SL/R230: CONSUMER BATTERY VOLTAGE DRAINAGE SOLVED.
Thread Starter
Senior Member
Joined: Oct 2021
Posts: 493
Likes: 101
From: Benton, Louisiana, USA
SL 500 2004, Ford F-150-2014
CONSUMER BATTERY VOLTAGE DRAINAGE SOLVED.
Guys, I though I share this with you....My SL is not a daily driver and sometimes sits unused for a couple of weeks. I used to have a blade type disconnect to prevent drainage. Until I found a magic relay called smart battery relay.
The device is sold in Amazon for $15 and it is amazing. It connects to the negative pole, with a small cable going to the + pole. When the voltage reaches 11.8 volts, it automatically disconnects the ground pole. So, it comes with a little remote
to connect or disconnect the battery from the - pole. When parked for a long time, I just push the ON button and power is restored and voltage is high enough to energize the necessary components. Also, it can be used as an anti-theft device since you can purposely tun it off and no one should be able to open or start the car without opening the hood! I also got another one in my Corvette and works great, since my daily driver is a Ford Truck.
The device is sold in Amazon for $15 and it is amazing. It connects to the negative pole, with a small cable going to the + pole. When the voltage reaches 11.8 volts, it automatically disconnects the ground pole. So, it comes with a little remote
to connect or disconnect the battery from the - pole. When parked for a long time, I just push the ON button and power is restored and voltage is high enough to energize the necessary components. Also, it can be used as an anti-theft device since you can purposely tun it off and no one should be able to open or start the car without opening the hood! I also got another one in my Corvette and works great, since my daily driver is a Ford Truck.
MBWorld Fanatic!
Joined: Mar 2018
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From: Netherlands
2005 R 230 SL350 (M112 3.7). Sold the 1966 W113 230SL recently
You mention 'the hood'. Is that device on your starter battery? You must never let the accessories battery be connected with the starter battery disconnected.
Super Member
Joined: Sep 2021
Posts: 534
Likes: 115
From: Ann Arbor, MI
2004 SL600
These are easier to find on Amazon if you search for "remote battery disconnect switch".
They sound like a good idea if the car is going to be parked for more than 2 weeks in a location where there is no 110V power for a trickle charger. And you don't want the hassle of a blade type disconnect.
They claim under 10mA draw which means a healthy battery should be OK for well over 6 months.
They sound like a good idea if the car is going to be parked for more than 2 weeks in a location where there is no 110V power for a trickle charger. And you don't want the hassle of a blade type disconnect.
They claim under 10mA draw which means a healthy battery should be OK for well over 6 months.
Junior Member
Joined: Sep 2020
Posts: 44
Likes: 10
2009 SL550
Over the 5 month winter storage I sometimes use 2 trickle chargers, one for starter and one for the consumer battery, only the consumer battery is essential though, starter battery doesn't discharge, or just a bit over the storage period.
Last edited by tallguy2020; Jan 11, 2026 at 07:21 PM.
Newbie
Joined: Oct 2024
Posts: 14
Likes: 1
SL600 R230 2004
[QUOTE=elMacko;9251809] Thanks for sharing! Sounds like an appealing solution. I winter store my SL600 R230 in a parking garage without assess to electricity, so I try to take it out bi-weekly when weather allows it. But the trips often start with low battery warning and electric consumers disabled. Are there any downsides in disconnecting the consumer battery? I have 2 things in mind of potential concerns.
1) I think that if the voltage is too low (something like 10.8 V) it will start to draw power from the starter battery at least when running? Could this be drained by deactivating the consumer battery?
2) Does cutting the Consumer battery power affects any of the electronics in a negative way?
1) I think that if the voltage is too low (something like 10.8 V) it will start to draw power from the starter battery at least when running? Could this be drained by deactivating the consumer battery?
2) Does cutting the Consumer battery power affects any of the electronics in a negative way?
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Senior Member
Joined: Nov 2021
Posts: 469
Likes: 119
From: North West England
SL500
My 2p's worth...
When you disconnect/reconnect the battery what do you usually see/experience?
I see a spark, jumping 5-10mm if I am sloppy!
