In reply to wilkie14c:
> ... car ...
This makes a lot more sense! Go for it. Make sure to get a high quality gel or AGM lead acid leisure battery and not a "wet" one. It'll last longer, suffer less degradation when discharging more than a paltry fraction, release less explosive hydrogen gas under normal and fault conditions, and pose less of a spill risk for sulfuric acid. These are more of a concern when it's the cabin compared to the engine bay. A battery box is meant to isolate these hazards from you and from ignition (fire) sources, so ideally you wouldn't put the electronics (voltage converters etc) in the battery box itself.
I'd not write off charging in the car itself during journeys. Less invasive (to your car's wiring) than a split charge unit is to use your battery charger fed from a cheap 120W inverter running from a cigar lighter socket. The efficiency loss pales into insignificance compared to the engine and driving, and it sidesteps a lot of problems with modern alternators and split charging, as well as giving you an inverter to supply low power mains from either battery (if you fit a cigar lighter socket to your leisure battery system.). Compared to split charging with a relay it's trivial to control the charge current to a safe level for your battery by getting the right capacity charger.
If you end up using crimp connectors, buy a proper ratchet crimping tool for £20-£30 and save yourself the misery of the cheap pressed steel crimpers.
Look at the depth of discharge vs cycles data for your battery and make sure you don't abuse if to death. At a minimum you'll need a voltmeter to watch it or to fit a suitable "battery guard". Some cheap panel mount led voltmeters on Amazon.
As mentioned earlier 12voltplanet are great. Everything you could need apart from batteries and battery boxes. Marine suppliers often stock the later.
If you're looking to burn a hole in your pocket then there are some lithium ion 12v batteries with built in safety/charge control/management systems. Lighter and smaller per unit energy, and you can discharge them much more without damage so even smaller still in practice.
Post edited at 20:49