At last I've been able to finish a 'mini-project' that's been outstanding for months on the Traub Lathe.
There are three 3.6 v NiCd rechargeable batteries within. One backs up the RAM in the controller and is absolutely vital, as the RAM holds not only the parameters, but also various secret squirrel option flags set by Traub and not disclosed to customers. The other two are less critical, they back up the machines 'absolute position' so if they go flat the machine has to be re-referenced.
Although these batteries are brand new and good quality they will only last 5 or 6 days, so I've been running the machine for an hour each morning to keep them charged - a pain

So - what to do. Well the scheme I settled on was to arrange a trickle charge to each battery when the machine is powered off. Three phase power comes in direct to a large isolator switch however the door interlocks are powered by a 415 : 24 volt transformer directly across two phases. This got me thinking. Between Phase 1 and neutral there is 240v even when isolated pick this up, drive a battery maintainer, and arrange that it is switched off when the machine is turned on.
So how to implement? I bring the 240 single phase through a 6 amp MCB to a relay box. Relay contact is normally closed passing mains to the battery maintainer. When the machine is powered up, I pick off 24 v from it's internal supply to drive the relay, which isolates the maintainer.
So first we need to make up the relay box: