Battery drain in cold weather

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55AMGCLK

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Hi all

As the weather has turned cold, the car battery has started to drain pretty quickly and after just couple of days the car struggles to start and I have a blank dash.

Battery and alternator have been tested and are fine. I had the same issue last winter and had to disconnect the battery to save it from draining.

Any ideas as to why this would only happen in winter? Car can be left a couple of weeks without use in summer with no issues at all. This kind of rules out the usual suspects such as seat/door controllers and alarm system.
 
That's a really odd one.... My immediate thought from the title was that its just the battery dying - usual thing to happen to an old battery when the weather turns colder. Also the motor is harder to turn over the colder it gets further adding to the problem. But, as you say it stays up fine if you disconnect it (assuming its still outside on the car), that kinda rules out the battery, as also confirmed by the test. So it sounds like you have some system which only drains juice when it gets cold... Only thing that comes to my mind is that I know some Mercs have a separate heater system which runs off the battery - and also a residual temp facility on the heater system. Maybe one of those has gone rogue and is kicking in when it gets really cold - or possibly a heated screen\mirror coming on when temp drops? I think you are going to have to do some current draw tests on the battery to ground and across the fuse terminals to try and identify where it's leaking juice... I feel your pain as I have parasitic drain on my R129 - but at least its not temperature sensitive and I'm managing to narrow it down..
 
You could try pulling the radio fuse. My 07 GL and 07 CLK 63 had the same problem and it turned out to be the head unit CD constantly spinning up. I also use a CTEK trickle charger....
 
The inherent internal drain of a healthy battery is lower during the winter so there is a problem.

Either the car is drawing current and running the battery down which you could measure, or it could be the battery is failing as the internal battery drain does appear to increase when a battery is on it's last legs.
 
If your battery is old or weak it will already have reduced performance but may have been masked by the recent wamer weather.

When it is cold the chemical process that produces and effectively stores electrical- energy inside the battery is slowed down.

Temperature when cold also reduces the ability of car batteries to hold a charge.

Cold temperatures quickly show slower charging rates then once charged - holding onto charge for long durations, etc.

Whenever I replace batteries, always seek out the CCA (think it means Cold Cranking Amperage) for my diesels as the effect of temperature, thicker oil when colder and generally colder components require more of a "kick" from the battery. (Rather like me in the winter from the wife)
 
That's a really odd one.... My immediate thought from the title was that its just the battery dying - usual thing to happen to an old battery when the weather turns colder. Also the motor is harder to turn over the colder it gets further adding to the problem. But, as you say it stays up fine if you disconnect it (assuming its still outside on the car), that kinda rules out the battery, as also confirmed by the test. So it sounds like you have some system which only drains juice when it gets cold... Only thing that comes to my mind is that I know some Mercs have a separate heater system which runs off the battery - and also a residual temp facility on the heater system. Maybe one of those has gone rogue and is kicking in when it gets really cold - or possibly a heated screen\mirror coming on when temp drops? I think you are going to have to do some current draw tests on the battery to ground and across the fuse terminals to try and identify where it's leaking juice... I feel your pain as I have parasitic drain on my R129 - but at least its not temperature sensitive and I'm managing to narrow it down..

That is a good line of thought. My car does have a the heater system which will keep the car warm for period of time after switching it off, to activate this, you click on the rest button. Maybe it is this system which is playing up. I have no idea how to isolate this though. If anyone can shed any light on how to turn this off for troubleshooting, that would be great!
 
You could try pulling the radio fuse. My 07 GL and 07 CLK 63 had the same problem and it turned out to be the head unit CD constantly spinning up. I also use a CTEK trickle charger....

I have just ordered a CTEK mxs 5. Great price in Amazon at the moment £56 delivered if anyone is after one.

I will try a the radio fuse, will look up which one it is but the reason I have not tried this is as the issue only occurs in winter.
 
The inherent internal drain of a healthy battery is lower during the winter so there is a problem.

