I need a battery charger so that we can charge one battery whilst other one is on van, any ideas which one to buy and what do I look for when purchasing a charger?
Ring Smartcharge and Ctek (they are very similar) 'intelligent' models are fantastic. You can leave your battery connected permanently, and the charger will maintain it in peak condition. The more expensive models can even restore old batteries, and you can select the charging rate.
Have a look on ebay, eg. RING CTEK