I hope someone can help, I have an electric problem that is starting to drive me crazy.

A week ago my wife's EB broke down. Turns out it was a completely dead battery. I determined this by jumping the battery and then disconnecting the battery terminals - the car ran beautifully on it's own alternator power. Changed it over and all was OK for a week.

Last night she broke down again, checked the new battery and it was completely dead.

I have it home now and put a multi meter on the terminals.

12.49v on the battery with the engine off
14.92v on the battery with the engine & lights going.

Seems like the alternator is working fine, but why would two batteries completely flatten?

Please help!

For future reference never ever disconnect the battery from the electrical system in any car when it is running. The battery is required to regulate the system voltage and with it disconnected, the voltages can be unpredictable.

The problem is that something is draining that battery whilst the car isn't being used. I'm a bit concerned about the 14.92 volts whilst the engine is running. Sounds a little high. I think my NF was 13.98 with all on last time I checked.

Check for any lights that may be stuck on including the boot. You have a multimeter so check for excessive current draw from the battery. Disconnect each fusible link and check to see how much current is flowing. Place multimeter in series (setup for milliamps) with each link and see if you have any high current flow. Anything more than 50mA would be a concern. Keep in mind that the alternator itself may be the culprit. One of the fusible links goes to the battery so this would checked in the above step.

Thanks for the advice.

My wife was driving on the Freeway at around 90kmph for about 20 minutes when the instruments, lights and motor started to flutter. She drives a minimum of 1 hour a day usually at 90 to 100kmph. The battery should be easily charged to full with this. When i came to her rescue the battery was completely flat.

I will start checking as you suggested, but if there is charge there from the alternator at the moment doesn't the problem indicate inconsistant charge for long periods of time?

Is the regulator part of the internal circuitry of new alternators? Could this be a problem? You already think the voltage is a little high. Maybe this somehow stops for periods of time allowing the battery to flatten and the car to stop when there is not enough power in the battery.

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Edit...

have had the engine running since last posting and have just checked the voltage again - back down to 12 v. When i disconnected the jumpers the car stopped. I think this indicates the alternator???

have you driven the car with this problem? might not be charging while your driving, which means the regulator, mine is having the same problem were the regulator wont switch the alternator on untill i draw a decent amount of power through hard acceleration, it drains the battery dead if i dont accelerate over 3k rpm then the alternator kicks in and is fine till i switch it off

The batteries going flat would be caused by a defective diode pack in the alternator. Your voltage regulator is also over charging( should not go above 14.5V or you will boil the battery).

Your problem sounds like a loose/worn drive belt off the alternator pulley.

Thanks everyone - Your advice is appreciated. I bit the bullet and picked up a new unit and fan belt and dropped them in.

It would have been good if I could pick them up tomorrow as parts are very expensive from the places that sell them on Sunday. At least it is going and the wife can get to work tomorrow.

Thanks again.