Battery or alternator?
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Newbie
Joined: Sep 2020
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2015 S550 4Matic
Battery or alternator?
2015 S550 - Every few days the battery gets too weak to turn the car over. I can charge it with a charger using the connections under the hood and it seems to take a charge fine and lasts maybe 15 starts then gets too weak again. I assumed this meant the alternator wasn't doing its job, but do not notice any issue with the electronics in the car while driving as I would expect with a faulty alternator. I took it to the auto parts store and their tester said the alternator is ok and the battery is bad. This is good news, if true, but why will the battery take a charge from an external charger but not from the alternator?
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Newbie
Joined: Sep 2020
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2015 S550 4Matic
Super Member
Joined: Jan 2009
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From: In The Woods
2019 W222 S560 4Matic/2025 Yukon Denali
You can get any agm battery that is rated H8 AGM.
dealer charge is like $100, after market is like 60 or less.
I think yours is in the dash or hood....later models are in the right side trunk.
dealer charge is like $100, after market is like 60 or less.
I think yours is in the dash or hood....later models are in the right side trunk.
MBWorld Fanatic!
Joined: Jan 2015
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15 S550 4Matic, 86 560SL
As long as you buy an AGM they are all fine. I just went though the same thing last month - I did a lot of research and it turns out all the AGM manufacturers are reputable. The OE MB one is made by Deka, which is hard to get outside of the dealership. I bought one from AdvanceAuto/Carquest (made by Johnson Controls Inc, who makes a majority of the batteries in this country) around $260 and it works great. 3-year free-replacement warranty which doesn't mean much since our original ones lasted 8 years!
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Newbie
Joined: Sep 2020
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2015 S550 4Matic
Thanks for the replies, I'm going to replace the battery with a $250 Superstart platinum from O'reilly. I don't want to drive 2 hours abd pay $200 more if there's not a significant difference.
Still curious why my charger will charge the battery and the alternator won't. Oh well, perhaps some things in life aren't knowable.
Still curious why my charger will charge the battery and the alternator won't. Oh well, perhaps some things in life aren't knowable.
Senior Member
Joined: Mar 2022
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From: Atlanta, GA
2020 S450 4Matic, 2022 GLS450 4Matic, 2015 C300
I can't speak to MB Batteries, as I have never had to replace one. But the last battery that I replaced was on a BMW 525, when I spoke to the tech at the dealership, he said that the alternators only really provide enough power to keep the battery topped off, in addition to running all of the electronics, but they don't have enough space to also charge a failing battery. Assuming that this is true, the tech in our W222's is even more than I had in that 525, which is why the charger can charge, but the alternator does little more than maintain.
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Joined: May 2011
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From: Fort Worth TX
2020 S560, 2014 SLK55 (Prev. 2016 S65, 2013 SLK350, 2008 CLS63, 2006 E350, 2006 CLS500, 2005 C230K)
The alternator in your car can power your burmester at full volume while running a 850W suction fan, I can assure you your indie does not know his electronics.
The reason for the difference is a battery on the way out will take charge more slowly, in addition many chargers have a "condition" cycle which also helps attain a better charge on a bad battery.
The reason for the difference is a battery on the way out will take charge more slowly, in addition many chargers have a "condition" cycle which also helps attain a better charge on a bad battery.
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