Alternator or battery

[NigelP]

I have a 2008 Mercedes c200 which is dead in the driveway. It starts perfect and quick when jumped but does immediately after being switched off (doesn't turn back on unless jumped). I tested the voltage with a multimeter and got the following

Stress no accessories 1.5rpm 13.77
Stress with accessories 2.5 rpm 13.67
No stress idle 13.76

Battery off 10. 13

I am inclined to assume battery, but I have noticed lately that 6 bulbs have blown which may be alternator. What's does everyone think?

 

[Mike Johnson]

Battery has had it, take it to your local dealer and he will load test it.

 

[James]

Battery is dead.
pop in a new one an you will be good to go.

 

[ruston]

Are you expecting it to start after it has been turned off after just a jump start with little or no charge , or have I misunderstood your post.

 

[NigelP]

Are you expecting it to start after it has been turned off after just a jump start with little or no charge , or have I misunderstood your post.

I have jumpstarted it, driven for 30 minutes and it runs fine, I turn it off and it won't start again after 5 seconds.

 

[ruston]

Is it completely dead or does it crank, 30mins driving is not a lot depending on your engine size, especially if you have AC on . Does it crank when you try to restart it, and have you tried to charge it with a conventional charger?

 

[NigelP]

Is it completely dead or does it crank, 30mins driving is not a lot depending on your engine size, especially if you have AC on . Does it crank when you try to restart it, and have you tried to charge it with a conventional charger?

The starter motor attempts to turn once then the power is gone, the computer then blacks out and the lights all turn off if I twist the key again then about 3 or 4 minutes the battery is showing 8-9 v again, directly after turning the car off without turning it back on the battery dips to 12.2 V but won't start form that, it attempted to turn the start which chugged for a bit then the voltage drops to about 8 and it stops. Once jumper leads are put on it jumps with ease. The first trip I took was probaly closer to an hour with ac blaring and daytime running lights on and the second 30 minutes. No issues at all especially when driving.

 

[ruston]

Your battery will not charge great from flat in the conditions you describe , and it would use a good percentage of the charge to run the accessories unless it has a second battery fitted , some Merc's do.
Give it a good conventional charge, depending on the charger and amp hours of the batteries this could take up to 48 hours.
What is the size of your engine?

 

[NigelP]

Your battery will not charge great from flat in the conditions you describe , and it would use a good percentage of the charge to run the accessories unless it has a second battery fitted , some Merc's do.
Give it a good conventional charge, depending on the charger and amp hours of the batteries this could take up to 48 hours.
What is the size of your engine?

It is a 1.8L supercharged. I don't have a battery charger and am stuck with the issue being that a new battery will probaly not he far off buying a new charger. I honestly am not to fussed about buying the battery but I am more wary I could buy it and it could end up being the alternator. I always assumed if the alternator was operating over 13.5 V then it should be fine, but given the symptoms lately of fast blowing light bulbs, I am only thinking of it as a possibility.

 

[ruston]

Is this a one off problem , or has it developed to a point it will not start?