12 volt Conversion

One of the best conversions you can do to your C15 is to change it from 6v to 12v electric’s. This results in better lights, giving a 30 to 50 watt headlamp and an improved charging rate control at low revs. A further advantage is an ignition and direct lighting system without a battery for competition & trials machines, which is easy to set-up the ignition timing and you can have luxuries like electronic ignition, indicators, etc.

Background

Before describing the how to do it, let us have a brief comparison of the two systems. Both use a generator which has six output coils. In the 6v configuration the output is divided so that when the machine is running on ignition alone one third of the output is used and when the lights are switched on the rest of the power is brought into use.

The 12v system differs in that the full output from all six coils is used continually. To prevent overcharging and damage to the battery a device called a Zener Diode is fitted into the system, which absorbs the surplus current and converts it into heat which is dissipated through an alloy heat sink. The result is a much simpler wiring harness and cheaper, but more reliable switches. This is the type of system fitted to later models of BSA. You can even change the ignition switch to a standard automotive part with a proper key.

Making the change

Conversion from 6v to 12v is quite straight forward and well within the scope of the average person using normal home workshop equipment. 

The first thing to consider is the alternator. The 6v which has three output wires has to be changed to 12v which has only two. Note that the colours of the three output wires are White/Green Stripe (WG), Green/Yellow (GY), and Green/Black (GB). Using a double bullet connector, join the Green/Black and Green/Yellow together and you now have a 12v unit. On early models the wire colours are light green, mid-green and dark green, in which case join the mid-green and dark green together. 

The next thing is the rectifier. If it is a black three-bladed type, Lucas 49072, then it is OK for 6v or 12v use. There is also a small square solid state component which is not of Lucas manufacture which is also suitable for 12v use. (See tips below)

You will need to obtain a Lucas Zener Diode, part number 49345 and a BSA type heat-sink, part number 68-9428. When in use the diode produces quite a bit of heat and the heat-sink which should be mounted somewhere in the air stream helps to dissipate the heat. If you can not get the correct heat-sink, you can make one using several pieces of alloy sheet sandwiched together with washers and a bolt.  Two popular positions for the heat-sink are under the front fork yoke or the petrol tank. Make sure the mating surfaces between the diode and the heat-sink are clean and be careful not to over tighten the nut on the diode. A separate earth wire should be attached to the heat sink which should be Red 28/0.30mm automotive cable.

A new 12v battery will be required, which are easy enough to get hold of nowadays, and you will also have to obtain a 12v coil and horn, along with new bulbs. The ignition points and condenser can be re-used, or you could fit electronic ignition which will increase reliability and reduce points maintenance.

Next the cabling has to be dealt with. You can modify the existing loom or this may be the right time to make a complete new harness. To modify the loom, remove the two wires from the rectifier and make up two new leads Green/Yellow & White/Green (GY & WG) to connect your two alternator wires to the outer connections on the rectifier using 14/0.30mm automotive cable. It does not matter which one goes onto which connection. Any other green/yellow, green/white and green/black wires that go into the harness or connects to the switches can now be cut and insulated, as they are no longer required. Next a brown/white (NW) wire needs to be connected into the ignition side of the wiring. This can be done at the ammeter were the brown/white wire is connected to it. If in doubt disconnect the wires from the ammeter and check for power, as the battery lead will have power, but the rectifier side does not.

You can now fit the new battery and due to the extra current concerned a 35amp in-line fuse should be fitted. Having changed the bulbs and horn, you are now ready to go.

For competition a Lucas capacitor 54170009,2MC can be fitted. It should be mounted in a spring to absorb vibration with the terminals pointing downwards. It has two terminals, the small 3/6" blade has a red spot near it and is connected to earth. The other ¼" blade is connected to the centre connection on the diode, which is also connected to the coil, and lights if direct lighting is fitted.

Study the wiring diagrams. It is not as difficult as it appears when you read it. Click on diagrams to expand.

Full Wiring

Basic Wiring

Competition Wiring

Simplified Wiring Diagram

Tips

Alan Brooksbank

a) In a non battery system, use a capacitor (2200uf 63v ) from www.maplin.co.uk at a cost of £1.99 (Part LE01B 2200uF 63V)

b) 12v conversion, if you put 2 x 2.2 ohm wire-wound resistors in parallel (1.1 ohm 20w handling ) in line with your 6 v coil, you get a nice healthy 7 volts, total cost 38p. Maplin part no H2R2, saving 15.00 on a coil, plus a nice (7v) juicy spark. Be sure to h/sink them down.

c) The rectifier to use, again from Maplin is part No. AR87U, this is a std KBP 3504 electronic package, 4 terminal type rated at 400v and 35a. A large square metal type with a bolt through the centre costing £1.29p and no heatsink is required.

d) Also handy from them is 7 core trailer wirer ,which can be separated and used as necessary, saves buying rolls of different coloured cable.

Ian Giles

Had a problem with the rectifier so I have fitted a solid state one I had seen the ones advertised by various companies at £20-00 plus,  but  you can buy a 16amp bridge rectifier from Maplin electronics for £2-93 then all you need is some heat sink which was just over £4-00 so a good saving.