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Registered 1997 SLK230
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Discussion Starter #1
Apologies in advance if that has already been covered...

but does anyone know what the drain current should be (roughly) for my R170?

It's an early model, no extras, and it has a Mercedes fitted (but I guess 'after market') alarm

I've got a new and an old battery, they both go flat in less than a week - I think the alternator is okay

I tested the current drain today... initially it's over 300ma... then it drops to 200ma, and then it seems to settle at 150ma - I left it for 5 minutes maybe I should leave it longer?

anyway - is 150ma to high or normal?

I suspect the alarm, it's a daft infra red thing, must take current... if I knew how to disconnect it, I would!

TIA
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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Premium Member 2001 SLK320/2000 SLK230
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After everything settles out it should be less than 50ma (20 ma in most of mine). Should be fine for a couple of weeks. Note: MB in general are very sensitive to battery condition. I buy only AGMs.

150ma is three times the normal limit. If an aftermarket alarm I'd disconnect that first otherwise may just need to pull fuses (with key off and out) until it goes away.
 

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Premium Member 2001 SLK320/2000 SLK230
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I just use a little DC clamp on ammeter here clamped on negative battery cable (.03A = 30ma) - does not matter if on positive or negative side/ do not care about polarity. Down this low is just ballpark (+/- 10 ma) but all I need for a quick check. For more accuracy would need to break the circuit and use a wired ammeter
 

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Not much more to add really. Padgett is right with the normal standing current draw of less than 50mA and a good charged battery should be alright for 5 maybe 6 weeks at these levels. @Turdo2 there is no drain voltage as such. The voltage drives the current so the lower the voltage gets the less current will flow.
 

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Super Moderator UK 2002 SLK320 Blue
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I've got a new and an old battery, they both go flat in less than a week - I think the alternator is okay
That strikes me as a fairly substantial drain if you're draining a car battery in under a week - probably more than the 150mA you measured!
@Padgett is right in his estimates of car current draw in a 'sleeping' car, so your drain should be falling to 20-50mA after the car is fully asleep (about 20mins) It's difficult to check this once the car is asleep as any opening of doors etc. would wake the car up again. That said, it's worth checking to make sure that:
  • the inside dome lamps are off (check the boot/trunk lamp too)
  • check the parking lamps.
  • Is your radio turned off?
  • Have you wired in a dashcam or similar?
 

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Premium Member 2001 SLK320/2000 SLK230
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" It's difficult to check this once the car is asleep as any opening of doors etc. would wake the car up again. "

Not really, just connect DMM to battery lead on 10A scale and leave hood open for a 1/2 hour. Just do no do ANYTHING else until you restore the cable. With a clamp on I just reached in, opened the hood, and clamped.
 

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While it could be several different things, the usual suspects are the alarm system and audio. Pulling fuses while measuring draw is an easy way to test (as described above), Make sure you have your radio code prior to pulling the radio fuse however. I store mine in my phone number contact list.
 

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Registered 1997 SLK230
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135 Posts
Discussion Starter #12
Thanks for all the input, much appreciated

I obviously need to leave the meter on for longer... when it stops raining I'll do just that and report back.

At night you can see two red neons blinking in the car, one is the (standard) CD player, the other is the (chief suspect) alarm. There's no aircon, no other things like dashcams, and I've checked the glove box light and boot light.

I have thought about the battery conditioner... however... having had a mate blow up the computer in his van and having to fork out hundreds of pounds to get a new one, I am somewhat nervous about attaching any charger to the car while the battery is still 'live' to the vehicle.
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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How did u check to see if the boot light stayed on? :D
did u put your smartphone camera in there or crawl inside? :D
 

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Registered 1997 SLK230
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Discussion Starter #14
No... I took the bulb out! I never drive it in the dark anyway!

I did have a funny creaking noise recently, I did leave my camera recording video in the boot with a torch on and went for a drive... needless to say this did not really help.
 

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Registered 1998 SLK230
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A good and accurate method to monitor current draw is to use an appropriate multi-meter. A "Fluke" 77 model has two ampere ranges : 10 A and 300 mA. Fit leads with crocodile clamps to the multi-meter. Disconnect the battery live terminal and connect the multi-meter in series between the lead and battery terminal.
I wrap polythene sheet around all connections and secure it with insulating tape.Then, using the current, 300 mA connection terminal and V DC range, you have a direct and continuous display of current drain. The meter can be switched-off overnight and current then monitored when required. Other "Fluke" models may not have this mA range, so it's important to get the correct version. I found that my radio was still drawing current, even with the ignition key removed!

David
 

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Registered 1997 SLK230
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135 Posts
Discussion Starter #16
Okay... I braved the cold wind and performed a proper test on the drain current, being less impatient this time!

Initially the current goes shooting up to more than my 300ma scale, but rapidly drops back down to 200ma, then after a few seconds drops to 150ma (as reported before)

But after 20 minutes or so I checked again and it had fallen to about 40ma, just under the 50ma limit mentioned in previous posts.

So... maybe I have a bad battery, it's quite new but maybe faulty I guess, a garage would tell me I suppose.

(Unless anyone knows a way to test a car battery using a multimeter?)

Thanks again for all the input on this
 
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