ammeter question

[Rod Petrie]

I had a new repro ammeter in my 26TT. With the third brush almost fully retarded it would read +10 while driving. I just acquired an older ammeter (don't know how old it is) and it seemed to match the looks of the interior better so I installed it. Now while driving it reads +20 and not running and the lights on -20. What is going on?

[TXGOAT2]

It looks from here like the old ammeter may be very inaccurate. It's unlikely that your lights would draw as much as -10 amps, let alone 20 amps, and unless you have a lot of accessory lights or some other added load, a -20 draw is very unlikely. I think (not sure) that the third brush moved all the way toward the engine block is the maximum output, so your reading of +10 amps with the engine running at high idle or about 10-15 mph road speed is probably about right. I think the stock type ammeters, old or new, are probably about plus or minus 10% of the reading as far as accuracy. If the generator is actually pushing 20 amps, that's too much. I'd expect to see about -1 amp draw with the engine and all lights off and one coil buzzing. With 21 cp headlights and one tail light on, I'd expect to see about 4-5 amp draw with lights on and engine off. With 32 cp headlights and one tail light, I'd expect about a 5 to 6 amp draw with lights on and engine off. A stuck generator cut out would show a very heavy discharge with the engine off, regardless of whether the lights or ignition were on or off.

[TonyB]

With 21w lights the current draw will be 2*(21/6) which is 6.6 amp plus the tail lamp of 12w will total 8.6 Amps.
With 32w headlamp the draw will be 2*(32/6) plus the tail lamp 12.3 amps.
I think Pat forgot that we normally have two headlamps.
Mind you the moving iron ammeter is notoriously inaccurate even when new, never mind when 100 years old and been subject to all kinds of abuse.

[Scott_Conger]

Careful Tony!

You're right about the notoriously inaccurate gauges, but don't fall into the CP = Wattage trap!

Wattage is purely an equation starting with filament resistance. Candle Power as with Lumens, is an entirely different thing.

Years ago Gary H. White - Sheridan, MI provided the forum with a general guide which might prove to be a little more useful in the calculation. It's only a guide and until the exact bulb(s) are measured for resistance individually, true W cannot be determined (but this is a good start for seat-of-the-pants calculations)

2W ~ 3cp
6W ~ 9cp
10W ~ 15cp
16W ~ 21cp
25W ~ 32cp
35W ~ 50cp

Finally, fun fact: resistance increases with heat, so the measurement is sort of a moving target and explains why incandescent bulbs have a tendency to burn out at turn-on and last best with minimal cycles.

[TXGOAT2]

My T with 32 cp bulbs and one tail light shows about 6-7 amps draw with lights on and engine off and no coils buzzing. How accurate the ammeter is I don't know. It does go to zero when everything is off. I don't have a master ammeter to check it by. According to the ammeter, the generator is putting out about 5-6 amps at road speed with lights off and running on Mag. Turning on the headlights while running at speed results in a discharge of around 1 amp. I can adjust the generator to increase the charging rate, but hesitate to do so because the car has a fancy red battery and I don't think it would like the higher rate and also I'd rather not add to the parasitic load the generator places on the engine. I don't drive much at night because of deer, wild pigs, and having only one tail light on a black car. I'm considering turning up the charge rate to about 9 amps and adding switchable resistance in series with the generator to ammeter wire with an under dash switch to shunt out the resistors to regulate the charge rate. I'm thinking 2 or 3 of the 1960s style ignition resistors in parallel with one another might be suitable. I'd like to see about 2-3 amps charge rate running at road speed on Mag with lights off. At night, I could flip the switch to up the rate sufficiently to power the lights and still maintain about 3 amps charge rate.

[Art M]

I set my generator at 2 to 3 Amp charge with no lights, running at 30 mph on mag. I have 32 / 50 c.p. headlights bulbs. I seldom drive at night and I am never more than 2 hours from home. With a good battery, I figure I won't run out of juice until I get home. ImOther may have a different opinion on the low charging amperage.
Art Mirtes

[Scott_Conger]

Art

with a standard Group 1 6V battery at 640CCA, you're going to be able to drive on tour for example, for hours and hours (and hours and hours) while burning headlights, and then start up the next day and have a charged battery 1/2 way through the next day.

In general, folks seem to feel like they dodged a speeding freight train just driving an hour at night with a discharge on the battery. Nothing could be further from the truth, but it is a commonly held belief.

Operating your generator the way you do will make that thing last another generation or two and you will ALWAYS get home and have battery to spare.

[Scott_Conger]

Well, after hearing from Tony, and then rereading his post, it seems that it was I who inserted "CP" for "W", though my memory seems certain that it originally said CP and thus my response. In any event, his calculations are correct. The caution still stands in general though, as CP is not the same as W and each type of bulb needs to be measured directly to make a power calculation.

Sorry Tony!

[TXGOAT2]

Circuit voltage at road speed is probably 7+ volts.