When full beam is on at 5k revs, I get a reading of about 14.3V from the battery.

When full beam is on at idle it drops instantly to about 12.5V and then continues to drop sharply, it goes below 12V and then I stop the test. The bike has died on two occasions and I believe this is the cause.

I have ruled out a faulty reg/rec. Is the generator most likely at fault or is there somewhere else I need to look?

I did have an issue with my reg/rec plug but looks like I have sorted that.

14.3 volts at battery at 5000 rpm does not sound like a dud alternator/charging system, more like a dud battery. Charging system at idle will always show no more than 12.5-12.8 volts with a fully chatged battery. If voltage falls below 12.5-12.8 volts it means the battery cannot store enough energy, at higher rpm, to maintain system voltage at a nominal 12 volts when electrical load exceeds generating capacity at idle or low rpm.

magg wrote:14.3 volts at battery at 5000 rpm does not sound like a dud alternator/charging system, more like a dud battery. Charging system at idle will always show no more than 12.5-12.8 volts with a fully chatged battery. If voltage falls below 12.5-12.8 volts it means the battery cannot store enough energy, at higher rpm, to maintain system voltage at a nominal 12 volts when electrical load exceeds generating capacity at idle or low rpm.

Thanks - so what should it read at 5k with lights on full beam? It is ~14.8V @5k with no lights on.

Also, just noticed my rear lights are not as bright as they should be. Fine when braking, otherwise they are too dim.

If you can get 14.2 volts at 5000 rpm no lights on you should get similar voltage with highbeam on. The charging system is designed to support all normal electrical loads and charge the battery at 5000 rpm.

Rear lights always been dim, wrong wattage. Recent development, may be poor ground return or one filament failed, dual lamp circuit can give interesting lighting issues.

sounds like you have a dud battery / posisbly some bad earths.

if you fully charge the battery up and then leave it a few days how many volts does it read?

as above - charge the battery then read the voltage after a few days. 12.3V and up (max around 13V) is good, less than 12V says duff battery.
can get a decent battery for around £30

Battery does not appear to hold a charge if voltage at idle falls as quickly as described, to the point that the engine stalls. A battery with such low capacity should have trouble starting the engine, unless it is kept on a charger. How does it go at starting the engine?

If still want to do further testing on the battery, charge battery and turn on headlights. A good battery should hold more than 12 volts for 30 minutes or more. From your description, the battery will drop below 12 volts very quickly.

magg wrote:If you can get 14.2 volts at 5000 rpm no lights on you should get similar voltage with highbeam on. The charging system is designed to support all normal electrical loads and charge the battery at 5000 rpm.

Rear lights always been dim, wrong wattage. Recent development, may be poor ground return or one filament failed, dual lamp circuit can give interesting lighting issues.

So the voltage should not drop when I put full beam on? Or is a little fluctuation (0.3V) expected?

There was poor grounding on one of the lights, put a new spade on but that has not resolved it. I'll look elsewhere and at wattage.

@Neosophist, I will try this and see what happens. Though did do this recently and it seemed to lose some voltage, however I wasn't sure what was un/expected. I will record it properly this time.

@Magg, The engine starts perfectly. The two times it didn't start, I just got a clicking/buzzing sound. However the battery was 12.17V when I checked it out so I was a bit confused. It should start at 12.17V shouldn't it?

Does it start perfectly? Two times it did not, when recently? Clicking/buzzing sound could be a starter solenoid issue or a flat battery. Battery voltage was 12.17 volts after it did not start, what was it during the "did not start"?

How did the battery load test go, greater than 12 volts for 20-30 minutes with high-beam on?

I have not tried anything yet, was planning on doing it this weekend, but the bike wouldn't start this morning so had to get the tube to work

It turned over, attempted to start but wouldn't. With my limited knowledge this does not sound battery related..gonna try some stuff tonight and then hopefully be back where I was when I started this thread.