What a nuisance. Take a look at this website (apologies if you have already visited it) Ducati ST FAQ
A few owners have reported electrical problems with their ST (Electrical problems on a Ducati? Oh no….!). Many of the problems are related to loose connections, or an occasional faulty regulator. Another theory is that the older regulators are not able to handle the heat of the newer items, and being buried inside the fairing, there’s no airflow - the result being cooked (or partially cooked = flaky) regulators.
Older ST models have a battery light which will shine if there are problems. If your bike has problems with a constant flat battery, but the battery seems ok, or you have other gremlins… check the regulator.
The following tip comes from AJ: “If you have meter, try measuring the voltage when you ride, it should read something around 12.5-13 on idle and <1500rpm, then above 2000rpm it should settle around 14.1-14.4. If the voltage looks flaky then you should look at the regulator.”
The following hint comes from Chris Kirk:
“Did a bit of hitchhiking after the ST2 wouldn’t re-fire when I stopped to clean my visor after motoring through a brief but fierce hailstorm. Thought the regulator was cooked since it was showing no output so Steve @ SLMS is sending me a replacement. When doing some R&M while the fairings were off I did a thorough check of the regulator wiring & found corrosion had fried the internals of one of the two plug in regulator connectors to the alternator and a wire that looked connected in fact was not. Last night I hard wired the three yellow wires between the alternator/stator with soldered connections and protected the connections heat shrieked insulator wraps. The output is now marginally normal with13.88v @ 3K
So next time you guys have the fairings off, I’d suggest it might be worthwhile checking & cleaning the alternator/stator to regulator connections. A little dielectric grease on hand wouldn’t hurt.”
The regulator problem seems to be most prevalent on the 98 model ST2. Some owners have had the regulator replaced a number of times. An alternate to the standard regulator is the item by “Electrex”. Some work will be required to fit the non-standard item, as the mounting points are different.
Ruudje provides another alternative:
“Yes your regulator will be gone soon. But.... it can be operative for a year or more...
Replace it for peace of mind and stock it for future use in emergency situation. That is what I did. I take it on longer trips.
The Electrex RR51 may die on your ST also. Even sooner as the original one did.
The Ducati replacement is better but expensive. Use a SH232-12 or SH538-12 from a totalled Honda, models CB125/250 of VF500. Other Honda models may have used the same regulator but from these models I know for sure. A used regulator will cost you not a lot more than $50 as the Duc replacement
goes for about 200.”