Td5 discovery starting problem

Hi looking for some advice or if anyone has had the same problems.
A few weeks ago the battery died in my disco so first thing I did was attempted to jumpstart it to which it refused to even crank over even when tried with multiple vehicles. So took the battery off the truck and charged battery over night put it back on fired straight up works fine. After running it for 30 mins ish then turned it on and off a few times still had no hesitation to start perfect. So next day I go out to the truck and nothing all the light all the noises except any cranking so I done the same again took battery off and charged it up and so on… meanwhile i went and bought a brand new battery assuming maybe the battery was at fault and put the new battery on, nothing. The only was I can start the disco is by disconnecting the battery letting it drain all residule power from the ecu etc, and then starting with a freshly charged battery a hour or so later. Any help or suggestions appreciated thanks

Is it the imobiliser thats stopping you from starting? Lots of information on immobiliser problems in the forum or You tube.

Railman said:

Is it the imobiliser thats stopping you from starting? Lots of information on immobiliser problems in the forum or You tube.

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I thought the same myself that it might be immobiliser at fault but then once it has a fresh battery it starts and runs fine multiple times it’s only after left for 5 mins or so with key out and door shut that it then won’t start again