2007 XK 4.2L 110k miles

I'm a new owner and quickly realized the voltage/battery gremlins associated with the car.
Background:
- Installed CTEK - fully charge every 2-3 weeks
- Drive car only on weekends - 5-20 mile trips
- Battery 1 year old
- Have a voltage meter installed in cig lighter (actually an FM transmitter which reads voltage.
- I lock the car after each use

Here's my question.
1. After a full charge, the voltage typically reads 12.6v when ON (before starting / running) and always reads 14.1-14.3 while running.
2. The next day the voltage might read 12.3 when ON
3. The next weekend the voltage typically reads 11.8-11.6 when ON
4. The following week or 2 the voltage might read 11.0v at which point I'll charge with CTEK. I will also get some error codes for low voltage.

Is this unusual or typical for the car's battery/voltage quirks?

 

The 14.x is normal but the 12.3 let alone the 11.x makes me think the battery is on its way out or there's a current drain (perhaps both).

 

Mine is playing up funny. been on ctex for weeks months was reading 13.1v the other week its now fell down to 12.9 battery is 2 and half years old. might remove the trunk lights . that was an old drain problem see if it goes up

 

That's typical for our cars. I'd recommend keeping on the Ctek when not in use, as that's it's purpose. Don't wait for the battery to weaken. So each time you use it, it will read 12.6 volts.