12v Conversions

12v Conversions

Some very early vehicle electrical systems were only 4v but it wasn’t long before most manufacturers standardised on 6v. This lasted quite a few years until the number of electrical components on a vehicle started to increase and the standard system went up to 12v. This is what most vehicles have today though heavy vehicles such as buses and lorries use 24v systems and even higher voltages have been considered by some manufacturers. There is no doubt that system efficiency improves as the voltage goes up – that’s why the National Grid carries hundreds of thousands of volts though this is not likely to appear in our vehicles! Increasing the voltage is not the total answer as it then creates its own problems - switch contacts burning out is just one. So, the question is: should we convert to 12v?
The fact is, a standard ‘6v’ dynamo is only 6v because the regulator is set up to work at 6v. The same dynamo is perfectly capable of generating higher voltages so fitting a 12v regulator will give a 12v system. This is a simple view and there are other factors to consider. The voltage output of the dynamo is dependent, amongst other things, on its speed of rotation so it will need to be turning faster to charge a 12v battery than it would for a 6v battery. What this means in practice is that 30mph in top gear might be enough to generate 6v but you may need to be doing nearer 40mph to get 12v. This may be fine if riding out on the open roads but could become a problem if doing a lot of town riding. On some machines it may be possible to get round this by altering the dynamo drive ratio so that the dynamo runs faster at a given road speed. How about actually converting the dynamo so that it now generates 12v at the same speed that used to give 6v?
Dynamo theory tells us that, to convert from 6 to 12v, the number of turns in both the field coil and armature needs to be doubled. Unfortunately there just isn't enough room in most dynamos to be able to achieve this. Yes, a thinner wire could be used but that creates another problem. The wire's current carrying capacity is directly proportional to its cross sectional area so halving the cross sectional area would also halve the current carrying capacity. Now, power in watts is equal to volts times amps. The standard 60 watt Lucas E3L dynamo is 60W because it was designed to produce 10 amps at 6 volts. If converted to 12v in the way just described, the thinner wire halves the current carrying capacity so we get 12 volts times 5 amps which works out to be the same 60 watts. Bearing in mind that the brightness of the lamps is measured in watts, you can see that there is no increase in power gained by converting to 12v in this way so the lights would be no brighter. That in itself is not a problem and certainly the lower amperage improves the efficiency of the wiring loom because there is less power loss in the loom with lower currents. Unfortunately, halving the cross sectional area of the wire still would not reduce the diameter enough to create enough room to double the number of turns. The wire would need to be reduced in diameter by a considerable amount which would reduce its current carrying capacity to an unacceptable level. In practice, this type of conversion could not be done – there simply is not enough room. Armatures and field coils ‘converted to 12v’ are a compromise on this theoretical ideal.

It is often the case that a poorly performing 6v system is the reason for choosing to change to 12v. When this is done, consider what the new system consists of. A newly overhauled dynamo ‘converted’ to 12v, a new 12v battery, a new 12v regulator, new 12v bulbs and so on. Chances are, while these parts are being changed, poor connections are noticed and corrected – maybe even a complete new wiring loom is fitted. In short, a new 12v system is bound to be so much better than the poor, worn out 6v system it replaces. A similar overhaul on the original 6v system is likely to produce very similar major improvements for a fraction of the cost.
Our recommendations for obtaining a good 6v system are:
  1. Service/overhaul the dynamo and if of the 3 brush type, convert to 2 brush operation.
  2. Fit an electronic regulator – far more efficient than the old mechanical regulators. 
  3. Fit a battery of at least 8Ah. Gel batteries are good as they require less maintenance and hold their charge for longer when only used infrequently.  Lead acid batteries fitted into hollowed out rubber battery cases will be of too small a capacity and will not last.
  4. Keep the battery fully charged. A so called 6v battery will be nearer 7v when fully charged but will drop to zero if not looked after. The dynamo will have a hard job powering the lights and attempting to charge a flat battery at the same time.
  5. Fit LED bulbs in the rear lamp and instruments. On a 6v system, an 18W/5W filament bulb taillight takes nearly one amp when the lights are on and another 3 amps when the brake is applied. LED bulbs are just as bright but require around a tenth of the power. That frees up valuable power capacity which is better used in the choice of headlamp bulb.
  6. Don’t be too ambitious on the chosen headlamp wattage. A 60W headlamp, though superb when first switched on,  will probably flatten the battery and dim to practically nothing after a few miles whereas a 35W or 40W headlamp will give an acceptable light all the way home. Quartz halogen headlamp bulbs give better light output.
  7. Ensure the headlamp reflector is in top condition
  8. If rewiring, use cable with a large cross sectional area and as few connectors as possible. This will keep resistance to a minimum, thereby allowing more power to actually reach the bulbs. For the same reason, run a separate heavy gauge earth wire from the headlamp to the battery earth terminal so that the earth return is not trying to get through greased head bearings or along control cables.
  9. Remember that current has to flow along the wire to the bulb but also has to flow from the bulb holder, through the frame and back to the power source (battery and/or dynamo) so make sure ALL connections are clean and tight – this includes bulb holders, switches and fuse holders.
  10. The dynamo body itself is an important electrical connection. Make sure that a paint finish on the dynamo doesn’t prevent a good contact. Similarly ensure that the engine is electrically connected to the frame. It may be necessary to scratch off some of the paint under engine mounting bolts or run a separate earth wire from engine (or dynamo) direct to the battery. 
  11. If possible, consider changing the dynamo drive gearing so that the dynamo runs faster at a given engine speed. 
The resulting 6v system will be perfectly adequate for short night time journeys. If you are contemplating running for hundreds of miles of night time riding on a regular basis, then yes, a 12v conversion is the way to go but it should ideally involve a change to an aftermarket 12v alternator rather than  an attempt to ‘convert’ a Lucas E3L dynamo!

These notes on 12V Conversions relate to the use of  Lucas and Miller dynamos as used by the majority of British made motorcycles. They may also be relevant to other applications. The notes are our own views and and we appreciate that others may think differently. 

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