Car off battery drops to zero if connected - 2001 Opel Astra

Get a multimeter and set to amps. Disconnect the ground cable at the battery. Connect your multimeter between the neg battery post and the negative cable. What is the amp draw? It should be close to zero.

Then one at time you pull each fuse and check the display on the multimeter. If it drops then that circuit is drawling power.

On my SUV it was the factory amp for the stereo system.

Often that is a sign of a bad connection between the battery and the starter, or a ground. You might try using a jumper to the starter post to verify the problem.

Compare cranking speeds cold to warm most diesels require about 375 RPM's cranking to start. NOW possibly a bad starter (dragging) voltage drop (high resistance) in cables. loose or dirty battery cable ends. To test voltage drop requires 2 people one cranking engine other watching volt meter. Place one meter lead on battery cable end and other lead on the other end of cable. Do this to all 4 battery cables. What voltage that can not go through cable will go through meter. So the closest to zero is the best. good readings are .02 volts for connections and .05 volts per foot of cable.

You still have a poor connection between the battery, starter, and ground. A bad cable, or cable terminal, or a loose connection at ground (of your battery cable), or loose connection on the starter..

If the battery voltage gauge goes to zero, it may be a bad connection at the battery.
Batteries generally last an average of only 3 years, but they will go bad gradually until the voltage drop on starting will drop below levels that the car can run the starter.

Try a new battery, they're not too expensive at Walmart or Sears or a local auto parts store.
Try the largest CCA capacity battery you can buy and you won't have regrets, in my experience.

Good luck on this repair.

G'day mate there is also a main fuse with high amp fuses located near battery check each fuses by pulling out and checking for condition ie loose wobbly also weird as it may seem i have found a number of vehicles in which the ants have made a nest in the fuse box/s, dont know why just found it od , what they do is ends up with tracks to earth from nest for positive curciut, try that.

If you have a multimeter, set it on x10 ohm scale. read resistance between engine and body, then negative terminal of battery to body. should be zero ohms. if not connect a ground strap from engine to body (make sure metal is clean at connection points) then one from negative of battery terminal to body. check again with meter should be zero ohms.

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

Alternator......period.

Charging system grounding out, poss inside alternator

First, check battery post to cable connection: positive meter lead on battery positive post, negaitve lead on battery's positive cable clamp. Crank the engine and note the reading. A good connection should have zero voltage drop. Second, check the positive cable: positve meter lead on positive battery clamp, negative lead on starter terminal connector. A good cable will show a voltage drop of .2 volts or less while cranking. Third, check the starter connection: positive meter lead on positive battery terminal on the solenoid, negative meter lead on actual starter stud. A good connection will have a voltage drop of near zero volts. Now to check the negative side of the circuit. Total drop on the ground side should be .3 volts or less and can be checked by placing positive meter lead on starter housing and negative meter lead on battery ground post. Take your reading while cranking the engine, and be sure your connection at the starter is solid and clean. If total voltage drop on this side of the circuit is excessive, complete testing at all connections in the same fashion as the positive side of the circuit. Check the following: between battery post and clamp (zero voltage drop), cable end at battery to cable end at engine. (.2 volts or less), cable end at engine to engine itself (near zero voltage drop), and finally between starter housing and engine block (.1 volts or less).

To read the entire procedure for all automotive systems read this artile:
http://www.engine-light-help.com/voltage-drop.html