Round Aluminium Plate getting hot ?

Cheyenne · Jun 18, 2017

There is a round disc attached to the battery box side of my bike ( 1969 Commando ) and notice it gets warm/Hot when key is on. What is it's function ?

Stephen Hill · Jun 18, 2017

Sounds like you are describing the heat sink for the zener diode. Early bikes used a thin round aluminum plate, later bikes used cast aluminum z plates. The zener diode dumps excess charging current to ground, and the heat is dispersed through the aluminum plate. If the plate is always hot when the key is on, you should test the zener diode to see if it is faulty.
(Cue the electron scientist dudes for the real answer......)

Stephen Hill

Cheyenne · Jun 18, 2017

Stephen Hill said:
Sounds like you are describing the heat sink for the zener diode. Early bikes used a thin round aluminum plate, later bikes used cast aluminum z plates. The zener diode dumps excess charging current to ground, and the heat is dispersed through the aluminum plate. If the plate is always hot when the key is on, you should test the zener diode to see if it is faulty.
(Cue the electron scientist dudes for the real answer......)

Stephen Hill

Great thank you

concours · Jun 18, 2017

Always run your headlamp for safety :mrgreen:

dave M · Jun 18, 2017

Concours, I have found on bikes with standard charging systems sometimes they discharge the battery over a couple of hours if you run with a headlamp on all the time. I run mine with the pilot light on unless it's night time. This can be mitigated by running LED lights that draw less power and also one of the electronic ignitions like a Pazon that use less power.

concours · Jun 19, 2017

dave M said:
Concours, I have found on bikes with standard charging systems sometimes they discharge the battery over a couple of hours if you run with a headlamp on all the time. I run mine with the pilot light on unless it's night time. This can be mitigated by running LED lights that draw less power and also one of the electronic ignitions like a Pazon that use less power.

Original single phase alternator, rectifier, & zener. 32,000 miles with a 45W halogen headlight on for every mile of it. Never a low battery.
I suppose, if it's inner city, low RPM duty, it could go low. YMMV :mrgreen:

dave M · Jun 20, 2017

I do live in Hong Kong, Concours, so not much highway running here and sometimes quite a bit of traffic.

concours · Jun 20, 2017

dave M said:
I do live in Hong Kong, Concours, so not much highway running here and sometimes quite a bit of traffic.

Got it.

:mrgreen:

ewgoforth · Jun 23, 2017

concours said:
dave M said:

Concours, I have found on bikes with standard charging systems sometimes they discharge the battery over a couple of hours if you run with a headlamp on all the time. I run mine with the pilot light on unless it's night time. This can be mitigated by running LED lights that draw less power and also one of the electronic ignitions like a Pazon that use less power.

Click to expand...

Original single phase alternator, rectifier, & zener. 32,000 miles with a 45W halogen headlight on for every mile of it. Never a low battery.
I suppose, if it's inner city, low RPM duty, it could go low. YMMV :mrgreen:

My experience has been with everything stock the battery will get chargd pretty well with the lights on, even around town. With a halogen headlight turned on it will charge the battery pretty well at highway speeds, but not in around town riding, but it would take many hours of running to kill the battery. If you're killing the battery after a couple hours with the lights on it sounds like the charging system isn't working at all, you're just running off the battery.

dave M · Jun 23, 2017

The charging system was working fine and showing a decent charge when checked with an ammeter. the problem went away when a Pazon was substituted in place the old Boyer ignition.

Round Aluminium Plate getting hot ?

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