Six volt vs. twelve volt wiring

[marty in Ohio]

Last week, at our "A Team" meeting, we talked about the difference between the battery cables on a 6 volt and a 12 volt system. I'm no electrician so I can't follow why a 6 volt system needs a heavier cable than a 12 volt system.
I'm sure someone here can explain it so I can understand.
Marty

[Kevin in NJ]

V=IR

That is voltage equal current times resistance.
A heavier cable is needed to carry more current. At a higher voltage you need less current to flow so you can get away with thinner wires. This is why the electric company jacks the voltage way up to transmit it long distances as there is less loss.

So back to 12 vs 6. If you double the voltage in the above equation you halve the current flowing. I am assuming you remember some algebra.

Essentially you end up halving the amount of current needed to get the same amount of energy across. BUT, it also means you can get away with crappy connections as there is less loss on bad connections or a few broken strands of wire. So 12 volts can also be a band aid fix for poor wiring and poor grounds.

[eagle]

A watt is actually a unit of measure for work done. Current (amps) X Voltage = Power (watts) Therefore if you double the voltage (from 6 volts to 12 volts) the current (amps) is cut in half in order for the same amount of work (watts) to be done. However, if you do not increase the windings of the starter, (use 6V starter on a 12V system) you still get the high amount of current AND the higher voltage. Therefore higher power (watts). Thats why the engine spins faster. This also can burn out the starter quicker because the higher watts creates a lot more heat. Fortunately the A starter is very heavily built so will handle the extra heat as long as you keep from running it too long. This is a very simplified explanation, there are other things going on too with 12 volt conversion but hopefully gives you a general idea.

[steve s]

Maybe this will help. I copied it over from an earlier thread.

Water flow/pressure is a common analogy used to explain electricity for folks who are are uncomfortable thinking about electrons and dealing with units of energy, etc.: Amps would be like the amount of water going over a water fall, while volts is like the height (or pressure) of the water fall. The amount of work the water fall can do for you depends on the combination: the more height you have, the less flow you need, and vice-versa.

In reality, amps is a measure of how much charge (such as electrons in wires) moves past a point in a second, and volts is the difference in energy per charge that the electrons have after traveling between two locations (such as battery terminals). The units of charge and energy involve funny words like coulombs and ergs, and there's no need to go into them here.

The point is, just as the amount of gravitational work you can get out of a waterfall depends on both the height and flow of the water fall, the amount of electrical work you can get out of a battery depends on both the voltage and amperage of the battery. And, thus, for example, the wiring in a 12V system doesn't have to be as heavy as in a 6v because only half as many electrons are going to have to make the journey from one terminal to the other to release the same amount of energy and do the same amount of work.

By the way, you might have noticed the key word, DIFFERENCE, in the definition of voltage: There is no such thing as the absolute voltage at some point; it always must be measured relative to the energy the electrons would have at some other point.

[Cool Hand Lurker]

If you are converting from 6V to 12V and wondering if the wires will carry 12V, don't worry. The 6V wires are twice as thick as would be needed for 12Volt. There will be no problem.

[marty in Ohio]

Sponsored Links (Register now to hide all advertisements)

No, I'm not going to go to 12 volts Cool Hand, I just didn't understand the difference. Thank you Steve. I think I got it by way of your explanation. It's the first time I've had it explained with the waterfall. Kevin and Eagle certainly know what they're talking about, I guess. I've just never been electrical.
Thanks for the help.
Marty

[steve s]

Thanks, Marty, I'm glad to help. Nobody is born "elecrical." Electricity physics is eye-glazingly complicated and attempts to boil explanations down to quickie mathematical derivations, simple statements, or analogies almost always involve erroneous statements. It's even worse when the subject of heat flow and thermodynamics comes up. Fortunately, people can make their peace with the complexities and come up with rubrics that permit rules that are both practical and true enough for our use, like, "lower voltage needs higher amperage and heavier wiring."