Either the car is drawing current and running the battery down which you could measure, or it could be the battery is failing as the internal battery drain does appear to increase when a battery is on it's last legs.

I did have the battery and alternator tested and all was fine. In addition, I did disconnect the battery and found very minimal discharge over 2 days. I do think something is draining the battery.
 
I was thinking of getting one of these to do some testing:


Does anyone have any experience on how good these are or any recommendations?
 
No dash cam or other accessories fitted?
 
I did have the battery and alternator tested and all was fine. In addition, I did disconnect the battery and found very minimal discharge over 2 days. I do think something is draining the battery.

How was the battery tested? Was it load tested off the car and checked against the CCA? ( I had a fully charged battery showing 12.6v tested and it failed the CCA test and was replaced)
If it was then it does point to a battery drain. Good luck pulling fuses.
 
How was the battery tested? Was it load tested off the car and checked against the CCA? ( I had a fully charged battery showing 12.6v tested and it failed the CCA test and was replaced)
If it was then it does point to a battery drain. Good luck pulling fuses.

I had the battery tested by Kwik Fit, I was expecting them to say I need a new battery and try and sell me one but they said all of their tests indicated the battery and alternator were fine. I was hoping it was just a faulty battery.
 
I had the battery tested by Kwik Fit, I was expecting them to say I need a new battery and try and sell me one but they said all of their tests indicated the battery and alternator were fine. I was hoping it was just a faulty battery.

Just to add, the mechanic I use also tested it and came to the same conclusion.
 
That is a good line of thought. My car does have a the heater system which will keep the car warm for period of time after switching it off, to activate this, you click on the rest button. Maybe it is this system which is playing up. I have no idea how to isolate this though. If anyone can shed any light on how to turn this off for troubleshooting, that would be great!
There will be a fuse for it. On the R129 it's fuse #10 in the engine bay fuse box. Yours will likely be different - take a look in your handbook, or look inside the fuse box, there'll prolly be a legend inside the cover. From general reading around on battery drain when parked up you shouldn't be getting much more than 40 - 80mA draw, that to measured between negative battery terminal and ground with the negative ground strap disconnected. Mine is pulling about 250 - 300mA, enough to pull it down below 12.6V after about 3 - 4 days. My problem seems to be the IR remote control module. The fuse current tester you point to later is a good cheap solution for testing the later blade terminal type fuse holders.
 
There will be a fuse for it. On the R129 it's fuse #10 in the engine bay fuse box. Yours will likely be different - take a look in your handbook, or look inside the fuse box, there'll prolly be a legend inside the cover. From general reading around on battery drain when parked up you shouldn't be getting much more than 40 - 80mA draw, that to measured between negative battery terminal and ground with the negative ground strap disconnected. Mine is pulling about 250 - 300mA, enough to pull it down below 12.6V after about 3 - 4 days. My problem seems to be the IR remote control module. The fuse current tester you point to later is a good cheap solution for testing the later blade terminal type fuse holders.

Good luck in fixing yours, I will find and take the fuse out and see if it makes any difference. I will measure the daily drain beforehand. Just need it to stop raining for a bit so I can fully charge the battery beforehand.
 
I was thinking of getting one of these to do some testing:


Does anyone have any experience on how good these are or any recommendations?
I have one of these, definitely worth having & accurate
 
I had the battery tested again, results look ok apart from the starter test which I thought should be around 11v. Look forward to any thoughts...

D4C64CCA-447E-4FD6-9A7D-4C829CDA9F20.jpeg
 
That looks like a report from a device that injects an AC waveform into the battery and checks the impedence. In my experience a load test is a more accurate method of determining battery condition.
Also the charging test looks to be low. 13.7-14.3 used to be the acceptance criteria but these days I would look at anything under 14.0 volts with a little suspicion.
Also, if it is a wet battery, I would check the s.g. with a hydrometer to determine the state of charge.
That should give you a good idea of battery condition/state of charge.
 

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