In air it takes approximately 1000v (1kV) to jump 1mm, so that spark is between 5kV and 10kV in magnitude (admittedly with little energy)..
Now I do not want to subject the cars electronics to these voltages any more than absolutely necessary, so I only disconnect a battery if I absolutely must.
I am an electronic engineer, and take great precautions when working on my gear to avoid static, which is a similar voltage, though I presume there is plenty of protection built into the ECU's around the car, but I am brought up on being over cautious.
When you disconnect/reconnect the battery what do you usually see/experience?
I see a spark, jumping 5-10mm if I am sloppy!
In air it takes approximately 1000v (1kV) to jump 1mm, so that spark is between 5kV and 10kV in magnitude (admittedly with little energy)..
Now I do not want to subject the cars electronics to these voltages any more than absolutely necessary, so I only disconnect a battery if I absolutely must.
I am an electronic engineer, and take great precautions when working on my gear to avoid static, which is a similar voltage, though I presume there is plenty of protection built into the ECU's around the car, but I am brought up on being over cautious.
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Junior Member
Joined: Sep 2020
Posts: 44
Likes: 10
2009 SL550
My 2p's worth...
When you disconnect/reconnect the battery what do you usually see/experience?
I see a spark, jumping 5-10mm if I am sloppy!
In air it takes approximately 1000v (1kV) to jump 1mm, so that spark is between 5kV and 10kV in magnitude (admittedly with little energy)..
Now I do not want to subject the cars electronics to these voltages any more than absolutely necessary, so I only disconnect a battery if I absolutely must.
I am an electronic engineer, and take great precautions when working on my gear to avoid static, which is a similar voltage, though I presume there is plenty of protection built into the ECU's around the car, but I am brought up on being over cautious.
When you disconnect/reconnect the battery what do you usually see/experience?
I see a spark, jumping 5-10mm if I am sloppy!
In air it takes approximately 1000v (1kV) to jump 1mm, so that spark is between 5kV and 10kV in magnitude (admittedly with little energy)..
Now I do not want to subject the cars electronics to these voltages any more than absolutely necessary, so I only disconnect a battery if I absolutely must.
I am an electronic engineer, and take great precautions when working on my gear to avoid static, which is a similar voltage, though I presume there is plenty of protection built into the ECU's around the car, but I am brought up on being over cautious.
To prevent sparks when reconnecting a battery, connect the positive (+) terminal first, then the negative (-) last, ensuring your tool doesn't touch ground, and use a pre-charge resistor (or even a pencil/light bulb) briefly in series with the negative cable to slow current to charging capacitors, especially for electronics or inverters, then connect firmly and cleanly.
Super Member
Joined: Sep 2021
Posts: 534
Likes: 115
From: Ann Arbor, MI
2004 SL600
My SL600 sometime ends up stored in a warehouse or even outdoors for weeks at at time where there is no electricity. Generally I delegate storing the car and then nothing gets disconnected and the rear battery dies. Couple times the front battery died too. If only the rear is dead the car will start. If both are dead, I place quality jump packs (Noco) on both batteries and never had a problem. Windows need to be calibrated by running them down and up; sometime ESP error until I turn the wheel enough.
Yes, I know its not recommended to jump the rear battery, but never a problem with modern computer controlled jump packs. I would avoid those old heavy roll-up jump starters as they can easily feed 20+ volts into the cars.
I also experiment and work on the car a lot and often have both batteries disconnected. Barely takes 60 seconds to get everything working again.
When possible I have a trickle charger on the rear battery.
I like the idea of using a "pre-charge resister" to reduce the arching when reconnecting a battery.
Yes, I know its not recommended to jump the rear battery, but never a problem with modern computer controlled jump packs. I would avoid those old heavy roll-up jump starters as they can easily feed 20+ volts into the cars.
I also experiment and work on the car a lot and often have both batteries disconnected. Barely takes 60 seconds to get everything working again.
When possible I have a trickle charger on the rear battery.
I like the idea of using a "pre-charge resister" to reduce the arching when reconnecting a battery.
