Ian,
You can buy ready made RSUs from Swanage Models or London Road Models. They disappeared from the market a few years ago when the blasted CE rules came in, but have reappeared, presumably after the makers spent a fortune getting certified. I have one and it is fantastically useful for O gauge work, and I’m sure it would be for P4. I use it with Carr’s solder cream.
A battery charger does not have enough current capacity for this job (well I supppose an industrial one for truck batteries might have, I don’t know) and given the ones above only cost £120-150 it’s doubtful whether it’s worth the effort of making your own.
For ordinary soldering I use a temperature controlled soldering unit. I have 2, a 50W one and a 100W one, the latter I acquired as 50W is a bit feeble for larger O gauge items but should be fine for P4. I use Carr’s 145 degree solder almost exclusively with these.
I disagree with Chris on one thing: avoid Powerflux and Baker’s fluid, they are too corrosive unless you are extremely careful about removing them afterwards. I use Carr’s Yellow flux for brass, but there are some totally acid-free fluxes around which are suppopsed to be very effective.
Edited By David Littlewood on 30/12/2011 18:57:21