Further discussion about electronics

Question:

I have been following this discussion in your magazine and have had this question in my mind since it started. I am not an electrical engineer, but I have some knowledge of Ohm’s Law, which states E = I x R, or Voltage = Amps x Resistance.

So let’s assume we use the starter circuit as an example, and specifically the large wire going directly from a 6-volt battery to the starter and we check the resistance from the battery to the starter and come up with 1/8th of an ohm. (Probably way high but it’s an example.)

Now plugging that info into the formula for Ohm’s law we get 6 divided by .125 and that gives us 48 which can also be shown as 6 = 48 x .125. And then we swap out the 6-volt battery for a 12-volt, using the same wire to the starter; we get 12 = 96 x .125. So basically we have doubled the amps on the same wire by swapping to the 12- volt battery. I am not sure this would be a cost saving for the auto manufacturers by way of using less copper, as it seems to me it would require more copper and larger wire to handle the higher amperage. What am I missing?

Answer:

I am not an electronics engineer either, Jeff, but I have a good friend who is, so I sought his advice as to your calculations. He has helped me out on more than one occasion when it comes to things electric. He says:

Voltage is like pressure. Doubling voltage with no other change will increase the current, but normally a 12-volt circuit will be designed to have higher resistance than a 6-volt circuit. The key is the amount of work to be done. Work is expressed in Watts. In a battery-powered DC circuit Watts are called VA or Volts x Amps.

The 12-volt cars use a smaller wire because they need to carry only half the current to do the same amount of work as in a 6-volt car. The 12-volt light bulbs have higher resistance than 6-volt lamps and use less current for a similar rating in watts. You will note that modern starter cables are #4 gauge and old 6-volt cars use #0 gauge, a much larger diameter. Hence the savings in copper for the car companies.

The calculations you used assume replacing a 6-volt battery with a 12- volt battery with no other changes. That doesn’t work. One well-known exception is using a 12-volt battery with a 6-volt starter. Since the starter is typically used for a very brief time and has a large mass to dissipate heat, we can energize it with 12 volts and get that wonderful quick spin provided we don’t grind away on it endlessly.

But most other things need to be changed. Ammeters work fine with 6-volts or 12-volts. The gas gauge may need a series resistor though. Most old cars have temperature and oil pressure gauges that are not electric except for the light bulb for night use, but any electric sensors will need to be replaced as well. All light bulbs including headlamps will need to be replaced with 12-volt equivalents too.

The radio, heater, electric wipers, horn, signal flasher and cigar lighter will also need to be protected by a step-down device such as a ballast resistor.

And if your classic is 6-volt positive ground, you will need an inverter for the radio if you go with 12-volts and a negative ground. In addition, you will want to use a ballast resistor for your breaker-point ignition because ignition points will burn when subjected to the full 12-volts.