Siemens MP3 Door Chime

[severn]

Hi, I have just installed Siemens MP3 door chime kit (DCW20). It is wired to a bell push.The chime comes with 8 pre-loaded melodies, and there is the option to add your own.The chime can be wired for battery operation or transformer operation. I have wired it for transformer. However, for both operations, the instructions specify that batteries must be installed, which I have done.
The chime works perfectly, but I don't understand the relationship between the transformer and the batteries. What job is each performing? How will I know that the batteries need replacing? I wanted transformer operation so that I wouldn't have the problem of flat batteries. I have emailed Siemens support but they did not answer my question.
Thanks in advance.

 

[OwainDIYer]

the batteries may be required to keep the user-loaded melody in the memory; following a power failure the thing might try to play a non-existent melody and you would miss your latest delivery of much-loved pre-owned collectibles from ebay.

 

[ericmark]

Reading the manual it would seem the mains operated door push option is purely to allow the unit to replace an existing unit without the need for rewiring.

The transformer supply is only sent to unit when the door push is pressed and it seems likely it's a simple relay one does wonder why a 1 amp supply is required for a relay?

But it's only the door push which is mains operated the bell is worked by 2 batteries. Clearly unless already wired for mains operation then rather pointless using mains.