FAQ for drained battery problems.
For diagnosing these problems accurately you are going to need a multimeter. I paid about £10 for one on ebay.
1. DONT TOUCH ANYTHING!
Before anything else carefully check controls in the car to see if you have left a light or other equipment turned on. This could save you a lot of time diagnosing problems later if you don't just start fiddling with stuff.
2. Check charge in the battery.
Set multimeter to 20volts. Plug your black probe in COM and red into Volts.
Place black probe on negative terminal and red on positive.
Read multimeter - A battery should be knocking out about 12v. Even 1v less could mean a flat battery and the 6 engine needs every last drop to start it seems.
If the reading is less than 6v, skip it - The battery is dead.
If 7v or more then you can re-charge the battery using a charger (£38 from halfrauds).
3. Check for parasitic drain.
Remove the negative terminal from the battery.
Set your multimeter to 10 Amps.
Plug the black probe into COM and red into Amps.
Place the black probe onto the terminal on the lead to the car, and the red onto the negative post on the battery.
The MM should read a mili-amp figure.
Around 20-25ma or less should be fine - this will be clock, hifi battery, ecu etc. (Mine was around 16ma).
25ma and greater and you may have a drain somewhere.
4. Identify drain.
Using step 2 to check drain after each step, remove fuses from the fusebox one at a time - noting what they affect - until the MM reads 25ma or less. When it does - check the wiring carefully of the components that fuse supplies.
See Fusebox FAQ's for full list of fuses and their function.
5. Check earth points.
Check all these earth points round the car are securely fastened and clean.
one by the abs pump
one by the drivers side headlight
one on the gearbox
one in the drivers footwell
one under rear bench seat (i think)
one behind the cover in the boot (the side with the boot light)
6. Check battery terminals.
Make sure the posts and the terminals on the leads are clean so they make a good connection.
DO BELOW ONCE A CHARGED BATTERY IS IN PLACE.
7. Check lights.
When battery is charged, check all interior lights go out. Fold the rear seat down to check the one in the boot while it is shut.
8. Check alternator
Start the engine and turn on lights and fan and hold at 2k revs (easier if someone else does this than trying to use throttle and MM at the same time!).
Set MM as before when checking battery charge.
Test the voltage across the battery terminals with the engine at 2k and you should see a figure of around 14volts.
If it is still only 12v then your alternator is not charging the battery. Check the alternator belt is tight. You may require a new alternator. It may also be a faulty regulator.
(I found I didn't even need to rev the engine, it read 14+ volts when running regardless of engine speed. Your results may vary).
If you have any inbuilt charging e.g. phone charger cradles set fuse for lighter to ignition only, see fuse faq.