So new owner 30 year old defender 90. Go to out today on the motorway and pull over at the shops. Jump back in a turning over really slowly and won't start. I'm outside halfords and they use a jumper pack and start it for free. Get home take the battery out 10 mins later fully charged, it would appear not a flat battery as when I put it back in cranks over fine. The alternator looks brand new and no visible signs of a problem. Although I have noticed that sometimes it cracks over slower than other times but most of the time it starts fine even if I haven't used it for a week or so? Ideas