i had that problem, but i had it with the 24v ( 2.5), for me it was the alternator, i had to replace it, i know its not a "in-depth" answer, but i hope it helps!
Have you checked the alternator?
Something is on drawing power. Can be any light on the vehicle or a relay stuck.
It may need replacing or it can be the alternator is overcharging the battery. Have the charging system checked out. The alternator should output from 13.5 to 15.5 volts with the engine running at idle.
Check the battery with a vom meter while the engine is running. It should read about 14.5 volts and if so the alternator is working, than have the battery checked Now if the meter does not read 14.5 volts, have the battery, and the alternator checked. If they both check out okay, the problem is in the battery or the alternator circuit on the vehicle.
Dead cell in battery, some light is on and is drawing power when the car is not running, or a relay is stuck. This is assuming the alternator is good.
The battery does not have sufficient voltage to start the car when it's cold. Have the battery checked out. If it still goes click click after running the car for a while, have the alternator checked also. A bad battery will not hold a charge and a bad alternator will not recharge the battery, (That is the reason to have the battery checked first). Make sure the connections at the battery are clean and tight.
Sounds like the alternator isn't working at all. Have it checked. Good Luck!!
If it is running on the battery only then that means the alternator is not charging the battery.
Get your charging system checked out as it sounds like your alternator is going bad or the battery is shot.
I would suspect a bad alternator and battery, have them both checked at the parts store. A car with a fully charged battery and a bad alternator will run until the battery is drained and then quit.
The battery light indicates the system is not charging. Have the charging system checked. If the alternator is not charging, the engine will quit running and leave you stranded.
Why are you disconnecting the battery with the engine running? Never mind, you've probably already figured this out by now, but there's a good chance your alternator is fried and your spark plugs are running on battery power. Get your alternator checked out. New ones aren't that expensive (approx. $50) and can be changed by anyone handy with tools.