Electrical theory please

[martinbrowning]

Good morning,
I've been trying to trace an electrical fault on my son's Opel diesel van for some time now. Problem manifested itself as a starting (or lack of) issue. Over the last month or so I have replaced the battery, starter motor and alternator with new quality items. That has not solved the problem and the battery still has a drain on it.
Yesterday I disconnected the negative lead and charged the battery. Got a 12V reading on my meter. Left the negative cable disconnected and left the van overnight. This morning - no start and voltage down to 7V.
Obviously a drain somewhere but question is - how can that happen if the negative cable was disconnected? Didn't think that a component could draw the current with that cable disconnected.

Cheers

Martin B

[PeterK]

If you are 100% confident that the battery was effectively disconnected, no small cables still attached, and the battery still lost its charge overnight, then I'd be looking at a failed cell in the battery. A new battery would sort.
Peter

[c42]

If the voltage is dropping with no lead connected you have a battery problem.

Regards
John

[mbell]

Repeated flattening of a battery is not good for them and might of caused it to fail.

When I was hunting a battery drain on my car I modified (broke the fuse and soldered some wires on) a fuse so I could connect my meter and measure the current through each circuit. Allowed me to identify the circuit causing the drain and then disconnect items until the problem is found (AC control panel in my case).

The only tricky part is in a modern ish car you need it to be in deep sleep mode to test. Getting it in deep sleep mode and getting to the fuse box with out waking it up will require some vehicle specific hoop jumping.