Battery being drained

[BorderVolvo]

Remember when doing a voltage drop test between battery positive and alternator terminal with engine running, the alternator needs to be under load..... so switch on as many consumers as possible ;- lights, heater, HRW etc
Same applies when doing the earth circuit.

[Beadybc]

Hey guys so me and my Dad just did some tests.

All tests done with engine running.

When I say ‘everything in the car’ I mean: the stereo, fans (no ac fitted), rear heated window, mirror heaters, heated seats and all lights.

Between the alternator body and negative on the battery:

Everything in the car off - 0.527v
Everything in the car on - 1.324v

Between the alternator body and positive on the battery:

Everything in the car off - 14.16v
Everything in the car on - 13.83v

Between engine block and negative on the battery:

Everything in the car on - 0.559v

Trying to get a true reading from the positive terminal on the alternator to the battery terminals was quite challenging as the numbers were all over the place.

It did however look as if it stopped for a second or two at readings between 5v and 8v from the positive on the alternator to the positive on the battery and around 5v to the battery negative.

[Clan]

Originally Posted by Beadybc

Hey guys so me and my Dad just did some tests.

All tests done with engine running.

When I say ‘everything in the car’ I mean: the stereo, fans (no ac fitted), rear heated window, mirror heaters, heated seats and all lights.

Between the alternator body and negative on the battery:

Everything in the car off - 0.527v
Everything in the car on - 1.324v

Between the alternator body and positive on the battery:

Everything in the car off - 14.16v
Everything in the car on - 13.83v

Between engine block and negative on the battery:

Everything in the car on - 0.559v

Trying to get a true reading from the positive terminal on the alternator to the battery terminals was quite challenging as the numbers were all over the place.

It did however look as if it stopped for a second or two at readings between 5v and 8v from the positive on the alternator to the positive on the battery and around 5v to the battery negative.

It is generally taken that volt drop across a cable on a car should not exceed 0.2v .
I would renew the battery cables, they will be heating up with that resistance which will make the problem worse.

Charging voltage is just about good enough but 14.2v + would be better.

[Rversteeg]

See attached part of diagram, this is most probably the culprit. The charging cable is not a straight run from alternator to battery, but via the starter motor. It is this common joint that deteriorates over time, causing increased electric resistance that you have measured as a voltage drop.
Best solution is to replace the complete cable. Instead of replacing this cable I have added an additional wire between alternator and battery. Note this wire should be able heavy as it must be able to carry the full 100A from the alternator in case the original cable fails completely.