How long does it take for a fully charged battery to be incapable of starting the engine when installed in the car without being driven.

A typical 12v 60 amp car battery has a capacity of 720 watts. The speed with which this discharges is a guide to the power loss.

Normal ancillaries - alarms, radio, central locking etc are low draw - a car could remain unused for a month or more before problems with battery charge.

A couple of standard headlights will use ~120w - if they were left on the battery would be flattened in ~6 hours.

If the battery is drained within (say) 15 minutes this would be the equivalent energy to a 3kw electric fire - a lot of energy being lost somewhere. Sparks would fly and a vehicle fire a real risk.

This may help identify what is causing the charge loss.

The alternative is that it appears discharged after having been driven - this is more likely to be caused by a loose or corroded connection becoming detached due to vibration etc when the engine is running and car moving.

If the voltage is dropping like that there's a short, surely? But why is it not present when the car is running, and why has it not blown the main fuse?

I wonder if the voltage is being measured at the actual lead battery post?