Before starting you need to decide, what sort of wattage you want to burn, how long a burn time you want from your battery pack and what sort of batteries you want to go for.
Picture shows my battery pack made up from 10 X tagged NimH D cells they are in a 3-3-3 'toblerone arrangement'with the 10th length ways on top, this leaves a convenient space for switch wires/plugs etc.
For my money once you've used a 50W halogen you won't want anything less. For batteries, the options really are, lead acid, NiCAD or NiMH. Lead acids are by far the cheapest are readily available and can be bought 'ready to go'in appropriate voltage and amperages they are however bulky and heavy and will require quite a large canister to deliver a decent burn time. NiCADs and even more so NiMH are considerably more expensive and packs will need to be assembled from single 1.2 cells in multiples of 10 to give the correct voltage but they are much more compact and lightweight and mean a much smaller canister than the equivalent output lead acid one.
I decided to go with NimH batteries, they don't suffer from the 'memory effect' like nicads (you can top them up without fully discharging first) and you can make a powerfull battery pack that is an acceptable size/weight.
I bought 10 X Greenpeak 900DH cells from Budget Batteries, each one gives 9aH so in series I have a 12V 9aH battery pack. You will want 'industrial' or 'tagged' cells that will allow soldering together for packs. Alternatively you can use standard cells and have solder tabs spot welded but this is just extra hastle.
Pictured is my battery pack heat shrink wrapped with cable and Tamiya connector.
I use the Maha c-777 Plus II universal charger, it retails at about £80 form Nevada in Portsmouth this pumps out 800 mah and will charge from completely flat in about 11 hours.