Battery backup and charge circuit for 6v and/or 12v DC

[SpookyOwl]

Hi,

I am trying to find a circuit diagram for a battery backup and charging circuit that will need to provide either 6 or 12 volts DC (this is dependant on what I decide to attach to it).
Ideally I want to provide a DC voltage to the circuit and for it to then supply the same or slightly lower voltage to a load, whilst maintaining a charge in a backup battery without over charging the batteries. If there is power loss I want the batteries to kick in and provide power to the load until power is restored.
I have found a few circuits but most seem to be based on an AC supply or don't give any details with regards to how the battery charge is maintained.
I have also been offered suggestions of using diodes to setup what was described as a steering circuit, this made sense but I was a little concerned at the lack of control with regards the charging of the batteries, either there being a risk of them being cooked or not charged enough and failing to provide enough power when needed.
I plan on using several NiMh AA batteries connected in series as the backup battery as I have plenty of these, this is due to their dimensions being ideal to fit in the project I am working on.

The most interesting and explanatory article I have found so far is this one:
http://www.supertex.com/pdf/app_notes/AN-D9.pdf
Again this is AC based, I *could* switch to using an AC feed if need be, the only other problem with this circuit is that I can't seem to find anywhere to purchase the TN2504N8's in the UK (I am based in the UK). I think they may not be ROHS compliant and have been discontinued.

The reason for needing this is because I am building an automated train that is to travel around a model railway track taking several hundred photographs, that are then to be strung together to create a very high res video. The unit is likely to have to spend anything from 5 - 10 hours travelling a full circuit and is likely to encounter power dead spots on the track along the way. It may be possible to get away with just having a precharged power pack that can switch in but I am not entirely sure, again though I am a bit stuck for circuits to acheive this as my electronics knowledge is patchy!

The train unit will consists of a small microcontroller based I/O board and motor driver (a BV4113), a small to medium sized stepper motor and a bluetooth module. There will also be a digital camera to take the shots, but the camera's internal battery works fine as a backup power source when external power is lost so does not need to be driven by the circuit.

If anyone could offer any suggestions regarding this I would greatly appreciate it as I have limited time to get this thing built and working. If I can get it all working I will of course post details of the project for others to benefit from.

Thanks for your time,
Paul.