hornethandler

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.

Streamliner

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?

hornethandler

During several main battery charge cycles I saw no voltage change in the small battery.

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).

crconsulting

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"

hornethandler

Crconsulting,
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.

rmorin49

I have a second CTEK that I connect to the auxiliary battery using a pigtail installed directly on the auxiliary battery. This CTEK has always gone to a complete cycle within a few minutes. The other CTEK connected to the main battery usually takes a couple hours to move through a complete cycle.

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.

rorywquin

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.