2004 Renault Megane black smoke from exhaust, revving it's self and flat battery

Lots of possible causes i.e EGR valve etc. etc. Probably best to get it checked out in garage unless you want to try and do some work yourself . This site gives you some idea of causes ,



http://www.renaultforums.co.uk/archive/index.php/t-20389.html

Sounds very much like the dreaded Ren-Oh "Blown Turbo" syndrome. Where sludge build up due to extended 18k oil change intervals and too many short-range journeys eventually leads to collapsed turbo bearings, engine then feeds on its own oil (quite feasible for a diesel), producing clouds of dense smoke, till either: A) It runs out of oil and stops/siezes of its own accord; B) it goes BANG big time, totally wrecking the engine, or C) the driver manages to stop it by stalling the engine in gear shortly before (A) or (B) happens. Much more detail here: http://www.parkers.co.uk/advice/forum/forum.asp?FORUM_ID=29

Regretably, its quite a well known problem for 2002-2004 dCi engined cars. Later versions had modified oil feeds plus either reduced oil chnage intervals, and/or stronger warnings for user to reduce intervals where usage involved a lot of short journeys / school runs, etc. Scenic seem particularly prone due to likely use for this kind of driving. Cars that thrash up and down motorways all day seem less prone?

Sad to say, but the flat battery may be the least of your worries? But I'd be happy to be proved wrong?