I did a lot of research on this when I built a battery supply for my Naim CDX2. I'll assume you have basic electronic skills and a multimeter. You'll need a system of switches or connectors so you can switch the batteries from the preamp to the charger.
To determine what size battery supply you need you first need the voltage output of the supply (obvious) and also the current draw to determine the capacity of the batteries you need/want. Higher capacity batteries will last longer will cost more.
Once you figure the current draw multiply that by how many hours you want it to last between charges to get the capacity in amp/hours.
Next determine what type of batteries. Go here
http://www.buchmann.ca/toc.asp to get a good overview but you are basically looking at a few things:
1. cost, this includes the initial cost, how many charge cycles they can take before needing replacement, and the cost of a charger.
2. Ease of maintenance, what efforts are required to charge and maintain a proper charge for maximum life
3. Discharge charecteristics, do they maintain their voltage until depleted or does it begin to drop as soon as they are loaded down?
You are looking at NiCad, NiMH, and sealed lead acid (SLA). Disposables are cheaper up front but too expensive in the long run.
I settled on NiCad. Cheaper in the long run because they have the most discharge/charge cycles if properly maintained. The most finicky about this though and short life if not properly maintained with a quality (read $150) automated charger/discharger, but you can use it to maintain your other NiCads too. The memory thing is pretty much a thing of the past and not a concern at all if properly maintained. They also maintain a steady voltage until almost completely discharged and like a deep discharge if you don't run them until dead, which can kill them. That is not likely IF you remeber to turn it off when not listening since it will begin to distort when they get too low.
SLA are cheap to buy and easy to charge so you see them used a lot, but the voltage drops as soon as you start to use them and they don't last as long, especially if you deep discharge them repeatedly. They are best for short bursts of high current like starting your car and then being topped off again.
NiMh are good and second to NiCad for your needs but about half as many charge cycles and generally a bit more expenseive, and you still need the good charger.
Go here to compare prices.
http://www.robotmarketplace.com/Good luck. It made a significant improvement to my CD player which was no slouch to begin with.
