SL/R129: AGM battery considerations?
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From: Utah
2000 SL500, R129
AGM battery considerations?
I’ve read a bit about AGM batteries. Want learn if there cconcerns with it replacing a conventional lead acid battery on a. 2000 SL500. It seems AGM may have stronger cranking power, might that impact control modules, or would wiring potentially need upgrades? Any experience with Marxon brand from Amazon?
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From: NJ
2009 SL63 AMG
I've run AGM batteries in many cars that originally came with a conventional battery without issue.
Duralast Platinum AGM batteries from AutoZone have been my preference. Good quality and an easy warranty process should there be any issues.
Duralast Platinum AGM batteries from AutoZone have been my preference. Good quality and an easy warranty process should there be any issues.
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SL600
Marketing aside, AGM batteries are identical to normal, wet lead batteries. The difference is a glass fiber fleece that absorbs the acid, meaning that the lead plates, even though pressed together, are still in contact with the acid. As a result of the pressing, the plates do not deform even when deeply discharged and retain their capacity for longer. As a result, AGM batteries can withstand greater discharge and last longer with almost the same electrical data.
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Newbie
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From: Utah
2000 SL500, R129
The cost is not the main issue here; I believe but haven’t researched extensively, that the AGM style costs slightly more than conventional lead acid. But my understanding is that AGM responds better in a low use environment (if I can drive my SL 2k/ year I will be lucky) and has a stronger starting capacity for high performance engines, and a longer life. I guess the latter speaks to cost a bit but it is really more about the hassle of changing batteries to me. I will use a “battery tender” though, mainly in the winter months.
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From: SF Bay Area, CA
190E 2.6L 1989, 190E 2.6L 1988, BMW 535dXdrive 2014, BMW 428i 2015
My Duralast AGM lasted only the warranty period of 3-4 years. I was not very happy with that especially because it cost 30% more.
I went back to Interstate lead acid. Interstate has both lead acid and AGM. For 30% more cost and lasting 50% less life, no thank you.
Also if you have a lead acid old time charger it will not work with an AGM. Because the charging of a lead acid is different then lead acid.
If there is a deep discharge in an AGM, it probably puts a lot of strain on a charging system designed for AGM.
Also AGM seems to need a higher charging voltage than lead acid. And the charging system never provides that in our cars.
Does AGM batteries work in a system designed for lead acid? Sure that has been proven times over but the question is why switch?
I went back to Interstate lead acid. Interstate has both lead acid and AGM. For 30% more cost and lasting 50% less life, no thank you.
Also if you have a lead acid old time charger it will not work with an AGM. Because the charging of a lead acid is different then lead acid.
If there is a deep discharge in an AGM, it probably puts a lot of strain on a charging system designed for AGM.
Also AGM seems to need a higher charging voltage than lead acid. And the charging system never provides that in our cars.
Does AGM batteries work in a system designed for lead acid? Sure that has been proven times over but the question is why switch?
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G550
I put an Optima Redtop (an AGM battery) in my '95 Jeep 12 years ago, and it always performed just fine with a charging system designed for a conventional flooded cell lead acid. I changed it out for another Redtop a couple weeks ago only because I thought 12 years was pushing it. I would be reluctant to replace an AGM battery in a newer vehicle (that places greater demand on its battery) with a conventional lead acid type that might not be up to the demands of start/stop and electronic systems than never shut off.
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SL63
My understanding of the AGM, in addition to all of the above, is the crashworthy aspect. If the battery is compromised in a crash at a track type setting, or any other, it is less dangerous than a traditional lead acid.
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FWIW most basic modern day trickle chargers have settings to specify a regular 12V battery vs a 12V AGM battery. They can still die if you have some sort of parasitic draw somewhere or an aftermarket sound system/nav things of that sort, but will hold up better than traditional lead acid batteries if sitting for awhile. Regardless I would still suggest hooking the car up to a trickle charger with a 12V AGM setting.
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SL600
AGM batteries normally do not need a different charging voltage. However, since they can withstand cyclical charging and discharging processes better, they are used in such situations (start-stop operation). With this cyclical usage pattern (and only there), the need for charging voltage increases significantly in order to prevent the otherwise unavoidable sulfation, which is why cyclical use requires charging at around 2.45V/cell (14.7V for a 12V battery): the "AGM charging mode".
Normal lead batteries would also have to be charged with a higher voltage, so in reality it does not depend on the type of battery but on the kind of application. Since the R129 does not have a start-stop, an AGM does NOT need the AGM charging mode there. Quite the opposite.
Normal lead batteries would also have to be charged with a higher voltage, so in reality it does not depend on the type of battery but on the kind of application. Since the R129 does not have a start-stop, an AGM does NOT need the AGM charging mode there. Quite the opposite.
Last edited by AndreasHannover; Dec 25, 2024 at 06:32 AM.
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From: San Francisco Bay Area, CA
@AndreasHannover great information thanks!
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From: NJ
2009 SL63 AMG
AGMs also tend to have better warranty periods. For a few extra bucks, the margin is worth it for me.
As for duration of use, I've observed 5+ years on the Duralast Platinums.
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190E 2.6L 1989, 190E 2.6L 1988, BMW 535dXdrive 2014, BMW 428i 2015
It would be interesting to find out how a 80's and 90's lead acid car charging system behaves compared to a modern car charging system. The curves suggest the AGM battery is charged to a higher voltage for this charger. I suspected this back when I had the AGM. Just comparing the system voltages of my BMW's to my old MBenz's. I used to get 7+ years from quality Lead Acid batteries like Die Hard. Not sure about today's batteries though.
My wild guess is, if one does not have a trickle charger on an AGM and one does not drive the car often and for longer periods the AGM does not last as long.
That is my one and only data point. I could have ended up with a poor quality/defective AGM. Oddly, I had switched to the AGM just because I thought for the scenario above it would do better.
BTW, just visited my in-laws who have a 1965 Datsun Fairlady in the garage. I left the car there 2 Xmas's ago with a new lead acid. 2 years later I figured the battery is toast and completely drained.
To my surprise it still had 11.2V on the terminals and I could charge it to the full 12+ voltage and it holds. I did not think a lead acid lasts that long. I was wrong again....
My wild guess is, if one does not have a trickle charger on an AGM and one does not drive the car often and for longer periods the AGM does not last as long.
That is my one and only data point. I could have ended up with a poor quality/defective AGM. Oddly, I had switched to the AGM just because I thought for the scenario above it would do better.
BTW, just visited my in-laws who have a 1965 Datsun Fairlady in the garage. I left the car there 2 Xmas's ago with a new lead acid. 2 years later I figured the battery is toast and completely drained.
To my surprise it still had 11.2V on the terminals and I could charge it to the full 12+ voltage and it holds. I did not think a lead acid lasts that long. I was wrong again....
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SL600
Modern charging systems try to keep the charge level as low as possible. This saves gas and ruins the batteries.
And again, there is no AGM charging mode, the charger manufacturers just don't trust their customers to understand the term "high charge-discharge cycles". AGM is simpler, nobody questions the causes and everyone is happy.
Doesn't the voltage drop immediately when loaded?
And again, there is no AGM charging mode, the charger manufacturers just don't trust their customers to understand the term "high charge-discharge cycles". AGM is simpler, nobody questions the causes and everyone is happy.
Doesn't the voltage drop immediately when loaded?