[Jim Brierley]

I agree with all the above, however, I got into a serious discussion in a magazine a few years ago about this same thing as regarding running a 6v starter on 12 v. My opinion was half the amps. After several rounds of this argument, I put an induction ammeter on 2 of my A's and found that while the amp draw was definitely less, it was more than half. In my feeble mind I figured that it was because the power output had increased with 12v input. Just FYI

[steve s]

Originally Posted by Jim Brierley

I agree with all the above, however, I got into a serious discussion in a magazine a few years ago about this same thing as regarding running a 6v starter on 12 v. My opinion was half the amps. After several rounds of this argument, I put an induction ammeter on 2 of my A's and found that while the amp draw was definitely less, it was more than half. In my feeble mind I figured that it was because the power output had increased with 12v input. Just FYI

Jim,

I don't know what, if anything, was different between your two A's, but, just because theoretically half the amount of 12V current is needed to do the same amount of work as done by 6V current, doesn't mean that exactly half the amount of current will actually be drawn if you just swap a 6V for a 12V battery. In fact, according to Ohm's law, E = IR (or V = IR) IF the resistance the current is flowing through stays the same, TWICE the amount of current will flow at 12 V as at 6V.

Back to your two cars: A real analysis would have look at the TOTAL electrical energy expended, both as mechanical work spinning the starter, and that given off as heat due to mechanical friction and electrical resistance, AND doing that would require allowing for any difference in times one's foot was on the starter buttons.

Calculations get messier fast if the device the current is flowing through has some capacity to store electrical charge (capacitance) or to build up a magnetic field (inductance), and the voltage source of the current is alternating in time (AC current).

An additional common source of confusion in these discussions is the differences between energy, heat, work, and power. Heat and work are means of energy transformation and both have units of energy, which include calories, btu, ergs, joules, but NOT watts. Power is the rate of energy conversion or "doing work", so it has units of energy/time. Examples are horsepower and watt. One watt = 1 joule/sec. So, when the electric company wants to bill us for ENERGY useage, they can do so in units of kilowatts x hours, where the units of time now cancel out leaving only units of energy.

[barkleydave]

So why did cars go to 12 Volt.
Lots of reasons.
1. Cost savings Lighter ga. wire required
2. Lighter weight for vehicle
3. Smaller AMP batteries required so again less weight and cost.


So Military is 24 volts
Lots of push think of current as pressure like water in a hose
Less amp required so more efficient in combat environment...corrosion moisture etc.


Lots of other reasons..
But those are some of the main ones.


On our A's
Not much difference except less chance of burning wiring with 12 Volt amp loads through the heavy 6 volt wiring.


Can add modern acc. such as radios, lights etc.


Vehicle can be jumped started by any modern car without damaging any of the Model A components.


Brighter modern lighting


Now some will argue however that is only opinion not supported by physics. Stay OE or change either way they will all go down the road.

[Cool Hand Lurker]

Yup.... what Steve S said.....

[Crankster]

Manufacturers went to 12 volts because the era of high output high compression engines was dawning, along with a host of power hungry electrical accessories. There was simply no way a 6 volt generator electrical system was viable heading forward.

The key to maintaining any automobile electrical system whether 12 volt or 6 volt is to ensure clean, tight grounds and connections. Low voltage DC is susceptible to voltage drop due to corrosion, even invisible. By the time it is visible the connection has usually failed altogether. This is why people end up brushing battery terminals off in a dark parking lot on below zero mornings.

But it doesn't take much resistance to slow cranking speed. A related problem is the ability for the charging system to bring the battery back up to 100% to ensure fast tickover - a negative (heh) feedback loop. Just 0.06 of an ohm - that is a few hundredths of an ohm, barely measurable, will cripple any alternator, by about 30%!!

[George Miller]

Originally Posted by Jim Brierley

I agree with all the above, however, I got into a serious discussion in a magazine a few years ago about this same thing as regarding running a 6v starter on 12 v. My opinion was half the amps. After several rounds of this argument, I put an induction ammeter on 2 of my A's and found that while the amp draw was definitely less, it was more than half. In my feeble mind I figured that it was because the power output had increased with 12v input. Just FYI

But you were running a 6 volt starter. So you were using more power than a 12 volt starter would. But you were getting more power from your 6 volt starter on 12 volts than you were on 6 volts. I think ?

[Synchro909]

Several different approaches have been used to explain this and as far as I'm concerned, none are wrong. Now that's unusual on a forum!
To slow the starter on my cars, I get into the stater and rearrange the field windings so they are all in series. This increases the resistance of the fields by four times meaning less juice gets through to the armature. It's a bit like placing a restriction in the pipe carrying water - less comes out the other end.
I figure this is like running the original 6v starter on about 8 or 9 volts. Plenty of grunt and not so brutal on the bendix.
From what I've seen, when using a 6v starter on 12v, you'd best have your foot on the brake or the fan will move the car.

[marty in Ohio]

Thanks for all your replies guys. I've learned something through all of this - I think, maybe, sort of.
Marty