Part 2
practical examples
Headlight bulb:
Say we have a headlight bulb, rated 50 watts @ 6 volts.
From Kirchoffs laws, P = VxI, we can calculate the current draw. Rearranging the equation, I = P÷V) (current = power ÷ voltage), = 50 ÷ 6, = 8.34 amps.
Now that we know the current, we can calculate the headlight resistance, R = V÷I, = 6÷8.34, = 0.72 ohms.
So what happens if you connect it to a 12 volt battery?
Again, I = V÷R, = 12÷0.72, = 16.67 amps (twice the 6v amperage!)
And power consumption? P = VxI, = 12x16.67, = 200 Watts!!! Now you know why a 6 volt bulb burns out immediately when connected to 12 volts.
Changing to a 50 watt @ 12 volt bulb is the only sensible thing to do. At 12 volts, a 50 watt bulb only draws half the current of a 6 volt bulb. 50÷12 = 4.17 amps.
What about running a 6 volt starter on 12 volts?
Common answers are "works fine", "No problems", etc. Well, those people are lucky, and I prefer not to do it.
From the above headlight example, it should be clear that 2 things happen when you connect a 6 volt device to a 12 volt battery:
- Current draw DOUBLES !
- Power consumption QUADRUPLES !!!
With 4 times the power consumption, a 6 volt starter is producing 4 times the torque that it was designed for (and 4 times the HEAT!).
While it may last a while, it's being severely overstressed. It should ONLY be used for very brief periods, with lots of rest in-between. If you crank on it too long it will get HOT, and you run the risk of burning it up.
Mechanically, it's producing 4 times the torque. It DOES NOT like that at all. Again, it may last a while, but you're damaging it.
Pictures are worth a thousand words, so here's what you can look forward to running one at 12 volts. A CJ-2A Autolite MZ-4113 starter. I have no idea how many starts this went through, but the installed 12 volt alternator was 1979 vintage. It was used this way until 1997. (Thumbnails link to higher resolution images)
| The starter armature woodruff keyway, for fixing the driven end of the Bendix drive.
It's clearly cracked through the front center, and through the lower left corner. This keyway will eventually break entirely. |
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| The bendix drive spring:
The one on the left was never run at anything but 6 volts. On the right, from the 12 volt conversion. The bolt eyes should be opposite each other. |
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| The 12 volt spring is severely distorted, and has been permanently constricted to the point that the bolt eyes are not in the proper opposite alignment. |  |
| This bendix drive came off the 12 volt conversion.
The centering sleeve/constriction stop has broken off completely, and the joining bands are missing entirely. A new centering sleeve is also shown here for comparison. |
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| On the left is a new centering sleeve.
On the right is what's left of the original. This centering sleeve was no longer serving it's purpose. |
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