Hi
Thanx for your replies. can I ask that you will post/send the circuit diagrams of them? MikeMl - I think that the maintainer will do.
The key to this project is starting with the correct Wall-Wart Plug-in Power Supply. Do you have one that might be suitable?
Ideally, you should find an older-style, real iron-copper (heavy) transformer-based (as opposed to a lightweight switching-mode converter). It should put out about 15Vdc (unregulated, but has rectifiers built-in) at about 1A (14VA or 14Watts). The input side should match your Wall Plug AC Voltage.
One that puts out ~14Vac (not internally rectified) will work too, but we will have to add a full-wave bridge rectifier to the circuit.
You didn't put in a country when you registered for the web site, so I do know how to help you obtain the Wall-Wart. Hereabouts, there are literally hundreds of these available at the local thrift shops and they are cheap (one fifth of the price of a cup of coffee at Starbucks).
What you are looking for is like
the Hammond BPD2EE. This one is available new, (look at the data sheet) but I would be looking for one from a surplus store or a thrift shop.
I have several versions of the circuit to go between the transformer and the battery, but the final details are dependent on the transformer you start with. Give me an idea of what you can find as the transformer, and I will customize the circuit when you tell me what you can find...