Amp Output for A Bosch Magneto?

[Barry Robin]

I just got my Bosch magneto rewired and the shop was very proud to inform me that it tested great! It put out a whopping 2.5 amps...!

Say what? That must be wrong; I seem to recall it putting out about 5-7 amps. Anybody know the correct amperage output?

-B

[schrader7032]

I'm probably out of my depth here, but I found an old copy of a German version of a Bosch manual. It looks like there are 30 and 45 watt versions. Divide these by 6 volts to get amps. So, 5 and 7+ amps. Looks like your memory is right.

[Barry Robin]

Whew! Nice to see that senile dementia hasn't kicked in yet.

Now, I know that the battery puts out something like 5 ampers. But that doesn't mean that the mag itself does. Looks like we're both waiting for the more acurate answer.

The real question is if I'll be OK on the output or do I return it to my mag guy?

-B

[schrader7032]

I wouldn't know how to measure amps realistically, so that might be something to check on. But with 2.5 amps, you have to wonder how much the ignition takes, plugs, lights, etc. Seems like you're battery would be drained faster than it could be charged.

[Barry Robin]

Yup, you got that right.

These prewar electrical systems are sorta famous for it. The system charges just fine on the highway or above about 3500rpm. But putting around town-from stoplight to stoplight-will drain the battery and kill the headlight in around 15 minutes. I've learned to drive at night by just using my parking light.

Time to install a fancy-pants LED set-up...

-B

[Beemer54]

You don't state which model, though I guess that they'll all be about the same for the pre-war models. When I got my R12 I sent the mag to RRC-Tuning for them to check over. Wolfgang wrote back

- the dynamo works perfect. I use a 35W bulb and a new battery for appliance and check the rest of Ampere, approx 3A is coming out in warm condition.

On that basis your 2.5A sounds marginally down but not enough to bother about.

[Barry Robin]

Its an R11. As far as I know, the bike uses the same Bosch D2 mag as the earlier R52/62 and the later R12 (though the R12 mag revolves in the opposite direction).

What do the rest of you think? Is that 3 ampers rating about the same for everyone?

-B

[caker]

Make way: high-school-electronics-class expert coming through...

Are we talking about the generator output? If so, 2.5A @ 6V is only 15W and will likely be insufficient to charge the battery when loaded with the headlight. The ignition load is handled by the magneto itself, so there is no worry about the bike not running even with a weak generator.

If they were talking about the actual magneto portion - I'm not sure how that would be measured and what the voltage would be. But, I don't believe amperage if/when measured on the primary coil is significant. It's the secondary coil output that matters as far as what will actually be at the spark plugs. The secondary coil ups the volts into the tens of thousands but reduces current to the milliamps.

-Chris

[Twocams]

It takes 1 volt to push 1 amp through 1 ohm of Resistance. That all I know
OHMS Law

[Beemer54]

Are we talking about the generator output? If so, 2.5A @ 6V is only 15W and will likely be insufficient to charge the battery when loaded with the headlight.
-Chris

Re-reading what Wolfgang has put I think my earlier statement was wrong and Chris is right. Wolfgang said that he tested the output whilst a 35 watt bulb was in the circuit. This would simulate the output when being driven at night ( less, of course, the slight draw from the rear bulb). So with the lights on there should still be roughly 3 Amp of power available to charge the battery