Battery Charger.

IF the battery is a healthy one but dicharged a little, the the charger will most likely output 14,5 v until it reaches the threshold voltage of the circuitry inside the charger and the voltage then reduces eventually, to a float charge of around 13.7v.

If the battery is sulphated and won't accept charging, the charger will try to hold it up near 14.5v because it can't detect a peak of increase. Maybe the chargers threshold is about 14.7v and the charger hasn't can't reach that voltage.

Need a bit more info to be able to decide anything meaningful.
What is the battery type, Deep cycle or start type battery?
How old is the battery you have which is connected to the charger?
Is the battery flat? You didn't mention if the battery was charged or discharged.
If flat, how long has it been flat/discharged for?

You may be able to connect the charger to only your car battery and if it is in good condition the charger should then charge it up fully and go into it's float mode when no more charge is required.

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From what I can determine, this model indicates as follows.....
Red light on = "Power On"
Green light on = "Charging in Boost and Absorption" modes
Yellow light on = "Charging in Float" mode

Note that the green light is "ChargING", not "ChargED", so your charger may be working correctly.

When you say that you "tested the supply at 14.55 volts", where did you measure that? Presumably it was at the charger output clips. If so, were they connected to the battery at the time? Or were they disconnected with the charger switched on?

Try leaving the charger switched on and connected to the battery. The light should changed to Green within an hour or two indicating that it has moved to the Float mode. If the battery is old and in poor shape, the charger may not move to the Float mode.

Edit: changed to... "connected to the 'battery' at the time".