# How the Current Flows in a Car?

My BIG question is : why does positive battery cable in a car is directly connected to the starter to crank the engine and does that mean the current actually flows from positive battery post to starter and then comes to negative post through a frame? Many car books usually state that it is the positive wire that gives power to the starter and not vice versa? Where is the flaw in this reasoning - it is certainly how electricity used in cars today practically, not theoretically? I started reading about Ben Franklin and then "real" flow of electrons and really got lost because i understand that usually it does not matter in simple circuits which way it goes to make it work but I still wanna get a little bit closer to my understanding: does this dc current (force, what ever you wanna call it) works in a car - a. travels from positive to negative b. from negative through a frame, starter and into positive c. people don't really know at this time so don't bother :)

UPDATE: Thank you for the answers and adding to this discussion - Franklin was probably wrong but car manufacturers of 21st century certainly have all the knowledge they need yet the THICK positive cable connects to the starter but probably thick cable connects from starter to frame as well and frame is pretty thick itself :) - it is just interesting to know that there are several circuits using the frame as the path, possibly at the same time - I'll have to read more on that : as far as "electron flow" i read that it may take 1 electron from negative post 4 HOURS to reach positive post so I'm not sure what kind of "flow" we are talking about and if the power that makes gadgets work is indeed the "flow" rather than some "force" or "field" that is instantly present once you close the circuit. I also read that change of frame polarity (positive or negative cable to the car frame) which happened sometime in the 1950s was to better deal with higher voltage systems (change from 6v to 12v) and rusting of car frame and bodies due to electrical current running through the frame. How did switch from positive wire to frame to negative wire to frame help with rusting ? - I still have to do more research to see if it was indeed one of the reasons to switch frame wires and whether it did any good but it seems that manufacturers' thoughts about the way current traveled through the frame certainly shaped their decision to make the change

• The starter, the main fusebox, and the alternator are all connected in parallel, in most cars. All of these devices have a positive and negative lead, and the negative leads are all also common to the chassis. Dec 31 '13 at 4:08
• I'm pretty sure there are no fuses connected in parallel with the alternator or the starter. Not for long, anyway. Dec 31 '13 at 14:56
• @JoeHass The fuses aren't in parallel, the fusebox (which houses all of the fuses, and routes all of the remaining power for the car, ie. ECU, lights, gauges, etc) is in parallel. That is to say, the starter, alternator, and the rest of the car's electrical system are all in parallel. Dec 31 '13 at 19:05
• Take for instance, the ECU circuit on the car. The circuit looks like: Battery (+) -> Fusebox (+) -> ECU Fuse -> ECU -> Fusebox (-) -> Battery (-). The individual fuses are all in series with the subcircuit they're protecting. Dec 31 '13 at 23:03
• Not to help your hijack, but the idea is that larger (0-2 gauge wires) or more ground wires will lower the impedance in the entire circuit, so you get less resistive losses in the ground wires. Jan 10 '14 at 6:15

When the battery is supplying power (discharging) to, e.g., the starter motor, the direction of the electric current is out of the positive terminal through the load and into the negative terminal.

Within the wire and frame, the electric current is due to electron current which is in the opposite direction of the electric current.

Within the (lead-acid) battery, the electric current is primarily due to proton (hydrogen ion) current which is in the same direction as the electric current.

So, there are at least three currents to consider: the abstract electric current (flow of electric charge), the electron current (flow of electrons, a carrier of negative electric charge), and the proton current (flow of protons, a carrier of positive electric charge).

Note that when the battery is charging, the electric current is into the positive terminal and out of the negative terminal.

• Thanks - I understand that people defined it as positive to negative first and they stuck to it, but in metals i guess it goes through frame first if there is such thing as first :) Dec 31 '13 at 17:33
• Interesting from your wikipedia link: "When analyzing electrical circuits, the actual direction of current through a specific circuit element is usually unknown" AND "In electronic circuits, the reference current directions are often chosen so that all currents are toward ground. This often corresponds to conventional current direction, because in many circuits the power supply voltage is positive with respect to ground." - I need to read more on that :) Dec 31 '13 at 17:36
• @user34890, keep in mind that the context of the quote is circuit analysis. When solving an electric circuit, one must first choose reference directions and polarities for the current and voltage variables that are being solved for. The reference direction or polarity for a given current or voltage variable is arbitrary. Just as reversing the leads of your multimeter reverses the sign of the measured current or voltage, reversing the reference direction reverses the sign of the calculated current or voltage variable. The physical direction and polarity are unchanged. Dec 31 '13 at 17:44
– Rev
Apr 25 '19 at 18:21

I can't say much about how the electrical system in a car works (as far as what does where), but in any electrical system you will have real electron flow from negative to positive potential (for equilibrium).

However, as you have stated, there is what has become known as 'conventional' current flow which is exactly opposite. This discrepancy between conventional and actual current flow was a result of early electrical engineers guessing which atomic element was moving in electrical systems; as you may have guessed, they got it wrong.

I have not read of any design in which it was necessary to consider the real direction of current flow, but I am sure there are some really low-level problems that require it.

• Explain the downvote. Dec 31 '13 at 4:11
• -1: electric current and electron current are distinctly different concepts neither of which is wrong. Dec 31 '13 at 4:12
• I never said either concept was inherently wrong. I said the early electrical engineers were wrong in their conjectures considering which atomic element was flowing; this is largely attributed to Benjamin Franklin. Dec 31 '13 at 4:17
• physics.stackexchange.com/questions/17109/… Dec 31 '13 at 4:26
• -1 I don't think Ben Franklin had the slightest idea of subatomic particles or "atomic elements". His notions of positive and negative were based on static electricity. Dec 31 '13 at 15:07

There is as far as I know no real technical reason to prefer one over the other.

HOWEVER, COMMA, it is FAR safer for everyone involved if ALL vehicles do it the SAME way. It makes it easier for people hooking up battery chargers and jumper cables to do it right and safely.

Today, the standard is 12 volts, negative ground. That wasn't always the standard. One of the cars my parents owned, a long time ago, had a 6 volt system. At one point, I owned a motorcycle with a 6 volt battery. I don't remember what the ground polarity was on either of them.