B.T.H Magneto-Generator Series 'A' Comet

[Michel]

Hello everybody and two questions for electricians.
The HRD is equipped with a magneto BTH combined with what was believed to be a dynamo. So we were on the idea of a similar Magdyno than Miller and other Lucas. Except that there is no regulator.
We seek to understand that BTH is permanent magnet and its operation is completely different from others dynamos . Yes, it is a magneto - generator . According to interviewed experts , no regulator was expected that only confirms the Electrical Manual (page 20) published in 1953 by Motor Cycling . The questions are :
1. How to avoid stopping the battery discharges to the BTH ? We put a diode but it also prevented the load. A switch would neither stylish nor original nor practical.
2. The battery does not risk exploding when load engine running constantly ?
For me it's all very new .
Have a nice Sunday.

 

[Michel]

I make clear,
the battery is a 6 V - 6 Ah and 30 A. I charged the battery overnight with a 6 V 1.2 A and this morning it was O.K. with light green. I then installed another charger (rather than maintenance) less powerful 6 V 0.9 A. The orange light indicated well the current load ... but the battery is discharged in a few hours! What deduct ? The mystery deepens .

 

[vibrac]

A google search for 'BTH combined mag dynamo' reveals some information and after looking down a lot of stuff about the new BT-H (How I wish people would put that hypen in, it would save a lot of time) I came across a pdf from 'Scott technicalities'. Having got a Scott I know these 'technicalities' are on a par with '40 years on' they do have something to say about something that sounds like your device and the fact you cannot use a CVC but I could be wrong it might be worth looking

 

[mercurycrest]

This is from a 45 year old memory of what was told to me then.....
" The charge position in the headlamp switch manually controls the charging of the battery. When it's charged you switch back to "off" or "lights" until it needs charging again."
I'd check to see if the field coil is connected to ground or to one of the armature brushes inside the generator cover and purchase a modern regulator to fit accordingly.
Cheers, John

 

[bmetcalf]

This might show how the dynamo is wired:

 

[mercurycrest]

This might show how the dynamo is wired:

View attachment 14466

Yep, that's the two ways generators are wired. "Internally" grounded and "Externally" grounded (referring to the field coil).

 

[nkt267]

I wonder if Michel has a 3 brush dynamo? that could change things a lot..john

 

[Simon Dinsdale]

The BTH mag-dynamo fitted to the earlier series A singles is a permanent magnet field. That means it does not have a wound electrical field winding like most later dynamos. I have one fitted to my A Comet. As there is no field winding to switch in or out with a regulator, the usual bobbin regulator or modern electronic regulator cannot be used. Originally a cutout was fitted to the top of the toolbox, and that is all it was, a cutout and not a regulator. This stopped the battery discharging back through the Dynamo, but a modern diode can be used.
The charging was controlled via the headlight switch. Off means lights off and no charge from the Dynamo to battery. Charge on the switch connected the Dynamo to the battery via a resistor so to keep the charge rate low. When switching the headlight on, the charge resistor was bypassed so giving full charge.
The Miller mag-Dynamo fitted to later series A singles was a different device.
Simon

 

[bmetcalf]

Could a zener diode be used from a late 60's BSA or such?

 

[Simon Dinsdale]

I dont think a zener would be a good idea. It would cause the Dynamo to run at full output all the time and then dump the excess to earth and so could damage the Dynamo.
The original system on my bike works fine. The BTH mag-Dynamo when going full charge generates 24 watts of 6v power and so with a LED rear bulb and small headlight bulb it just about keeps up. Never overcharged the battery yet. You just need to remember to switch to charge every now and again when not using the lights to top up the battery.
Basically if it gets dark and you need the lights, get home quickly as the whole charging system really cannot cope. What I would like to do is fit a LED dipping headlight bulb to reduce the load, but I've not found one yet that runs at 6v.