So as we all know, the charging system on these bikes is really marginal, especially at low RPM. I've seen a lot of talk about swapping in CB750A stators, going to permanent magnet alternators and all sorts of other stuff. Basically taking the brute-force approach to getting more power. But I personally thought there might be room for improvements in the charging system's efficiency, rather than just throwing more alternator at the issue.
The first thing that came to mind is the rectifier. Our bikes use a 3-phase bridge-rectifier to convert the alternator's electricity into DC. It does this by using 3 sets of diodes arranged like this:
The problem with this, is that standard silicon diodes have a weird characteristic voltage drop across them. For a standard diode, this voltage drop is 0.6-0.7 volts. So that means that while the amount of current through the diode can vary a lot, it's still going to have a 0.7v drop across it almost all the time. This becomes even more of an issue with a bridge-rectifier setup. For current to flow through a bridge rectifier, it most flow through two diodes, and the voltage drop of each diode stacks with each other. So a rectifier is going to drop 1.4v no matter what.
That means before the alternators on our bikes can even start supplying power, they need to be generating 1.4v MORE than the battery voltage at minimum.Since the diodes are shedding voltage, this means there is also a power loss through them that is directly proportional to the current flowing. Our bikes are quoted as having a 225w charging system, which at 14v means 16A of current. At 16A, a standard silicon rectifier is burning away over 22W of power as heat. That's a full 10% of the alternator's output!
There is a solution however (well two solutions, but more on that later): The Schottky Diode. This is a special type of diode with an extremely low voltage drop. Typically 1/3 - 1/2 that of a typical silicon diode. This means in low-voltage situations, you get a lot more of the power where you need it, and less wasted as heat: And as luck would have it, someone makes a fully integrated 3-phase Schottky Bridge Rectifier: The IXYS FUS-0045B -
https://ixapps.ixys.com/DataSheet/14b6ae00-24ef-4836-9531-dd26e2e1ed59.pdf . The best part, is that this little chip only costs $6. So for $6, I was willing to take a gamble and ordered one.
Step 1 was to solder and heatshrink some wires to it. 3 wires for the incoming AC, two for the outgoing + and - leads:
Next, I rummaged through my junk bin and found a heatsink to attach the rectifier to (even though it wastes less power, it still require a heatsink to stay cool), and crimped some terminals onto the wires.
Finally, I installed the whole assembly in the bike. Now this is just a "prototype", so if the rectifier proves itself, the little clip attaching it to the heatsink will be replaced with a proper screw-retained bracket, and the spade terminals will be replaced by a proper plug.
So then I put an ammeter inline with the connection between the main harness and the battery so I could see exactly what was going on. The results speak for themselves:
Before:1200 RPM: Battery discharging
2000 RPM: 3.2A battery charge current
After:1200 RPM: 0.01A Battery charge current (this means the charging system is matching the bike's electrical demands exactly)
2000 RPM:
4.8 A battery charge current.
So as we can see, this $6 part is giving me an additional 1.6A of charging current at 2000 RPM. That's 20+ watts of additional power! I'm going to put some miles on this rectifier to make sure it holds up, but this is looking like a huge improvement in low RPM charging system performance for just $6 and about 1 hour worth of time to put everything together.
This place has the rectifiers in stock for about $5.50:
https://www.futureelectronics.com/p/2009745?utm_source=eciaauthorized&utm_medium=CPCBuy&utm_campaign=ECIA_Ext_RefferalsI'm also looking into making another rectifier using technology called an "ideal diode bridge". This converts 3-phase AC to DC with almost 0 losses, but it requires about $35 in components and for me to make a circuit board. But with how much of an improvement just the schottky rectifier made, I think it may be worth investigating.
