Help with Alternator

[stickytoria2002]

Hi I will explain the issue and hopefully someone out there can help me narrow down the problem.
Right We have an BMW X5 4.8is 2006. The transmission fail safe programme came on one day and the car did not start.
the garage said it was the battery so we bought a new battery and paid for that to be fitted.
Now the car starts every morning no fail but the battery reads 11.5-6. then after about 10 mins and 30000 revs later the battery then reads 13.9-14.00, BUT it will then not go back down again until i turn of the engine and then when it is restarted and we go through the same thing again.,
Now the battery light has never came on until this morning it kept flashing on then it went off, then 30 seconds later came on. then when we got to the 3000 revs it did not come on again.
We were told this was a week alternator but why does it start to work after 3000 revs and continue to work until we turn off the engine?
We have a little battery reader that plugs into the cigarette lighter so we read the battery from there.
the light has never before come on only this once this morning.

Please can someone help to identify the problem. I dont want to pay the garage ££££'S for fitting a new alternator when it could just be a loose wire or something..
thanks

[stickytoria2002]

sunbeam alpine said:

If I've understood your post correctly, the battery is giving a reading of 11.5 - 11.6 volts when the car's not running, and 13.5-14 volts when the car is running (albeit only after you've run the engine to above 3000 revs) and it stays at that voltage until you switch off.

This suggests that the alternator is working fine and charging the battery as it should. A voltage measurement of below 12v when the car's switched off is a bit on the low side, but as you've said the car always starts fine, doesn't seem to be a problem.

Flashing charge light could just be a bad connection - check the battery connections - I know nothing about BMW's but I have a few classic cars which are quite sensitive in terms of bad electrical connections.

HI Sunbeam, thanks for this. but no it reads 11.5-6 when we start the car and drive. It then goes up to 13.8-9 once we drive at least a mile. We did do another test though and let the car run for 20 min before driving away and within minutes the alternator kicked in. so could this be heat related?
Battery light has not been on just that one time.

hanks for your help