2023 S500 Battery Tender Issue
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Joined: Apr 2022
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From: Virginia
1992 300E, 2023 S500, 2018 S450
2023 S500 Battery Tender Issue
I have a unique problem where my CTEK battery tender stops working (only a blinking power light) after multiple days (and years) of flawless charging. Once I see that the charger stops working I remove the charger leads and check the battery connections under the hood and the voltage drops to almost zero momentarily. I then wait a few minutes and clicked on my key fob (maybe this woke up the car..but not sure) and the battery voltage returns to ~12.5. I took my car to the dealer when this issue first appeared and they said they made some module software updates, as well as checking the battery. After the updates it appeared my problem disappeared, but now it has returned. Does anyone have any idea why my battery's voltage would drop to almost zero and then return to normal? My only thoughts is that some electronics in the car are causing this condition or I do have a bad battery (even though the MB dealer says the battery is fine). My battery is about 3.5 years old. I also talked to CTEK support and they think that somehow the car drops voltage to the under the hood battery leads when it goes into some sort of sleep mode. Thanks for the help.
Last edited by EBRANGER; Jun 1, 2026 at 09:19 AM.
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I know a lot about this topic in general though have 0 experience with CTEK. I have a lot of experience with Battery Tender products but the charging phases for battery top up devices are all the same.
It is impossible for a lead acid battery (or any battery for that matter) to "drop to 0" and then magically come alive again - let alone one connected to a DC DC converter and a 48V battery. I would start with grabbing another multi-meter to verify voltage. Short of that, when your multimeter is showing 0V, unlock your car. I bet you it unlocks, because if the battery was truly 0 then your car would not be responsive.
That said, I generally am against leaving a battery top up maintainer device for "days". The float voltage is usually around 13.8V and the current maybe 0.1mA or whatever. The intent is to offset parasitic energy loss but my issue with it is the voltage for float charge is dependent on temperature. So if your car battery is colder than the tender itself, you run into a problem. On my vehicles the batteries are all inside the cabin so when the ambient temperature rises the tender itself will raise the float voltage so the tender might be in 28'C ambient temps while the battery is still at 17'C as it's cold overnight. This is neither here nor there of course
EDIT: I see you edited your post. "Almost 0" and 0 are two different things. What is it dropping to? 11.8 is one thing, 0.2 is another.
It is impossible for a lead acid battery (or any battery for that matter) to "drop to 0" and then magically come alive again - let alone one connected to a DC DC converter and a 48V battery. I would start with grabbing another multi-meter to verify voltage. Short of that, when your multimeter is showing 0V, unlock your car. I bet you it unlocks, because if the battery was truly 0 then your car would not be responsive.
That said, I generally am against leaving a battery top up maintainer device for "days". The float voltage is usually around 13.8V and the current maybe 0.1mA or whatever. The intent is to offset parasitic energy loss but my issue with it is the voltage for float charge is dependent on temperature. So if your car battery is colder than the tender itself, you run into a problem. On my vehicles the batteries are all inside the cabin so when the ambient temperature rises the tender itself will raise the float voltage so the tender might be in 28'C ambient temps while the battery is still at 17'C as it's cold overnight. This is neither here nor there of course
EDIT: I see you edited your post. "Almost 0" and 0 are two different things. What is it dropping to? 11.8 is one thing, 0.2 is another.
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1992 300E, 2023 S500, 2018 S450
Thanks @superangrypenguin for your reply. I agree with everything you said. I will check the voltage once the condition appears again. I did a double take when looking at my meter. I checked a couple times and it was something like .4, which I know seems impossible with the battery showing 12.5 later on. Can you recommend a trickle charger that I should try. Maybe my CTEK is overkill. Thanks again.
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Thanks @superangrypenguin for your reply. I agree with everything you said. I will check the voltage once the condition appears again. I did a double take when looking at my meter. I checked a couple times and it was something like .4, which I know seems impossible with the battery showing 12.5 later on. Can you recommend a trickle charger that I should try. Maybe my CTEK is overkill. Thanks again.
As far as recommendations go it starts with looking at the manual. For example on the Audi A8 it specifically says that the max voltage cannot exceed 14.8 volts. This is why I didn't use the CTEK ones because they can go up to 15.4 when they enter the desulphation mode.
On Range Rovers the maximum voltage is 13.4 because it's a lithium iron phosphate battery and there's no charger that I know that fits that criteria
I don't know what the recommendation is for w223s as a result but just keep in mind that p0 hybridization must be respected
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When you first turn on the multimeter and you switch it to the right voltage range there's a internal calibration/reset that happens. Sometimes if you just measure at the posts before calibration is done you get a weird reading and I know mine does that every once in awhile.
As far as recommendations go it starts with looking at the manual. For example on the Audi A8 it specifically says that the max voltage cannot exceed 14.8 volts. This is why I didn't use the CTEK ones because they can go up to 15.4 when they enter the desulphation mode.
On Range Rovers the maximum voltage is 13.4 because it's a lithium iron phosphate battery and there's no charger that I know that fits that criteria
I don't know what the recommendation is for w223s as a result but just keep in mind that p0 hybridization must be respected
As far as recommendations go it starts with looking at the manual. For example on the Audi A8 it specifically says that the max voltage cannot exceed 14.8 volts. This is why I didn't use the CTEK ones because they can go up to 15.4 when they enter the desulphation mode.
