My battery voltage with the engine off reads 12.05v to 12.15v all the time, and always has done from Day 1. No change between ignition on or ignition off, same readings from three different voltage meters including my iCarsoft LR V2.0 code reader. Jumps to 14.5v or thereabouts when the engine is started. Only ever shows 12.6v immediately after a session on the trickle charger but then drops back to 12.05v to 12.15v within a few minutes of disconnecting the charger.
I have read in many places that a healthy battery is meant to show around 12.6v and if it is showing my readings it is "sick". So question #1 is, is my battery sick or not?
Thing is I have never had a single battery or electronics problem, not even a message. I disconnected the secondary "stop/start" battery on Day 1 which has never caused a problem.
So I tried a couple of experiments just now:
1. Hooked up the LR V2.0 and tried to run the BMS reset program, but no go, no matter what I tried "operation failed" (I did not reconnect the secondary battery).
2. Disconnected the main battery by undoing the earth lead, leaving it off for around a minute, then bolting it back down again - no change to the voltage readings and the BMS reset was again "operation failed".
I am hoping/guessing that the battery disconnection/reconnection will prompt the BMS into life once I drive the car a bit but not until then.
My thinking behind all this is that the BMS has never really worked as it is supposed to "trigger" once the voltage goes below 12.2v and allow the alternator to pump it back up to 12.6v, and it seems to me it has never done this. So question #2 is, is my BMS cactus?

 

If the alternator is putting 14V+ to the battery when the car is running, your BMS is functioning as designed. Sounds like your battery is indeed getting weak. I'd just check the standing voltage on a regular basis and replace the battery when it falls below 12V. BTW, are you checking the battery voltage with the battery disconnected? If not, that would depress the readings until the car goes to sleep.

 

All checks with the battery in the car and fully connected.

 

12.1 does sound low. I'd check voltage right after a drive with battery disconnected, and again with car running. Both should read higher than 12.1 I assume you verified tester is accurate?

If the battery is going out, the bright side is you can upgrade to a lithium battery, pull both old batteries and and remove about 60 lbs from the car...

 

I can pull my voltage down to 10.1 (Measured at the BMS and OBD Port)!

My battery is clapped, (2015 May build), but still charges when 14.8V is supplied from the alternator, or on the CTEK.

A sure sign is, if you use some of the electronics (The hood on a vert is a good tell-tale of battery state as the motors designed to do the final latch sap an awful lot of current - usually results in "Roof not latched" warning) with the engine OFF for a while, monitor the battery volts.

Have you tried leaving the -ve connected and disconnect the +ve to reset the BMS???

 

Update.
Seems I was fretting about nothing and my battery is perfectly OK and the BMS is working properly.
Just went for a short run down the auto parts store and then the shops, 3 km (2 miles) to the first stop then the same back to the shops.
Took my nifty little plug-in volt reader (plugs into the 12v socket in the console) and left it plugged in the whole time.
As usual it read 12.2v when first plugged in then went to 14.5v just after start up.
Spent 10 minutes in the auto parts store, came back out to the car and it's reading 12.6v!
Spent 5 minutes at the shops and same again. Pulled into my garage, switched off and watched the reader for 5 minutes - it slowly and gradually dropped to 12.6v then stayed there. Double locked the car then went back again 10 minutes later, now reading 12.4v - 12.5v so slowly dropping.
I think I have figured out why it reads 12.1v - 12.2v when the car has been sitting double-locked for a while, at least an hour or so - it's the BMS putting it in "quiescent" mode, and it doesn't wake up out of that mode until you start the engine.