BSA C15 upgrade to 12v electrics

Right then I'll set the ball rolling.

I note with interest the comments about upgrading to 12v electrics.

Is it really that easy?? Just join the two sets of wires? You don't have to upgrade the alternator or the coils or anything, they can all handle 12v?

I do have the Haynes manual with a wiring diagram for the Star, but I'm not sure its complete as I don't see an earth from the points nor a spark plug!

Earth connection for points/plug is through the metal of the engine/frame.....

You may need to change the coil for a 12v version or add a ballast resistor to drop the 12 to 6......

Dont forget the bulbs......they will glow pretty brightly for a short while on 12v 😃

I should mention that its a positive earth too although I can't see that making any difference

Yes but you should put a zener diode across the system to dump the excess current to earth (frame). Nowdays one can buy a control unit for the Lucas alternator that rectifies the output and dumps the excess current. It works well and only costs about £30. I have one on my Matchless G80. Obviously bulbs have to be changed but many keep the 6v horn. Electronic ignition is a worthwhile modification.

JA

I haven't converted a C15 so don't have anything specific to that model, but I have played with this sort of thing on both a Honda CB100 and a Triumph Blazer SS (a rebadged BSA and a descendent of the C15)

A lot of permanent magnet alternators on bike rely on the constant current characteristics of the alternator to limit the charging current. Where this is done they will often have two windings on the alternator. One will provide enough current to meet the normal current demands for ignition and to keep the battery topped up, and the other will provide enough current to meet the headlight and taillight load at night. This extra coil is usually switched in by an extra contact on the headlight switch.

With these permanent magnet alternators, the open circuit output voltage rises directly with speed. At low speeds the output will be too low to overcome the battery voltage, but as the speed rises it begins to charge. At higher speeds, the charging current cannot exceed the maximum that the alternator can provide, which is determined by the magnet flux and the number of turns. This provides a crude sort of regulation, eg one coil will provide an amp or so for daylight running, and the other will provide six or so for night running.

This sort of arrangement is fairly hard on batteries, since they are likely to get more charge than they need in some situations, leading to loss of water and harm to the battery. So some systems started to use various forms of simple regulator, usually shunt. Lucas in the early seventies started to use a Zener diode rated at about 120Watts in shunt. Around that time some larger bikes started to use proper excited field alternators like a car, with the equivalent type of regulator controlling the field. These are of course much better.

So what does this mean for conversion to 12 V? Well, at some low speed, the alternator is able to produce enough voltage to charge a 6 Volt battery. At some higher speed it will produce enough to charge a 12 Volt battery. The current it can deliver to the battery at higher revs will be about the same in both cases. So yes, you can just change the bulbs. It is preferable to add a regulator, especially if you decide to have both coils in circuit at the same time. The Lucas Zener would be fine if you can still get them.

In theory you could find that since it needs a higher speed to start charging, that you did not get enough charge when just going slow. In practice that does not seem to be a problem. However, if you are desperate for more lighting...my Triumph 250 was already 12 Volts, but the alternator developed a short from the winding to the frame. So I rewound it with a few less turns. This raises the speed at which it will start charging, but also raises the maximum current when it does. I was able to run a 120Watt spotlight, actually an aircraft landing light off a Cessna, on that bike, and it kept the battery topped up for long trips at night. With my CB100 converted to 12V I was able to run a 55Watt H4 halogen headlight, which was about adequate for the speeds that bike could do. Note that this was a better light than most of the superbikes had at that time, eg most early seventies superbikes had lights that were not adequate for more than about 50mph.

Note that you should not run the motor with the alternator and rectifier disconnected from the battery, as this is likely to lead to rectifier failure. One small Yamaha that I played with once was able to provide 85 Volts open circuit at maximum revs. The selenium rectifiers that the older bikes have will not cope with that sort of thing. Actually it is often worth replacing the selenium ones with a silicon rectifier, but that is another story.

John

55W halogen on a 100cc bike! I had 25W on a Velocette Venom, which would just about get to 100mph. Any more flatenned the battery as on a good day that was rated at 28W. Real men don't need to see where they are going!

On a more serious note, te big Zenner shunt always seemd more than a bit crude, couldn't some kind of switch mode arrangement be developed? I probably don't know what I'm talking about as they usually step up, but why not controlled step down?

