BMW boxers use a Bosch charging system. This article will describe how they work, as well as the ways in which they don't. There seem to be three basic ways in which they misbehave.
The first, and best known, problem is that if the rider doesn't keep the RPM's up, the battery won't charge. The solution is simple, as the joke goes, "Don't do that". Try to make sure that you keep the revs over 3000 or so for a significant part of the trip. Many people claim that this is easier on the engine bearings and timing chain as well, but this article is about the charging system, so I won't dwell on that. Avoiding short trips is probably a good idea too, but often that's just not practical. In situations such as this, a trickle charger can be handy. I use a Sears unit that has a 4 amp position and a trickle-charge position. If I can't get a good ride in on the weekend to compensate for my short daily commute during the week, I trickle-charge the bike for about four to six hours on the weekend. If you get a choice, ride -it's more fun!
The second problem is even more fundamental. The alternator on two-valve boxers puts out a rather modest 180 to 280 Watts, depending on the model. That's not much folks. For comparison, the newer beemers use 700 Watt alternators and 1000 Watt and larger alternators are not unheard of in automobiles. What this means is the owner of a two-valve R bike, especially the older ones, should not get too interested in the concept of adding a lot of electrical accessories. With a 65 Watt halogen headlight, the ignition system, and miscellaneous systems on the bike, we're talking about 100 Watts just to travel down the road. Subtract this from the rating of your particular alternator and you've got what's left over to charge your battery (to replenish it after starting the bike) and to run accessories. The numbers aren't too impressive.
Here are the figures, from the electrical chapter of the Haynes service manual. I'm not sure of the accuracy of these numbers. I have been told by another /5 owner that late `73 /5's, have the 280W alternator that /6's used. At least one long wheel base late `73 R75/5 (mine) does have the 180W alternator.
/5 180W R90s 238W 1000cc to `80 250W All else 280W
The third problem is one that is not inherent in the design of the system. It comes with time, age, and corrosion. Corroded/loose connections, worn brushes, and dirty slip rings can have a major impact on performance. Maintenance is rather simple, but tedious. Just keep all connections clean and tight and make sure to inspect the alternator brushes regularly. Kari Prager had an excellent article on this subject in the May 1993 of the BMWMOA "Owners News", so I won't go into detail here about maintenance.
By now, those of you with K bikes (and R1100's) will be snickering, but we'll just ignore you and go on to an explanation of how the charging system works. I don't have a K bike manual handy as I write this (In fact, not owning a K bike, I don't have one period), however I do think the basics are the same. Heaven knows Bosch uses this basic charging system on every alternator-equipped VW I've ever worked on, so the odds seem good that they'll use it at every opportunity.
Here's where we get to the part about how the charging system actually works, but first a note about the schematic provided. This schematic is highly simplified to make it easy to follow. It is basically correct, but it leaves out most minor year-to-year differences (mostly trivial, as far as I could tell from the manual), and more importantly, it leaves out a lot of connections. Connections which can become corroded and cause trouble. I've mentioned corrosion haven't I? Can't mention it enough in this context.
The most glamorous part of the charging system is the alternator. It is located directly under the front engine cover and is driven by the crankshaft. It converts mechanical energy provided by the engine into electrical energy.
The rotating portion of the alternator is called (fairly obviously), the rotor. In the Bosch alternator (and most others) it may also be called the "field", or the "field winding" since it is an electromagnet which provides the magnetic field the alternator needs to operate.
Electrical current is provided to the field by the brushes and slip rings. The slip rings are the two copper rings that are located just behind the front bearing. They rotate with the rotor and crankshaft. The brushes are the squared-off carbon rods which are pushed up against the slip rings by two springs, one per brush. The brushes are stationary and as they rub across the slip rings they maintain contact and allow current to be transferred from the stationary part of the alternator to the rotating part.
The outer part of the alternator is called the stator, once again a fairly obvious name as it remains stationary. The stator consists of three coils of wire (electrically, they are physically divided up) that surround the rotor.
As the rotor spins inside the stator, the magnetic field produced by it rotates with it. As this magnetic field moves across the stator coils, it generates electrical currents in them. This is the current that charges the battery.
At this point, there's only one problem. The battery, as well as the rest of the bike's electrical system is DC (Direct Current), and an alternator, as its name implies, provides alternating current (the direction of current flow reverse periodically). Enter the diode board, that assembly of electrical components that is located under the front engine cover above the alternator.
A diode is a two-terminal device which allows electrical current to flow through it in one direction, but not the other.
On the ever-so-infamous diode board, we find either nine or eleven diodes. The 180W /5 alternators have 9 diodes, while the later models have eleven. The schematic shows the "extra" diodes connected with dashed lines. The three stator windings connect to nine (or six on the older bikes) of these diodes which rectify the current they supply, thereby converting it to a form suitable for charging the battery and powering the bike's electrical system. The stator windings also connect to three smaller diodes that supply current to the voltage regulator and one side of the charging system "idiot light" (the other side of this light hooks to the switched power supplied to the bikes electrical system).
