heat. Heat problems usually occur in the wires or in bad connections. – itsproject Jun 29 '14 at 19:53

Conventional current flowing in the opposite direction to electrons causes a huge amount of unnecessary confusion. See Was Benjamin Franklin wrong (about conventional current)? and Does electricity go from negative to positive or vice versa?

For the rest of the questions, you need to be clear on a circuit; there must be a complete path through the battery, wiring and component in order to operate. Fuses are placed in this path such that excessive current causes the fusewire to melt, cutting the circuit and halting flow, rather than any of the wiring melting (which would start a fire buried inside the panelling of the car).

(In some ways, the important thing is the electrical field transmitted through the wiring that causes current to flow. Using the water pressure analogy, if you have a series of pipes and apply pressure to the end of one, it will be transmitted everywhere in the pipe system even if the water isn't flowing.)