Outdoor siren power and battery combo

Hi, I have just received the siren and in advance bought the optional doorbell battery pack and 6M power adapter. Does the power adapter keep the battery pack charged? Do I remove the D type batteries that came with it or leave them as an additional source. Do I need to take the optional battery pack off to charge it in six months to maintain a backup supply? The supplied instructions and online info isn't clear about the configuration when using the rechargeable batteries and a power adapter combo. Thanks I advance to anyone that knows. (Can't install it yet as waiting for the alarm gen 2 system to release at the end of April, I've pre ordered a full system worth for the house.)