At this point you might be wondering where the current for the field winding (rotor) actually comes from. It is supplied by the voltage regulator. The voltage regulator senses the battery voltage in a rather indirect manner through the diode board and uses this information to control the current to the field winding of the alternator. If the battery voltage gets too low, the regulator increases the field current which increases the strength of the magnetic field it produces and thereby increasing the alternator's output. If the battery voltage is too high, the voltage regulator reduces the field current which decreases the strength of its magnetic field and the output of the alternator.
Want to know about that "indirect manner" mentioned above? Well, it's not too complicated. The regulator needs to know the battery voltage (B+), but it is not connected directly to the battery. What happens is this: The stator provides current to the battery. This current results in a voltage drop across the diodes that connect the stator to the battery (B+). Typically, this will be somewhere above 0.7 Volts for silicon diodes. This voltage will rise with current, but not in a linear manner, if the current doubles, the voltage drop will only increase a few percent instead of doubling as Ohm's law would predict for a resistor. This is why diodes are classed as non-linear devices. Well, to get back to the subject, this voltage drop added to the battery voltage determines the peak voltage at the stator windings. The stator windings are also connected to the voltage regulator through the three small diodes. The voltage drop across these diodes is roughly equal to the drop across the larger diodes and causes the voltage they provide to the voltage regulator to roughly equal the voltage at the battery. And that is the indirect method...
At this point you might be asking, "What about the charge indicator lamp? Doesn't it provide a more direct path between the battery and the voltage regulator?". Well, as we'll see in later paragraphs, this does happen when the engine is being started, and when there is a problem, but when the system is running normally, the light is not lit. When it is not lit, there is no current flowing through it and it has no effect on the operation of the circuit.
The above description assumes that the engine is running and there are no electrical problems. What happens when the engine is off and the key is in the off position? What happens when the key is in the on position and the engine is not turning? When a part fails, how does the idiot light sense this? Read on! When the engine is not turning and the ignition switch is in the off position, there is voltage at the output of the diode board (from the battery), but the diodes block it from the stator windings, thereby keeping the battery from discharging. A leaky diode could ruin this feature, causing the battery to run down at night. Not too likely, but it's one of those things that could happen.
When the key is turned on, but the engine is not turning, power is applied to one side of the idiot light. The diodes (the little ones) block this current from flowing into the stator windings, through the bulb, just like their big brothers did all night. It can however, pass into the voltage regulator, and from there into the field winding (magnetizing the core of the rotor, thereby getting the alternator ready to do its job). This is why the light comes on before the engine starts. This is also why the charging system often won't work (or will work erratically) when the idiot light is open. With the bulb open there is no current through the field to get things started. Sometimes the iron retains enough of a magnetic field to get things going, sometimes not, it's a gamble. Always replace the bulb when it burns out! Bobby Bosch, what were you thinking? I often wire a 470 ohm 1W resistor in parallel with the lamp so I don't have to worry. I haven't done this on my R75 yet. I've done it on a lot of VW's though.
Once the engine starts spinning, the alternator starts working. The big diodes provide current to the battery and the little ones provide it to the voltage regulator. Since the lamp is connected between them, it has about 0 volts across it, and it goes out. Everything is normal at this point.
If the alternator stops providing output from the stator windings, the battery would keep voltage on the ignition side of the idiot lamp, but the alternator would not supply any through the small diodes to the regulator. Thus, current would flow through the lamp, into the regulator and mostly out into the field (rotor), thereby lighting the lamp and warning of the problem.
If an open rotor or, in some cases, a bad voltage regulator, were the cause of the above problem, the only path the lamp current would have to ground would be through the ground terminal of the regulator. It would be deprived of its normal path through the rotor. The lamp would likely glow rather dimly. According to my Haynes manual, the lamp would have about 1/2 volt across it at this point. A bad diode board can cause similar problem. If either the big diodes or the little diodes fail to produce the expected output, there will be a different voltage across the lamp. It will light, warning of the problem.
Actually any problem that causes the alternator output, as rectified by the small diodes, to be different than the voltage after the ignition switch, can light the idiot light. This means that if there are any bad connections in the circuit that cause a voltage drop between the output of the diode board (big diodes) and the point in the wiring system where the light hooks up, the light will glow dimly. If the connection opens completely, the light will be bright. Bad grounds can wreak havoc with the charging system as well.
The bad connections mentioned above are often the cause of a charge lamp that glows dimly all the time even though the bike runs normally. Often the bad connection in question is the ignition switch contacts. These may usually be cleaned with some lubricating contact cleaner. The connections at this stage of the problem are good enough that the battery still charges, but at a reduced efficiency. There is enough voltage drop across the wires, connections, and switch contacts that the voltage drop lights the lamp. Often it's so dim that it can only be seen at night, but once you notice it, it will bother you until it's fixed. As it should be.
While there are some inherent problems with the charging system on the two-valve boxers, the system does have the advantage of being relatively simple. Just keep in mind its modest power output and don't allow that dribble of energy to be wasted.