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Kevin, ASE Certified Technician
Category: Mercedes
Satisfied Customers: 8220
Experience:  Diagnostic technician
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I have a 2006 S320 Cdi reg C8EON. I believe the consumer

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I have a 2006 S320 Cdi reg C8EON . I believe the consumer battery has failed , it is a sealed lead acid unit . Does it have to be replaced with a VRLA AGM unit ?.
JA: Have you tested the battery with a voltmeter? And can you see if it's leaking or not?
Customer: Overnight the voltage drops to 7.18 volts
JA: What is the model of your '06 Mercedes?
Customer: W221 320CDI 3 ltr 642 motor.
JA: Are you fixing your vehicle yourself? What have you tried so far?
Customer: I have tried a new battery on the car and it clears all the fault codes that have been randomly popping up on the screen , I cannot ascertain if the battery is totally fail or there is a strong parasitic drain.
JA: Is there anything else the Mechanic should know before I connect you? Rest assured that they'll be able to help you.
Customer: I have been told a new AGM battery will fix the problem as opposed to a more reasonably priced SLA battery.

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Customer: replied 7 days ago.
e , sorry my typing skills do not match yours so I may be slow replying

Hi Ian I hope you are well. I'm sorry to hear you are having issues with your Mercedes and I totally understand how frustrating this is. So the answer is yes the battery could indeed be the cause of your issue. AGM battery is basically an ungraded lead-acid battery. The way it upgraded allows it to be far more powerful and have more ability to store charge, The car is listed as having an AGM battery fitted, and its likely with a cheaper battery the battery management system will not charge this battery correctly

Customer: replied 7 days ago.
I have been quoted numerous different prices for a replacement and also differing specs the current battery is a 72 ah SLA
it should be as close to the original as possible. Ideally a genuine part but I totally understand how expensive these can be
Customer: replied 7 days ago.
What spec is listed for a replacement ah wise.

i have 70 ah

Customer: replied 7 days ago.
O.K. thanks , this is the battery I bought the car with 4 years ago . Will it have failed because it is the wrong spec ?. Also I am concerned that one or more of the modules is not shutting down causing a parasitic drain . But it is not showing any fault codes with a new battery fitted.

the issue is we don't actually know how old the battery is. Typically the battery would last anywhere between 4-8 years on average so it's likely its just a battery failure but you have to get the right type of battery fitted and have the charging voltage tested afterwards

Customer: replied 7 days ago.
I have another car CLK 350 that has had a new battery a month ago so I have used this to test the s class , the charging voltage with this battery on the s class is 14.4v

that's a good charging voltage

Customer: replied 7 days ago.
O.K. I have left the car for 48 hrs now and it unlocked and started this morning without issue but the CLK battery is 105 ah so I would expect that . The voltage before cranking was 12 .4v

yeah that's normal also

Customer: replied 7 days ago.
I keep a spare 60 ah battery for emergencies so I may try this short term to see if it works before jumping in for a £250 AGM
Customer: replied 7 days ago.
Thank you for your advice , as I use the s class 2 to 3 days a fortnight I use a CTek battery charger/conditioner when not in use , is this good practice ?.
it’s good practice for sure
Customer: replied 7 days ago.
Thank you good bye .
Have a good day.
You are very welcome. Thank you again for visiting JustAnswer, please let me know if you need me to clarify anything or if you have any additional questions-I am happy to help!
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