Battery question

Battery indicator on my son's 2002 TD5 is quite dark which the hand book indicates charging needed. ( Green colour means all good )
So attached my CTEK MXS 3.6 battery conditioner and the maintenance mode light is on with the charge light flashing? Normally on other cars the charge light just comes on without flashing, so I disconnected.
Anybody know what might be going on here? Guessing the battery is nearing the end of it's life but started first time this morning with a hard frost.

Perhaps the Ctec's instructions, web site or the manufactures them selves will tell why the maintenance mode light is on with the charge light flashing.

Mine has never done it and I've used it on some iffy batteries in the past.