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The only place for a coil spring is up Zebedee's arse
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I've got the RF filter box so generally don't have any battery issues with the P38. It can sit for weeks or even months and starts no bother... Except Sunday!

Wife used it on Wednesday/Thursday, it's sat for a couple days, went to start it on Sunday and it managed about half an engine revolution, then nothing.

Grabbed a spare battery and jumped it and away it went. Drove 5mins to the park and it restarted fine after the park, and it's been fine since...

I want to give the battery a full charge, but are there any other common issues that can cause drain like this? Couldn't see anything obvious like lights left on.

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To go flat that quickly either the battery has reached it's use by date or something has kept the BeCM awake. This is a list of all the things that will wake it or keep it awake from the BeCM SID.

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or your alternator is done and the battery is just flat .

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Or a rectifier in the alternator has gone short circuit so it is draining through that. It's a 3 phase alternator so if the rectifier on one phase goes down it will still charge, but at a lower level, when the engine is running.

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It's been totally fine since and i haven't even charged it yet, so it's nothing terminal like the alternator or even the battery itself.

So I assume it must be something kept the becm awake...

The ignition barrel has been sticking a bit and it will often say "ignition key in" when you open the door. I usually poke the end of the barrel and it clicks out, I guess maybe the wife hasn't done that and it's thought the key has been left in for a couple days?

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If it thinks the key is in, that will keep the BeCM awake so that would seem to be the most obvious reason. On a later car it will stop the central locking from locking the car but will still allow it to be locked on an earlier one so you don't get an obvious clue. Blasting brake cleaner into the lock barrel will usually clean out the dust and dirt that causes the microswitch to stick.

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It often doesn't get locked anyway so we might not notice even if it refuser, but I've said to the wife about the beep/message and showed her what to do.

I'll have a go with the brake cleaner when I get a chance and see if that helps the sticking too.