I have a Mk3 2.2l diesel. Was a good runner when I parked it up in my mother in laws garage, (grateful to have a place to park it for a year) and a year later the battery is dead. The starter did click when I tried it first time.

Then my brother in law loaned me a charger booster he uses on farm machinery. Connected it and the lights inside and outside the car started flashing. Didn't start the car. Turns out the charger was delivering 20v! Checked fuses under the hood after and all look ok. But now the starter doesn't even click.

We tried jump starting it with another booster pack, and by towing it while we had the booster pack on it. Not sure how effective the booster pack on it was but dash lit up properly. But didn't start. Almost like the immobiliser is off.

Ordered an OBD reader.

Back at it today, with a better charger, and the battery is reading 6v! so might get it up to 9v or something. Who knows. While it was charging I put the OBD on it. The OBD connected to the ECU and it read no faults. This is obviously without it running, but with the ignition on. So I'm hoping this means the ECU isn't fried. Will see to tomorrow how many volts the battery holds if it will crank, or at least click.

I know buying a new battery is the first step to diagnosing this. But don't want to buy another battery yet, as the car is due to stay in storage for another year in the outdoor cold so will be toast by the time I'm ready to use the car again.

Do you think all it needs is a new battery, or has the 20v charger done something to the car that means this is now a bigger problem. With hindsight I don't trust any of the boosters and chargers my brother in law loaned me.

Is there a minimum voltage the car needs to disengage the immobiliser? Have I cooked an ECU somewhere with my brothers charger? Does it just need a new battery to start it?

Thanks

Can’t you take a battery out of another car and try that? I do realise it needs to fit in the space and the leads reach the correct terminals.

You need a new battery regardless of what else may, or may not be wrong. Unless you're seeing over 12v, you're going to struggle to start that engine.