Introduction to the Lucas charging system
Like pretty much every other component of a classic motorbike, the Lucas dynamo charging system is a pretty simple setup that does the job without too much fuss. Well, most of the time anyway!
It’s from an era before everything was computer controlled, so it’s back to solenoids, coils and bimetallic strips if you want to get your head around how it all works to keep your battery topped up and your headlights glowing.
There are a whole range of things that typically go wrong with the charging system. Some of these are simple wiring issues that can easily be rectified by the owner, whilst other issues may require the services of a specialist repairer. Either way, the key to tracking down the fault quickly is to have at least a basic understanding of how the battery, voltage regulator and dynamo work together to power the bikes electrics. And getting that understanding is exactly the aim of this article.
This article consists of the following sections:
- Introduction to the Lucas charging system
- The basic setup
- The voltage regulator unit
- The battery versus the dynamo
- A ‘tug-of-war’ analogy
- Scenario #1 – Charging the battery
- Scenario #2 – Powering up the lights
- Scenario #3 – Charging or discharging?
- Conclusions and your comments
The basic setup
Firstly, let’s start by looking at a basic schematic diagram of the system. You can see from the diagram that there are only four main components to the Lucas charging system if we ignore all of the switches, bulbs and other accessories for the time being. These are the battery, the ammeter, the dynamo and the voltage regulator.
The dynamo is a simple direct current (DC) generator which is driven by the engine. The faster the engine revs, the more output voltage is produced by the dynamo. At tick-over speeds, the dynamo’s output is likely to be only a fraction of a volt, but it rises quickly as engine speed increases and can reach 15 or more volts flat out. One dynamo maintenance guide I was reading (available to download here) even suggests that a dynamo can output as much as 25 volts when there is no load!But my bike’s electrical system is only 6 volts I hear you say! So why would we want a dynamo that outputs so many extra volts? The reason is that the rotational speed of the dynamo and its voltage output are linked in the same way that the DC motor spins faster when you increase the voltage supply. The increase in output is not linear as it will take a certain minimum rotational speed before anything will happen, and above a certain threshold the output will begin to level off. The dynamo speed is obviously not constant as it is related to engine speed which itself is constantly varying depending upon road speed and gear selected.
In order to gain enough dynamo output at modest engine speeds it is also necessary to have an output which is really far too high at faster engine speeds. This is why the standard Lucas system can charge the original 6 volt battery voltage with ease, but can also be used to charge a 12 volt battery instead; great if we wish to convert to use brighter and more readily available 12v bulbs (for more on this topic, see the “Converting to 12 volts” article).
Feeding these widely varying voltages into the fixed voltage setup of the bike could cause real problems with blown bulbs and boiled batteries. Luckily though, Mr Lucas had the solution – the automatic voltage regulator (AVR) unit, or just regulator for short. When we say ‘automatic’, don’t be thinking of flashy digital electronic devices though. This is automatic in the very mechanical (or rather electro-mechanical) sense!
The voltage regulator unit
The voltage regulator, as it names suggests, regulates the fluctuating voltages supplied by the dynamo into something more usable by the motorbikes electrical system. In the most basic terms it consists of two electrical contacts (switches) which are opened and closed (turned off and on) by a pair of solenoid coils. These electro-mechanical switches work in the same way as a relay; current passes through the coils creates a magnetic field which, when it is strong enough, pulls the lever of the switch towards it, thus completing the circuit and turning it on. The automatic bit comes in to play because the regulator is constantly comparing the voltage of the battery with the voltage being produced by the dynamo and opening and closing these two switches accordingly.
The first switch is the cut-out. When the dynamo is giving less volts than the battery, the regulator disconnects it from the system so that it cannot draw any current. A DC dynamo is basically a simple electric motor working in reverse, therefore if you connect it to a suitable power supply it will try to spin just like a motor (see my article “Testing a Lucas dynamo” for more info on this). The large current drawn would quickly sap battery power if the dynamo was left connected, so the cut-out isolates it from the rest of the electrical system.
When the engine attains sufficient revs that the dynamo output voltage exceeds the battery voltage, the cut-out switch closes to re-engage the dynamo into the charging circuit to help recharge the battery. However, when the voltage coming from the dynamo gets too high, the second switch closes diverting some of the output through a resistance thereby reducing the magnetic field inside the dynamo. The voltage output is also therefore reduced and regulation of the output is achieved.
This is a simplified overview of how the regulator unit functions, but is enough to know for the current purposes. However if you would like to read a more technical description of exactly how the Lucas voltage regulator device works, have a look at the article How a voltage regulator works.
The battery versus the dynamo
There are two sources of power in the motorcycles electrical system – the battery and the dynamo. The battery provides power when the bike is stationary and thus the engine not revving fast enough to generate enough output from the dynamo. The dynamo provides power when the bike is riding along to both power any lights that are in use and also to replenish the stored charge in the battery.
It should be noted that the system is rather finely balanced with the dynamo producing just enough power at normal road speeds to operate the headlamps and keep the battery topped up. This fine balance can easily be upset if, for example, you install a 60 Watt headlamp bulb. In such circumstances the dynamo may not be able to provide enough power to light the bulb by itself meaning that the battery will also need to help out even at moderate road speeds. In this way the battery is slowly drained and never replenished, eventually becoming completely discharged.
On the other hand, you can tip the balance slightly in the battery’s favour by reducing the power drain from lighting. Modern LED bulbs draw significantly less current than conventional incandescent (filament) bulbs, as discussed in the “Installing LED lighting” post. Reducing the electrical power load on the bike means that the dynamo has to work less hard to keep the battery topped up. This is especially important if you’re regulator isn’t performing at peak capacity anymore, or if you do lots of low speed stop-start riding around town.
