Converting Your 6-volt British Motorcycle to 12-volts
By Dave Porter, Customer and Technical Support at The Bonneville Shop
“The conversion requires not just an upgrade of charging system components, but also auxiliary electrical components. This includes light bulbs, horn, fuses, and in some cases, switches, ignitions, and ignition coils.”
   If your British motorcycle was made before 1966, chances are it was originally specified with a 6-volt electrical system. A 6-volt system is adequate for daytime running, but with limited lighting, ignition, and battery selection, the choice to switch to a 12-volt system has become more common. In most cases, the 12-volt conversion can be made without irreversible modifications. The conversion requires not just an upgrade of charging system components, but also auxiliary electrical components. This includes light bulbs, horn, fuses, and in some cases, switches, ignitions, and ignition coils.
If you are uncertain about which components to replace, give us a call at 720-570-7884. You can also email order@thebonnevilleshop.com
Motorcycles Fitted with DC Generator Charging Systems
“It isn’t suggested to do this however, since you would need to double the speed of rotation of the generator to charge a 12-volt battery.”
    For generator/dynamo models, the output of the generator is limited by the voltage regulator in place, and the speed which it can be spun. This is an oversimplification and there are other factors to consider. However, you could theoretically replace a 6-volt regulator with a 12-volt regulator and have a working 12-volt system. It isn’t suggested to do this however, since you would need to double the speed of rotation of the generator to charge a 12-volt battery. In other words, if charging capabilities of a 6-volt dynamo starts at around 2000 rpm, you would need to run up to around 4000 rpm to charge a 12-volt battery.
Some enthusiasts desiring 12-volt benefits while retaining a level of originality may consider altering the windings and field coil of the dynamo. This type of modification is best left in the hands of a specialist. Another option is to consider some of the modern aftermarket generators available. Suppliers of generators include Alton. Once you have addressed the charging capabilities of your generator, you can fit the appropriate 12-volt regulator. You also need the correct bulbs, horn, and battery.
Motorcycles Fitted with Alternators
     For 6-volt British motorcycles with alternators, you will need to source a 12-volt regulator/ rectifier, or what I call a charging module. Today’s charging modules are solid-state, compact, and vibration-resistant. Some charging modules with MOFSET technology are even heat-resistant. They can be installed in an enclosure without any regard for the cooling effects of airflow. As part of the 12-volt conversion, the charging module replaces the Zener diode (voltage regulator). It also replaces the selenium rectifier, which is tasked with converting alternating current (AC) to direct current (DC).
If a contact breaker points ignition is going to be used, the original 3-wire 8-amp stator fitted to most later 6-volt machines can be used to supply the current. Simply pair the green/black and green/yellow wires together to form one lead. Then connect it to one of the yellow leads from the charging module. The remaining white/green wire from the stator connects to the other yellow lead from the module. The brown or black lead from the module will be the power to the battery (through the ammeter, if fitted). The red lead is the grounding wire, at least on a positive ground system. Once the bulbs, horn, and battery are updated to 12-volt specification, you are now operating on 12 volts.
More Upgrades to Consider
Now that I have covered the most rudimentary conversion from a 6-volt system to a 12-volt system, let’s go forward to the next level of upgrades. The original Lucas 3-wire RM19 stator, as fitted to many 6-volt British motorcycles, had a current rating of 8 amps. That will equate to approximately 96 watts of power to run a simple electrical system. The majority of the late 1950s-early 1960s motorcycles were equipped with 96 watts. If an electronic ignition system should be retrofitted in place of the troublesome 4CA points ignition system, a more powerful stator should be considered. The Lucas 47205 RM21 is a 10-amp single-phase stator with 120 watts of power to better satisfy the demands on the battery. This stator is fitted to 1969-1978 models.
Since the stator only comprises half of the alternator, attention must be paid to the magnetic rotor. Rotors must be well magnetized to optimize the performance of the charging system. This is especially true for running at lower engine speeds. A new rotor to pair with a 10-amp stator will go a long way to keep the battery charged. It will also satisfy the demands of electronic ignition on the battery. When selecting a charging module for your 12-volt conversion, be sure to select the appropriate unit to match the output of the alternator. If deciding to further upgrade the charging system with a 16-amp RM23 single-phase stator, you’ll need a more robust charging module with 200-watt capability. One choice is Podtronics POD-1P-HP, which we offer under part number TBS-4035 (or in this kit with a stator, rotor and primary gasket).
Thanks for checking out this article. Please contact us if you need any further guidance with your 6-volt to 12-volt conversion.
Additional Resources
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