Recently, several new "hotrod" tricks for increasing battery voltage have appeared. Some of these parts are good: some are not so good, depending on how much you ride the bike. Here's the story behind these parts:
1. Rectifiers.
There are some modern rectifier diodes, specifically called "Schottky diodes", that can improve the power output from the older SOHC4 alternators. These diodes have a lower forward voltage drop, which becomes less waste heat, and in turn becomes more battery stoage power. These nice units are pricey: I have seen them selling for up to $60
on some websites, and they are a good way to recover some lost power from your aging OEM unit.
A related tip: make a piece of 16AWG wire (green is appropriate color) with a 6mm ring terminal on one end and an 8mm ring terminal on the other. Remove your rectifier's mounting nut from its post and clean everything there, then install this new wire to that same post, adding it under the nut, perhaps add a star washer with it for good cut into the bracket. Install it all back onto the bracket. Connect the other, 6mm, ring terminal to the frame ground under the seat on these bikes, maybe add a star washer there, too. The actual improvement: this provides a clean, direct, low-impedance electrical circuit where the old one is corroding. The original curcuit uses 2 or 3 bracket connections before reaching frame ground: this was OK when new, but doesn't age well. The improvement here is often about .15v to .25v in battery voltage. Also: replace the contacts in the rectifier-alternator connector (get the parts from
www.vintageconnections.com) for like-new results.
2. Regulators.
There are 2 types of non-OEM regulators out there: series-regulators (like the OEM circuit, but solid-state now) and shunt regulators.
The solid-state series regulators will regulate the field coil on these bikes' alternators in the same manner that they were designed for, but a little more efficiently overall. This will result in about a .15v to .25v battery voltage increase, and a more stable battery water level over time, than with the stock, aging regulator. I should mention, though, that the stock regulator can be easily tuned up for like-new performance, accomplishing the same thing for just some tinkering time.
The ("newer") solid-state
shunt regulators will immediately drive the battery voltage up to nearly 14 volts. While this seems attractive at first, it accomplishes this miracle by hot-rdding the alternator, much like installing 11:1 compression pistons increases engine output: there is a cost in terms of lifetime, because of heat. This isn't necessarily bad, if you only ride short periods of time, like commuting or weekends only. If you plan on a lot of long rides, this probably isn't your best choice, for reliability reasons.
Here's how this shunt trick works: A conversion-type shunt regulator turns the alternator ON at 100% by applying full power to the field coil all the time. The electronics in the regulator detect the system voltage through a zener diode of some sort and when it rises above maximum, shunts the alternator current back to the alternator by turning on a shunt thyristor (aka SCR) to "short out" the extra AC current, so it doesn't go into the battery. This makes some extra heat at both the thyristor (which has a finned heatsink case) and in the alternator. Usually, the thyristor can take the heat, but the (original) alternator's insulation will begin to suffer tiny cracks over time from this extra heat. Eventually, this insulation will start flaking off, shorting out one, then two, three, and more, windings in the alternator itself, until the output starts to drop. If the engine is only run for, say, 30 minutes at a time, it will take several years of daily riding before this loss becomes an issue. But, one week-long summer tour can toast much of the alternator, dropping its output by 25% after the trip. The evidence of this sort of damage is apparent: looking at the alternator will reveal darkened ("burnt") windings down inside the epoxy, obviously not caused by external heating, rather by internal heating. (In contrast, an alternator that was damaged by some sort of external impact, like dropping a wrench on it while it's open, will display burned insulation at the outer layers of the windings, or a burned field coil only.)
One of Honda's own shunt designs:
Some of you may remember the immensely popular Honda CX500 inline V-twin bike, rated the most-stolen single model in biking history, for its remarkable performance and nimble handling, and Phantom-jet exhaust note. This model had a shunt-type alternator-regulator design, and it cost Honda the whole bike in the end. Talk with nearly any tourer who owned one for more than 3 years: they will likely tell you that the alternator and regulator were replaced at least once, and Honda extended the warranty for several years to cover it (lawyers were involved): the engine had to come out to replace it and the melted wiring harness. By the time it became the Silver Wing tourer (and 600cc), Honda had created a series-interference type regulator to reduce the heating, but the reputation as a poor tourer was already on the street, and it failed in the marketplace. Too bad, it was a nice ride!
This is the "cost" of hotrodding the alternator without also spending a lot of money on the insulation in those wires to take the extra heat. For the most part, this design problem today is handled by improving the insulation or decreasing the wire size to increase internal resistance, while also increasing clearances, to let things cool off a bit more, if the shunt design is employed. Series-field regulator designs are more costly, but in the end, also more long-lived, and are usually found only on the upper-end bikes.
