I know everyone will be shouting stator at the screen but hear me out first please....it may of course still turn out to be that. Looking for additional checking guidance please.

Gen 2 Factory, 57 plate, 14TZZ battery, John Walker leads, always shows 14.2v at idle on both dash & multimeter. Habit means I always check the voltage when starting up waiting for the apparently inevitable. A couple of days ago I did a double take when bike was warming up, 13.6v and dropping to 12.4 over about 5 mins. Switch off, charge, repeat and this time it just hovers around 12.4. When I rev to about 3K, voltage increase to 13.1v.

What I've checked is:

- Brown & white connectors on reg rec- all good, no burning, damage etc

- Voltage across all 3 phases consistent - 27vac at idle, 58vac at 3K revs - checked several times.

- 0.4 ohm resistance, consistent across all 3 phases

- Phases to earth - all good

- RR Diodes - consistent open & closed in right directions

- Connectors under seat pillion good, 30A connections good

- rechecked all + & - connections  on engine - all good

- With 1 multimeter on battery & 2nd on white reg rec connector - same voltage readings on both meters with engine on (12.4) & off (12.6) so appears to be no losses in loom.

- To me it appears at idle battery is draining but higher revs creates some charge although low.

Stator values look OK, so must be reg rec! Swapped RR but NO, same voltages throughout as before. Nothing appears obviously stuffed. I'm no mechanic or sparky and have quickly run out of limited talent here, anybody have any other checks I could make?? Thanks.

Forgot to mention I'd tried another known good battery and all readings are the same as T's battery so had really dismissed the battery, although that may not be 100% conclusive. Agreed 13v is too low, it's always been 14.2 at tickover, it's the sudden voltage drop cause I'm lost with. Spoke to Griff, he felt the stator voltages were good and consistent and thought it heavily indicated the RR - of course he hasn't got the bike in front of him so his advice is appreciated but can only be limited over the phone. Changed the RR and no different. My small brain thought good AC voltage into RR meaning stator good, poor RR DC output, connections and wiring good therefore RR isn't controlling voltage properly and is failing. 

Maybe time to look at the stator itself.....unless anyone's had similar issue and has some other aspects to check...

I think your logic is good.

I'm not sure what the output connector at the RR is like, but can you get a multimeter in to take a voltage reading? If so, do one reading across the +ve and -ve at the RR and another between the +ve at the RR and the frame of the bike, just to see if there is a significant difference.

Is the stator one of those items best bought genuine? Will I need to sit down when pricing it 

Thanks for the input all.