SL/R231: Maintenance Charging By The Numbers
Thread Starter
Senior Member
Joined: Oct 2019
Posts: 293
Likes: 153
From: Nova Scotia, Canada
2019 SL450; 2023 M850i coupe
Maintenance Charging By The Numbers
I know some were wondering how often to charge their batteries in storage, so I started monitoring the rate of discharge of the Main and Eco start/stop (small) battery in my unheated, insulated garage. The inside temps. varied from just below freezing to a little above freezing, and the batteries are original OEM’s installed in a 2019 SL450.
Main battery full charge voltage: 12.7; Voltage after 10 days: 12.4.
Small battery full charge voltage: 12.7; Voltage after 3 months: 12.7.
Notes:
The full charge voltage is the voltage after the battery settles a few hours after charging.
Never allow an AGM lead acid battery fall below 12 volts.
Never partially charge an AGM lead acid battery as it will cause loss of capacity (memory effect).
This was good info. for me because I have a switchable--from inside the house—receptacle in the garage that I can now use to control the battery maintainer.
I hope you find the info. useful as well.
Main battery full charge voltage: 12.7; Voltage after 10 days: 12.4.
Small battery full charge voltage: 12.7; Voltage after 3 months: 12.7.
Notes:
The full charge voltage is the voltage after the battery settles a few hours after charging.
Never allow an AGM lead acid battery fall below 12 volts.
Never partially charge an AGM lead acid battery as it will cause loss of capacity (memory effect).
This was good info. for me because I have a switchable--from inside the house—receptacle in the garage that I can now use to control the battery maintainer.
I hope you find the info. useful as well.
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Joined: Oct 2010
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From: Corona Del Mar, CA
2019 SL450, 2019 E450 Luxury Trim Wagon, 2024 BMW I7 xDrive60
I know this has been discussed before, but does using a battery maintainer, connected to the posts under the hood, maintain both batteries, or just the main one?
Thread Starter
Senior Member
Joined: Oct 2019
Posts: 293
Likes: 153
From: Nova Scotia, Canada
2019 SL450; 2023 M850i coupe
As indicated in post 49 of the attached thread, there is reason to believe the small battery may take a charge from the posts under the hood once the small battery falls below some preset value.
https://mbworld.org/forums/sl-class-...ry-type-2.html
Since AGM lead acid batteries don’t like to fall below 12 volts, I keep an eye on mine and charge as necessary. In the three years since new, the small battery has never fallen to a point where maintenance charging was necessary; however, I usually top off before and after storage (5 month interval).
Last edited by hornethandler; Mar 12, 2023 at 12:42 PM.
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From: Nor-Cal
2020 GLC300 -2014 Sl550 - 1997 SL600
The auxiliary battery is “disconnected” at the Aux. battery relay once the ignition (N73) is switched off. That’s probably why it doesn’t discharge at any appreciable rate. From the attached wiring diagram (ECOStop wiring) below, there is absolutely no load on it. The only way it can charge when the car is off, is if you hook the charger directly to it.
Attached are some other pertinent documents from WIS
Note: The factory recommends charging Aux battery at the terminals see attached document "check condition of additional battery"
Last edited by crconsulting; Mar 12, 2023 at 02:10 PM.
Thread Starter
Senior Member
Joined: Oct 2019
Posts: 293
Likes: 153
From: Nova Scotia, Canada
2019 SL450; 2023 M850i coupe
The auxiliary battery is “disconnected” at the Aux. battery relay once the ignition (N73) is switched off. That’s probably why it doesn’t discharge at any appreciable rate. From the attached wiring diagram (ECOStop wiring) below, there is absolutely no load on it. The only way it can charge when the car is off, is if you hook the charger directly to it.
Attached are some other pertinent documents from WIS
Note: The factory recommends charging Aux battery at the terminals see attached document "check condition of additional battery"
Attached are some other pertinent documents from WIS
Note: The factory recommends charging Aux battery at the terminals see attached document "check condition of additional battery"
I want to take a moment to thank you for the documents you provided in this and many other threads. I created a library for them, and I greatly appreciate you for taking the time to provide them to us.
Last edited by hornethandler; Mar 12, 2023 at 03:34 PM.
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Joined: Dec 2015
Posts: 1,636
Likes: 522
From: Hagerstown MD
2015 SL400; 2019 X5M40i
The auxiliary battery is “disconnected” at the Aux. battery relay once the ignition (N73) is switched off. That’s probably why it doesn’t discharge at any appreciable rate. From the attached wiring diagram (ECOStop wiring) below, there is absolutely no load on it. The only way it can charge when the car is off, is if you hook the charger directly to it.
Attached are some other pertinent documents from WIS
Note: The factory recommends charging Aux battery at the terminals see attached document "check condition of additional battery"
Attached are some other pertinent documents from WIS
Note: The factory recommends charging Aux battery at the terminals see attached document "check condition of additional battery"
Previous owner installed the pigtail on the auxiliary battery as the car was leased for 3 years in Florida. It was stored every year during the warm months. I replaced the auxiliary battery at about the 5 year point. The selling dealer installed a new main battery during the CPO certification process.
Last edited by rmorin49; Mar 12, 2023 at 07:03 PM.
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From: God's County (Yorkshire)
Shanks’s Pony
Those small (typically VRLA) batteries are designed to go into installations where very low maintenance is desirable and can tolerate deep discharges well. Mine is 6 years old , car is rarely used (20,000 miles in 6 years) still no issues.
