Side effects of fault voltage regulator on Aprilia Leonardo 125 ST

About 6 months ago the voltage regulator failed on my Aprilia Leonardo 125 ST (2001) but I’m now confident that it was playing up before that time.

I’ve had the bike for 3 years with no problems and then the battery failed earlier this year. I assumed it was just old and so bought a new battery and it was working well for a few weeks until one day I came home and smelt boiling acid. This brought back memories as I’ve worked on very large battery banks (up to 10,000 AH at 50VDC for TELCOs) and so recognised that acrid smell of heavily working batteries.

I carefully popped the seat open and removed the battery cover. Acid steam was venting. I got water, goggles and gloves and unbolted the battery and removed it. Beh ! brand new battery cooked. The next day I re-charged the battery, put it back in the bike and it seemed OK but I doubt it would last.  Everything seemed fine though with an expected battery voltage.

I’ll jump to the end and what was happening was that the voltage regulator was failing but only intermittently. It eventually failed long enough for me to see a reading of 17 volts on the battery terminals !

The side effect of this though was the following…

  • the headlights would run normally and then would run brighter. I drive with the lights on all the time and so the higher voltage meant that they drew more current and this both contributed to them blowing faster and drawing more current through the light switch. I’d noticed this but never really thought much of it. I’ve blogged about the switch failure here.
  • when the battery failed then I had trouble starting the bike and so this meant more cranking. This probably contributed to increased wear on the starter brushes. I blogged this here.
  • the increased voltage meant an increase in current overall for all systems and so this probably helped the ignition switch to also slowly unsolder itself. I’ve blogged this here.

So: if you notice your lights altering in intensity a bit, or the radiator fan changing sound/speed, or certainly if  the battery is getting hot then you must check the voltage on the battery terminals is precisely as expected (which is usually under 14 Volts).

With intermittent problems there is not really much you can do because there are no engine management system over-volt indicators on such an old scooter so you just have to be alert for these subtle indications in the lights or fan sound before you end up with a boiled battery and a whole pile of other electrical problems.

The voltage regulator is the same across a wide range of Aprilia bikes so the part can be bought fairly cheaply second hand or you can invest in a new one. Once I put a new regulator in then I replaced the battery as the cooked one couldn’t be relied on.