Is my alternator broken?

XJ40
1

Hi all,

I have a question regarding the alternator in my 1993 V12 Daimler.

The battery in the car runs out of juice really quickly after turning off the ignition, even when disconnecting the ground wire. Therefor, I attribute this not necessarily to some leak current.

It is however possible the battery isn’t fully charged. I bought the car with a flat battery, and had it jumped to drive it home. This was a two hour drive in which the car basically ran fine, except for 2 seemingly momentary losses of power (which i attributed to the later discovered not 100% perfect tightening of the ground wire). I did however, after getting home and turning off the car, manage to start it again, so the battery does at least partially charge.

The current on the battery never really peaks above 12 volts and usually doesn’t even reach 12. It is more in the range of 11.something and rapidly drops.

The current with the engine running, in this current hot weather and the cooling fans running is about 13,4 volts. This seems a bit low, charging the battery would prefer a voltage around 13,8? As said, the battery does charge, but maybe too slow and not to full capacity.

My question now is, is my alternator broken and hence not charging the battery to full capacity causing the battery to rapidly lose current?

Or is my battery broken, hence why it loses current too quickly? I read somewhere batteries which havent been used in a while (and are in an empty state) can suffer from sulfate crystals which make an automatic charger think the battery is full even though it inst?

Or, worst case scenario, are both broken?

Kind regards,

Ramses

2

Could be either, have the battery tested first to discount that side of things.

3

Robin, thank you for your speedy reply.

I went to a company and they checked the voltage. This gave no concrete solution, as I know the voltage is low, just not what is causing it.

They disconnected the battery and performed some sort of test which said the battery was ‘ok’, yet even then it was rapidly losing voltage, so possibly not the most trustworthy of tests.

Is 13,4 an uncommon voltage or could it be attributed to lots of bits and pieces running in the car and a nigh completely flat battery?

4

13.4 to 13.7 is about where you should see when the battery is in good condition under charging situations, resting should be 12.7 ish

5

mmmm… That sounds suspiciously like a problem I had on my Jag battery some years ago. Battery supposedly checked out OK but after being fully charged and then left under no load off the car, the battery voltage would drop to around 10.5 V overnight.

Try that check.

My battery had one defective cell which was shorting internally, hence the rapid discharge.

6

Take the battery to a battery specialist for the test. They will check it properly.

7

Luckily, the battery was the culprit in the end. New battery and all issues regarding powerdrain are resolved.