Battery keeps going flat but . . .

[petesavan]

Hello everyone I'm new to this forum and I'm wondering if anyone can help with this problem. I have a 1968 bay with a 1972 1600 twin port engine from a Beetle, so the engine has an alternator, not a generator. The battery keeps going flat but only if I drive the van anywhere. I can fully charge the battery, which is less than a year old, and not use the van at all for a month and it will start first time. However, if I fully charge the battery and drive it straight away for only say 10 miles, the battery will be flat within two or three days. I have tried putting a multimeter in series with the battery on the most sensitive milliamp scale to see if there is a short circuit anywhere but I have not been able to detect one. On another forum it has been suggested that this might be due to one or more diodes having failed in the alternator which can cause current to leak after the van has been driven although I don't understand the physics behind that argument. I'm not sure if I dreamt this or if someone told me, but if the diodes are damaged I will need to find whatever fault caused them to blow before replacing them (or the alternator) otherwise they'll blow again. Does all this sound feasible? And could they have been damaged due to a missing or damaged engine to chassis earth strap or a short circuit somewhere - again I seem to remember someone telling me this might be the case or I might have dreamt it!?

 

[Trikky2]

Hello and welcome to the forum.

Yes your right - dodgy connections can cause diode failure on an alternator.

To check out your theory of it only draining after being driven, drive the vehicle then get someone to hold your meter probes on to the post and the terminal whilst you disconnect them eg the current flow will not be interrupted since its being bridged by the meter.

If the meter springs to life as you disconnect the terminal then your theory is proven.