Thread: A new one - battery voltage drops when car starts.

189500km on my D4, original alternator. Same error code and 11.8v or so (measured at the positive battery terminal and ground. Replaced alternator and ready for the next 200,000km.

dont muck around and replace your alternator.

Alternators don't seem that expensive, from memory.

Originally Posted by DazzaTD5

What he just said
And if you prat around too long you will be replacing the battery with the alternator

What he said. My alternator died and took my newish battery with it!

Originally Posted by DiscoMick

Alternators don't seem that expensive, from memory.

$680 for the Denso unit, including GST from Rovaparts in Perth. Took longer to gain access from underneath than it took to actually replace on my D4, straight swap over no issues, just a bit of my a squeeze to connect the wiring. I reckon that's pretty good considering the first one lasted 8 years and over 180,000km. The old one showed signs of delamination and insulation degradation, plus the rectifier housing was all brittle and cracked.

Originally Posted by Dagilmo

What he said. My alternator died and took my newish battery with it!

Hi Dagilmo, do you by any chance know how low your battery was discharge to?

When my alternator went, the cranking battery was discharged down to 7v, and while it seamed to be recharged back to a fully charged state and also held that state, but the battery lasted only a few weeks before needing to be replaced.

It was replaced to correct all the fault and error messages being displayed.

With a bit of luck, as Pinelli’s battery was only discharged down to 11.9v, his battery may be OK.

The unknown factor here is that because he was literally running his vehicle off the cranking battery, did the high continues current draw have any effect on the battery’s longevity.

In my experiance by the time someone finally works out that their alternator is buggered the battery is also toast as well.
Replacing only the alternator and leaving the defective battery in the car Must have a detramental effect on that new alternator.
A new battery is usually only a fraction of the price of a new alternator and it makes good sense to replace the battery when you fit a new alternator

Originally Posted by drivesafe

Hi Pinelli, D3 alternators will operate somewhere around 13.5v to 15.5v, but 15.5v is usually only in very cold conditions.

While D4 alternators can drop to 12.2v, this is normally only for a very short time and is something that only happens periodically, when the BMS wants to test the cranking battery’s condition.

So to be as low as 11.9v is well and truely below normal operating voltages.

As others have posted, I would start with getting your alternator tested.

Pardon my ignorance, but why does the battery voltage go up when it is cold? I have a Holden Rodeo with a Voltage reader in the cigarette lighter socket and on the colder days it has a reading of 14.4V which is the highest it has been.

Originally Posted by Kidbeen

Pardon my ignorance, but why does the battery voltage go up when it is cold? I have a Holden Rodeo with a Voltage reader in the cigarette lighter socket and on the colder days it has a reading of 14.4V which is the highest it has been.

Hi Kidbeen
The charge voltage is temperature compensated, higher at low temps.

Victron says for example

The recommended temperature compensation for Victron VRLA batteries is – 4 mV / Cell (-24 mV /°C for a 12V battery).
From this page Lead acid battery charging in cold weather - Victron Energy | Victron Energy


Ideally measured in the battery but usually a big compromise by measuring at the alternator, regulator or somewhere else.

Cheers

Peter