A ‘tug-of-war’ analogy
Time for an analogy perhaps! Think of the charging system as a tug-of-war competition. On the one side we have the battery and on the other side the dynamo and regulator team. The rope is a bit hard to see as it’s concealed inside the wires, but the ammeter gives us an indication as to which way it is moving.
When the battery is strong and the dynamo/regulator weak then the battery wins the privilege of powering the lights and other electrical accessories. When the dynamo is strong and the battery weak then the dynamo/regulator team come out on top. The strength of both sides is constantly varying; the battery strength is relative to it’s state of charge and the dynamo strength relative to engine speed. Like any competition we would prefer it to be fair with each side ‘winning’ about half the time. That way the battery doesn’t wear itself out and the dynamo isn’t overloaded.
Scenario #1 – Charging the battery
To help see how this all comes together in practice, let’s consider three different scenarios in which the Lucas electrical system might be operating. The first scenario is the one that most classic motorbikes will be operating in for the majority of the time – the bike is riding along quite happily during the day so we don’t need any headlights on.
The engine is at moderate revs as the bike cruises along, the dynamo is spinning quickly and a reasonably high output voltage (probably more than the 6/12v battery voltage) is being fed into the regulator unit. The regulator is then bringing this output voltage down to something more in-line with the bikes electrical system.
This is where the tug of war I mentioned earlier comes in to play. Both the battery and the regulator/dynamo are both pulling (or pushing if you prefer) current towards the ammeter. With the lights switched off there is nowhere else for the current to flow, so it all comes down to how ‘strong’ the battery is and how hard to dynamo is working. If the battery is fully-charged then it’s voltage will be equal to the voltage being fed to the ammeter by the regulator, so we have a draw and the current doesn’t go anywhere. The battery doesn’t get absorb any further charge, but neither does it discharge. Therefore the ammeter sits in the middle of it’s range indicating zero current flow either way.
If the battery is ‘weak’ (i.e. partially discharged), then the battery voltage will be less than the regulated dynamo voltage. The dynamo can therefore push current through the ammeter and into the battery, giving a positive reading (needle moves to the right) on the ammeter. The more discharged the battery is, the ‘weaker’ its voltage and so the more current can be ‘pushed in’ for a given regulated dynamo voltage.
As the battery gets recharged, so it’s voltage output rises and hence the amount of current that the dynamo and regulator can ‘push’ into the battery declines. When the battery is fully charged, the battery and regulated dynamo output voltages are approximately even. And so we reach the steady-state equilibrium position again when no charge is flowing into or out of the battery and the ammeter needle returns to the central zero position.
Scenario #2 – Powering up the lights
Let’s imagine that whilst we are out riding it starts to get dark, so we pull over to the side of the road and switch on the headlights. In this second scenario with the engine just ticking over, the dynamo isn’t really spinning fast enough to produce any significant voltage output. The regulator can only reduce over-volts; it can’t boost under-volts.
And so the dynamo and regulator aren’t really doing very much, which means that the power for the headlights can only come from one place – the battery. The ammeter needle swings to the left indicating that current is flowing out of the battery which is therefore discharging. The amount of current flowing will depend upon the load created by the light bulbs; it will be much greater if you have installed some blinding 60 Watt’ers compared to if you have the stock 25-35 Watt bulbs. But that’s all fine for now as the battery has plenty in reserve so long as we don’t sit here with the lights on at the side of the road for too long!
Scenario #3 – Charging or discharging?
And so with our headlights burning brightly we pull away again to head for home, which brings us to the third and final scenario I want to consider. So we’re riding along at a nice cruising speed, the engine is at moderate rev’s and the dynamo is spinning happily sending plenty of volts up to the regulator. The regulator is taking this dynamo output voltage, reducing it down where necessary, and feeding this up to the ammeter connections. Now this is where it gets a bit more interesting as we now have three different current paths for the battery, regulator and headlights.
Power can only flow from the regulator and it can only flow to the headlight bulbs. But the path to the battery is bi-directional so current can flow either way. It all comes back to that tug-of-war between the battery and dynamo/regulator voltages. If the regulated dynamo voltage is much stronger than that from the battery, the dynamo will power the headlights and any left-over voltage might be used to trickle charge the battery too. But if the battery voltage is strongest (for example when we slow for a junction), then the battery powers the headlights and the dynamo gets a brief rest.It would be nice to think that in normal riding conditions the battery and dynamo/regulator combination are both working together to power the headlights and thus sharing the load. But in truth, the voltage regulator unit is only a pair of relay switches which can be either on (outputting voltage) or off (disconnected from the circuit and doing nothing). The battery and dynamo might take it in turns to power the lights as one’s voltage momentarily exceeds the other, but they don’t ever really work together at the same time. That’s why I quite like the tug-of-war analogy as it seems to fit the constant power struggle that Mr Lucas designed for us!
Hopefully the above article has provided a good overview of the Lucas dynamo charging system fitted to many classic motorcycles and how this operates in various scenarios. For more information have a read of my various other articles on this website or look through some of the suggested further reading shown below. Finally please feel free to let me have your thoughts and suggestions on this article using the comments form below.
If you would like to read more about the Lucas electrical system, then checkout some of my other articles on this topic:
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There are also various classic publications in the Reference Documents section of this website which will be of interest. If you’re feeling particularly adventurous, the the Lucas ‘Workshop Instructions’ manuals to the MCR2 regulator unit or the E3L and E3N dynamos are a good place to start. Or for something a little less technical and more practical, the ‘Practical Dynamo Maintenance’ and ‘All About Dynamos’ publications are an interesting read.