On Range Rovers the maximum voltage is 13.4 because it's a lithium iron phosphate battery and there's no charger that I know that fits that criteria
I don't know what the recommendation is for w223s as a result but just keep in mind that p0 hybridization must be respected
As if OP still wants an alternative, the noco genius 5 is a good option and works on both AGM and Lithium.https://mbworld.org/forums/g-class-w...enius-5-a.html
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1992 300E, 2023 S500, 2018 S450
If it is an AGM battery, set it to AGM mode, set it to AGM car (not bike) then make sure Recon is disabled and it won't go above 14.8V.
As if OP still wants an alternative, the noco genius 5 is a good option and works on both AGM and Lithium.https://mbworld.org/forums/g-class-w...enius-5-a.html
As if OP still wants an alternative, the noco genius 5 is a good option and works on both AGM and Lithium.https://mbworld.org/forums/g-class-w...enius-5-a.html
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@W205C43PFL Thanks for the info!. My S500 battery is an AGM battery. I just verified in my CTEK manual that my charger does go up to 15.8v in the desulphation state, and I can't adjust it :-(. I'm going to look at the Genius 5 specs now. Thanks!
This is what you have right?
https://shop.superbikedepot.com/cdn/...025&width=1946
https://rallyways.com/wp-content/upl...ek_mxs_5_0.jpg
Edit: Press mode buttons until, the four leds at the bottom shows 2 leds illuminated, basically make sure the car is selected, AGM is selected and recond is not illuminated.
https://cimg1.ibsrv.net/gimg/www.mbw...2dc5e18995.png
Last edited by W205C43PFL; Jun 1, 2026 at 03:36 PM.
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From: Virginia
1992 300E, 2023 S500, 2018 S450
You're welcome but strange, MXS 5.0 should allow you to turn off Recond.
This is what you have right?
https://shop.superbikedepot.com/cdn/...025&width=1946
https://rallyways.com/wp-content/upl...ek_mxs_5_0.jpg
Edit: Press mode buttons until, the four leds at the bottom shows 2 leds illuminated, basically make sure the car is selected, AGM is selected and recond is not illuminated.
https://cimg1.ibsrv.net/gimg/www.mbw...2dc5e18995.png
This is what you have right?
https://shop.superbikedepot.com/cdn/...025&width=1946
https://rallyways.com/wp-content/upl...ek_mxs_5_0.jpg
Edit: Press mode buttons until, the four leds at the bottom shows 2 leds illuminated, basically make sure the car is selected, AGM is selected and recond is not illuminated.
https://cimg1.ibsrv.net/gimg/www.mbw...2dc5e18995.png
https://www.ctek.com/storage/AEB64A2...CPpC3roznG29Zn
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2025 AMG GT63, 2023 S500, 2022 GLE53
If the app is constantly telling you partially charged, I would bet that your battery is bad. Mine was in my 2023 and was replaced a few months ago at the dealer under warranty. You should go and insist that you're having voltage and partially charged issues with your battery before your warranty expires.
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The other thing to keep in mind is because your/our cars are P0 MHEVs, the likelihood of the 12V AGM dying prematurely is exceptionally low. These cars are wired to keep the 12V battery full via the DC DC converter while sacrificing the 48V battery. Why? I have no idea, other than the 4:1 "rule". For a 48V battery to step down voltage via DCDC converter to keep a 12V battery full is chump change (energy wise).
When I replaced the 12V battery on my A8 at the 6 year mark, it was because I'm paranoid. It started in the winter just fine. That said mine's a garage queen. It is worth getting the 12V tested by Medtronics or whatever name it is for battery testing that the German 3 automakers use at the dealership. I know Mercedes uses the same ones as Audi
When I replaced the 12V battery on my A8 at the 6 year mark, it was because I'm paranoid. It started in the winter just fine. That said mine's a garage queen. It is worth getting the 12V tested by Medtronics or whatever name it is for battery testing that the German 3 automakers use at the dealership. I know Mercedes uses the same ones as Audi
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From: Virginia
1992 300E, 2023 S500, 2018 S450
If the app is constantly telling you partially charged, I would bet that your battery is bad. Mine was in my 2023 and was replaced a few months ago at the dealer under warranty. You should go and insist that you're having voltage and partially charged issues with your battery before your warranty expires.
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@W205C43PFL Yes. That is my charger and I have been using the AGM mode as you stated, meaning the Auto and AGM lights are lit. I am not using the Recond setting. The MXS 5.0 goes through each phase (1-8) when I connect it to the battery and the first one shows 15.8v during the desulphation phase. Here's a link to the manual that shows the voltages during each phase (page 11).
https://www.ctek.com/storage/AEB64A2...CPpC3roznG29Zn
https://www.ctek.com/storage/AEB64A2...CPpC3roznG29Zn
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OP, I read a few posts of forum members posting about the MXS 5.0 although they are for different model MBs but basically the consensus is it will jump to stage 2 if it does not need stage 1. Basically what they did was not plug the charger in first and plug the positive side first and then negative side second then plug the charger to the power outlet, it will start at stage 2 if necessary. Is that the behaviour of your CTEK or is it broken? https://www.mercedes-benz.ca/en/owne...e-12-v-battery
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