Junior Member
Joined: Sep 2020
Posts: 44
Likes: 10
2009 SL550
My SL600 sometime ends up stored in a warehouse or even outdoors for weeks at at time where there is no electricity. Generally I delegate storing the car and then nothing gets disconnected and the rear battery dies. Couple times the front battery died too. If only the rear is dead the car will start. If both are dead, I place quality jump packs (Noco) on both batteries and never had a problem. Windows need to be calibrated by running them down and up; sometime ESP error until I turn the wheel enough.
Yes, I know its not recommended to jump the rear battery, but never a problem with modern computer controlled jump packs. I would avoid those old heavy roll-up jump starters as they can easily feed 20+ volts into the cars.
I also experiment and work on the car a lot and often have both batteries disconnected. Barely takes 60 seconds to get everything working again.
When possible I have a trickle charger on the rear battery.
I like the idea of using a "pre-charge resister" to reduce the arching when reconnecting a battery.
Yes, I know its not recommended to jump the rear battery, but never a problem with modern computer controlled jump packs. I would avoid those old heavy roll-up jump starters as they can easily feed 20+ volts into the cars.
I also experiment and work on the car a lot and often have both batteries disconnected. Barely takes 60 seconds to get everything working again.
When possible I have a trickle charger on the rear battery.
I like the idea of using a "pre-charge resister" to reduce the arching when reconnecting a battery.
I could not imagine owning a R230 myself without a garage with access to electricity, in my use case I mean, 2009 (18 year old vehicle) only used during the summer and mostly weekends so always on a trickle charger.
Just me of course but man, are these vehicles complex. No issue if its a daily driver of course, but then we are looking at the retractable roof, the water, it never ends! ;-)
Senior Member
Joined: Nov 2021
Posts: 469
Likes: 119
From: North West England
SL500
To prevent that spark do this:
To prevent sparks when reconnecting a battery, connect the positive (+) terminal first, then the negative (-) last, ensuring your tool doesn't touch ground, and use a pre-charge resistor (or even a pencil/light bulb) briefly in series with the negative cable to slow current to charging capacitors, especially for electronics or inverters, then connect firmly and cleanly.
To prevent sparks when reconnecting a battery, connect the positive (+) terminal first, then the negative (-) last, ensuring your tool doesn't touch ground, and use a pre-charge resistor (or even a pencil/light bulb) briefly in series with the negative cable to slow current to charging capacitors, especially for electronics or inverters, then connect firmly and cleanly.
However it will not prevent the sparking when connecting the negative terminal.
Pre-charging sounds a good idea, will try it next time and see but in reality who would do this everytime?
Super Member
Joined: Sep 2021
Posts: 534
Likes: 115
From: Ann Arbor, MI
2004 SL600
Related to this thread, I now use the "Power Probe PPDRAW Parasitic Drain Monitor" for both researching battery drain on cars and keeping the car "On" when the battery is disconnected.
The PPDRAW has in internal lithium 12V battery which can power the car with up to 10 amps when connected to the OBD2 connector.
This way no modules lose power when the (main/consumer) battery is swapped out. Windows, radio, etc do not have to be reset.
The PPDRAW displays how many mA are being drawn when everything is off as a parasitic drain tester.
The optional PPFUSE probe can then be connected to the little pads on each fuse to estimate how much current is going through the fuse. This way each fuse can be tested without removed it, as removing a fuse can signal other modules to turn back on.
Obviously this is not a solution for storage, just for testing and swapping batteries.
The PPDRAW has in internal lithium 12V battery which can power the car with up to 10 amps when connected to the OBD2 connector.
This way no modules lose power when the (main/consumer) battery is swapped out. Windows, radio, etc do not have to be reset.
The PPDRAW displays how many mA are being drawn when everything is off as a parasitic drain tester.
The optional PPFUSE probe can then be connected to the little pads on each fuse to estimate how much current is going through the fuse. This way each fuse can be tested without removed it, as removing a fuse can signal other modules to turn back on.
Obviously this is not a solution for storage, just for testing and swapping batteries.
Super Member
Joined: Sep 2021
Posts: 534
Likes: 115
From: Ann Arbor, MI
2004 SL600
We use these at my auto shops so modules and settings don't loose power when we temporarily disconnect the main battery, either for replacement or other reasons. We have had a few modules (not on MB) go dead when they lost power.
Again, not a solution for long term storage, but just FYI.
