Checking alternator output voltage (Audi 80)

By how much?

Any load being applied will make the output voltage change slightly. Not the AC compressor cycling?

The voltage reading was all over the place. It maintained 14.3 for a couple of secs and then suddenly drop to around 5v and back up again. I think I've narrowed it down to the battery. The cells were dry (2 in particular) so I topped up with dist water and gave it a charge over night. Tried it again today and the charging is now maintaining a steady flow of 14.3 without dropping. Would having dry battery cells cause the voltage to pulsate up & down like this? I know it wouldn't have done the battery much good but will keep an eye on it. The battery previously held a charge of 12.42v after been stood overnight, now it has water in the cells I will check this again tomorrow.

Crikey.

I assume they weren't dry, but just low in electrolyte? One which had no electrolyte wouldn't work at all.

Don't think so. The resistance of the cells would likely rise if low in electrolyte so present less of a load to the alternator.

Can't even hazard a guess as to what was going on, as a battery which works in any way just can't go down to 5 volts. Anything under about 10 volts open circuit is considered discharged - so even if the alternator was intermittent in output, this is the *very* lowest I'd expect to see.

I'd guess at a faulty in your meter or leads?

Yep, they have really weird behaviour when they're like that.

If it works after the frost this week you've probably won.