Just had the RAC out as my 2007 2.5si Z4 wouldn't start. I'd tried to jump start it using my company car but no joy. 
The car can sit for 2-3 weeks without being used and when it is used it's usually only for short journeys due to the company car. The problem of not starting first happened a few weeks ago and I had just assumed it was a flat battery due to it sitting in the garage for a while. Unfortunately it hasn't started a couple of times since but I've managed to jump start it no problem. Due to the short journeys I just assumed it wasn't being run long enough to properly charge the battery.
This afternoon it wouldn't even take a jump start. Just the clicking noises and lights flashing when the battery is dead. I've not had any warning lights on the dash.
The RAC guy measured the voltage across the terminals under the bonnet and it's approximately 15.5V. He informed me that most cars will run at 12 to 14 volts and that there may be a problem with the alternator or regulator which is causing them to "over charge" the battery.
I'm just wondering if this sounds right and if it is how much could I potentially be looking at for a new alternator from a dealers - the car is 5 months out of warranty.
Forgot to mention - the RAC guy got it started 1st time and with no problems using his industrial looking jump start equipment. He also said the possible overcharging is just knackering the battery and that's why it's not retaining any charge.
The car can sit for 2-3 weeks without being used and when it is used it's usually only for short journeys due to the company car. The problem of not starting first happened a few weeks ago and I had just assumed it was a flat battery due to it sitting in the garage for a while. Unfortunately it hasn't started a couple of times since but I've managed to jump start it no problem. Due to the short journeys I just assumed it wasn't being run long enough to properly charge the battery.
This afternoon it wouldn't even take a jump start. Just the clicking noises and lights flashing when the battery is dead. I've not had any warning lights on the dash.
The RAC guy measured the voltage across the terminals under the bonnet and it's approximately 15.5V. He informed me that most cars will run at 12 to 14 volts and that there may be a problem with the alternator or regulator which is causing them to "over charge" the battery.
I'm just wondering if this sounds right and if it is how much could I potentially be looking at for a new alternator from a dealers - the car is 5 months out of warranty.
Forgot to mention - the RAC guy got it started 1st time and with no problems using his industrial looking jump start equipment. He also said the possible overcharging is just knackering the battery and that's why it's not retaining any charge.
