Defender 300tdi not starting unless battery fully charged

bojackhorse said:

I need help diagnosing an issue where my 300tdi won't start unless my 100Ah battery is fully charged (12.8v).
Main points:
- Battery is under 2 years old
- Battery has 1000 CCA
- Battery SOC indicator is green
- Starter motor relay doesn't click like I would expect if the battery was dead
- I wait until the glow plug relay has switched off before turning ignition (I can see the battery voltage go back up indicating the glow plugs are no longer on, also the dash light goes out)
- If I remove the glow plug fuse (60A) the car will start even when my battery is at ~12.2v (but not good for cold weather). The same is true if I disconnect the whole glow plug relay.
- I have checked the negative cable from the transfer case to the chassis and the contact is good on both ends.
- Battery lead contacts are not corroded.
- Even with a battery jump starter (Noco Boost Plus 1000A) the car doesn't start unless the actual battery is full.
- If I manage to start it then drive around for 15 minutes it will start up again easily.

Any suggestions or tips for diagnosing would be very welcome, thanks!

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bojackhorse said:

Thanks for the reply.
Defender 300tdi as stated in the title of the thread.

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