Paul Goff''s site is worth a visit for all things electrical re motorbikes

www.norbsa02.freeuk.com

Usual disclaimer

Yes: it really was simple to do the conversion. I cannot recall which colour wires I joined, but I did join two together. I used a modern, solid state rectifier instead of a zener: the two wires from the alternator went in one one side, 12v DC comes out of the other, ready to feed into the rest of the bike. I did change the coil and bulbs whilst I was at it.

I originally used a standard lead acid battery, but after twoof three kept going flat with standing. I was given the gel burglar alarm battery, which fitted neatly inside the original battery carrier, and simply leave it. It came charged a few years ago, stands all winter and keep sperfect charge.

I bought my bulbs and indicators from Paul Goff (lives about 5 miles away): pleasant chap and very fair prices.

Good luck.

JAmes.

Well, the brakes on some of those older bikes meant there was no point in seeing what you were going to hit anyway. The little Honda would do 70mph standard, and after I put a 125 barrel, a hot cam, and a larger carb on it it would do 80mph. Admittedly it didn't do this for all that long before needing an overhaul but it was heaps of fun. I also put the forks and brake from a Suzuki 180 into it...8 inch twin leading shoe. To the casual eye it looked standard.

The zener is not instead of the rectifier, it is an addition. It limits the voltage. It is possible to use a more sophisticated design of shunt regulator instead, but my first attempt at one of these lasted about a week, the components all fell off their leads from the vibration. The second one was potted with silicon rubber. It gave a more accurate control over the maximum voltage so the battery did not get overcharged. You can't easily use a switch mode regulator since the problem is to get rid of excess power that you have already generated. Honda did use a design in the 175 twin that had an SCR in the rectifier bridge, so it could be controlled.

John

The zener is not instead of the rectifier, it is an addition. It limits the voltage.

I fitted a device similar to the one shown in this link, which replaced the rectificier and avoided any need for a diode. It is bolted to a thick sheet of aluminium, up under the seat where it stays reasonably dry and is in a free air flow. It has been on for quite a few years and miles with no problems.

**LINK**

James.

You only have to dump power if you allow current to flow in the first place. With clever electronis I'd have thought you could have it full wave rectifying up to a battery voltage of say 13V, then half wave recitifying up to 13.5V, then switching off completely. Battery has very low internal impedance so should mop up any ripple. Have I just re-invented the Honda system?

This is similar to how the old dynamo regulators worked, above some voltage the field current was reduced, all using a voltage sensitive relay.

James...yes, that looks ideal, and of course includes both the rectifier and the regulator.

Duncan, that would be pretty much the idea of the one on the Honda 175/200 twin. The disadvantage is that when you switch off the current, the open circuit voltage rises...as I mentioned earlier, some of those alternators can reach quite high voltages under those conditions. These days it would be easy, but back in the seventies power electronics that could stand high voltages and reasonable currents, say 10 amps or so, were not so common or cheap. Unlike now, where something really nice could be done with power MOSFETs. But these days excited field alternators are more common anyway

John

Back in the day I used to rewire quite a few British bikes for people. Apart from bulbs and coil really the only additional parts needed were a Lucas Zener diode and a 'battery eliminator' otherwise known as a 4700 micro farad electrolytic capacitor, which was great, as this was the biking on the dole era, how things have changed.

There were 2 altenators, the old 3 wire 6v altenator which would give 12v if the wires were joined as described previously ( G/w on one side of the altenator G/y +G/bk commoned together on the other). This would give about 100w at 12v . There was also the high output 150W 2 wire altenator. The best technology available then, the Zener diode acts like a weir, to hold the DC voltage at 13.8 volts, with any surplus shorting to earth through the Zener. A generous heat sink was a necessity. A battery could have course been used, but was considered an expensive luxury on apprentices wages/ the dole. Anything was better than the old 6v systems, not for nothing was Lucas nicknamed ''The prince of darkness', not fun being plunged into pitch black on a dark night. Modern systems using MOSFETs are I'm sure miles better

i convert about 20 bikes a year and keep a solid state regulator in stock these i sell to club members at cost (28),it is this easy just connect the two wires from the alternator (or three with dark and mid green wires connected together to make two) to the box you then have one red wire (pos) and one black wire*(neg) coming out to connect to your system the alloy case is finned and should be mounted so as air can flow round it as any excess current is turned to heat ,if you have any problems let me know and i will sort out a wiring diagram to suit your bike , will