Need advise on car battery

[Gaolow]

recently, I found that my car cameras (both front and rear) sometime turn off by itself when the car is idle like, traffic junction, stop or when doing reverse parking. So first suspect is that my camera UPS cut off and might due to low voltage. So i monitor a few days..and sure enough, my battery voltage when car idle or sometime reverse parking can drop to 11.8V.
So I did small troubleshoot to see whether the Alternator did charge the battery, when I rev to 2K plus, the battery voltage went up to 13.8V. When I stop the rev, it slowly go down to 13V and sometime 12.8V.
During next morning start up, no problem and I able to obtain the 14V before everything turn on. Once I turn on the air con, fans, ICE, headlight and fog light, the voltage drop to 13.5V. At this time, the car is still stationary and rev is still less than 1K.
When I drove out of the lot and keeping rev less than 1.5K, i observe that the voltage still retain and no dropping. So when I rev to 2K, it increase to 13.8V.
The drop to 11.8V dose not always occur, but it dose only when car is stationary or idle or when rev is not high (<1K) with Engine still on, but not always..the most is down to 12.2-12.5V.
So is this Battery problem? What cause a sudden draw of current? leakage? or something turn on ..like air con?
Any advise will be much appreciated. Thanks!

btw..this is my second batteries replaced. The first battery that came with car die after two weeks of not driving. about 9 months old.
The second battery, the cranking not good or smooth, so send in BM to check and they said battery not good, so they replace another one for me. So the current battery in my car is about 3 months old but I just found out that on side of the battery, it written 20/02/2013 on it. But this current battery that they replaced is on Sep. 2013. So i suspect that the battery that they replace is not new..might be recon. or old stock.

[Kb27]

Get your battery checked first.

If ok, most likely, it's the alternator fault, not able to maintain voltage properly due to varying loads.