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Discussion Starter · #1 ·
Is there an easy way to hook up a multi-meter to get a voltage reading on the 12v battery? I looked around and couldn't find any simple place to do it. It looks to be just as easy to tear up the passenger side floor and get the reading right off of the battery itself. I know that some of you routed wiring through to the front "hood" flap, but other than that is the battery itself the best option?

Len
2014 EV Coupe 20,000 miles
2014 EV Cabriolet 17,500 miles
 

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Discussion Starter · #4 ·
With the ICE cars, no big deal - I'd probably wait until the battery tells me something is wrong. With the EV a huge deal because of the bricking issue and a likely repair cost twice the value of the car. I posted this in the Electric Drive forum specifically for that reason. Just had a member where a 12v battery test on his EV may have saved him $$$$. We'll post those details in a few days.

Len
2014 EV Coupe 20,000 miles
2014 EV Cabriolet 17,500 miles
 

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Is there an easy way to hook up a multi-meter to get a voltage reading on the 12v battery? I looked around and couldn't find any simple place to do it. It looks to be just as easy to tear up the passenger side floor and get the reading right off of the battery itself. I know that some of you routed wiring through to the front "hood" flap, but other than that is the battery itself the best option
Routing a pair of wires from the battery to the hood flap is an easy job - and provides the ability to charge a dead battery with the car locked. This is important because from what I can tell, the backup key lock on the driver's side door is not a mechanical lock - it actuates an electric solenoid (same as remote unlock) to operate the lock. So getting into the car with a dead battery is not easy. There is a rubber gland in the firewall near the steering column for routing the charging/test wires. No hole drilling is needed.
 

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Discussion Starter · #6 ·
Thanks Yinzer. Were you also the one who talked about attaching a 12v battery monitor?

Len
2014 EV Coupe 20,000 miles
2014 EV Cabriolet 17,500 miles
 

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I have these plugged into the 12v outlet:


They only turn on when I turn the key to position 1 (at least in the 450s and 453) and when the car is running.
It should display 12.x volts or higher and display 14.4 volts when the ICE car is running to show the alternator is working.
It also shows how much voltage drop there is when you start the car ~1 volt to 11.x volts and surprisingly the biggest drop I notice is when you shift gears, ~1.5 volts.
I would think the initial voltage at least would show on an EQ, but the temperature display might be useful as well.

All for $9 each.
 

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Discussion Starter · #8 ·
The problem with that unit on the electrics is that as soon as you turn the key to position 1, the contactor closes and the HV battery connects with the 12v battery. Maybe the instant you turn it on, you might get an accurate reading, but you really want a reading with the HV battery out of the picture.

Len
2014 EV Coupe 20,000 miles
2014 EV Cabriolet 17,500 miles
 

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I put a jumper on the battery with a 2 pin quick detach plug that I got from Harley Davidson and have it come out the carpeting near the passenger door. Normally I keep a long extension run under the seat and plug in trickle charger if I don't drive it for several days. I have come out on four occasions in the last couple of months to find a dead 12v battery (between 3-5 volts each time). Can't seem to locate what is draining the battery and it happens randomly.
 

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Discussion Starter · #10 ·
BDFuqua - do you have an ICE or an EV? My guess is ICE, because if the 12v battery was doing that in an EV, I think you might have a brick by now.

Len
2014 EV Coupe 20,000 miles
2014 EV Cabriolet 17,500 miles
 

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Len,

It is a 2014 EV. Only has 6600 miles. Don't drive it much and now I keep a 12 V battery tender on it if I don't plan on using it for a day or two. I have panicked a few times opening the door and finding an apparently totally dead car but when I put a 15 amp charger on the battery it comes to life and the HV battery had not bricked. One 12 V battery replacement only lasted two weeks before I exchanged it after it was damaged from being drained so low. I can see a 1 amp draw from time to time but really don't hear anything turning on or off while I am next to the car. I have put split loom on a lot of the wiring to avoid the chafing issues others have had so I'm still stumped what is pulling battery down.
 

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Routing a pair of wires from the battery to the hood flap is an easy job - and provides the ability to charge a dead battery with the car locked. This is important because from what I can tell, the backup key lock on the driver's side door is not a mechanical lock - it actuates an electric solenoid (same as remote unlock) to operate the lock. So getting into the car with a dead battery is not easy. There is a rubber gland in the firewall near the steering column for routing the charging/test wires. No hole drilling is needed.
The driver door lock should be mechanical. I've been locked out several times with a dead 12V battery and had to use key to get in. I didn't bother to put the cap back on the lock since I've had to use it so many times.
 

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Discussion Starter · #13 ·
Wow - that's amazing. I'd be panicking too. I hope you have the latest service campaigns done. Some of us think that the latest one might help protect the HV against bricking. So are you also seeing a phantom drain of the charge on your HV battery if you let the car sit? If not, I wonder why the HV battery isn't trying to charge the 12v battery? Did you happen to read the thread I posted yesterday about the 12v battery issue another forum member had?

Len
2014 EV Coupe 20,000 miles
2014 EV Cabriolet 17,500 miles
 

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Hi, I’ve been trying to work out an easy access as well, the ED is very eager to switch the DC/DC on at all opportunities. As well as adding wires from the battery there are two possibilities I’ve found so far. If you don’t have heated seats then the last spare constant live accessories fuse on the SAM should be free. All five of mine are occupied so option 2 is pin 16 on the OBD socket with pins 4&5 as earth, I was planning on making a test lead up as I have three 450s and a 452 to check as well, the plugs are available on eBay for not much.

You can also get a key off battery check with an MB Star from the SAM/Actual Values section, only one decimal place though but it did match my multimeter.


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I don't see any phantom drain on the HV battery. I have let it sit weeks at a time without losing any appreciable charge. I leave it sitting at variable states of charge, but not below 40%. I bought it used in 2016 with 200 miles on it and have never taken it to a dealer. I have no clue what service campaigns have been done to the car.
 

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Discussion Starter · #16 ·
A cooperating dealer can tell you if your car has any outstanding service campaigns. All they need is the VIN. I think any Mercedes dealer can look up that info for you, but I believe that only a former Smart dealer can actually do the campaign - not positive about that. I would highly recommend that you find out and if a former Smart dealer is anywhere near you, I'd sure have it done - it's free.

Len
2014 EV Coupe 20,000 miles
2014 EV Cabriolet 17,500 miles
 

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I keep a small digital voltmeter in the 12v socket. I push it all the way into the socket and then turn the car key to first position. Momentarily it reads 12.4v then jumping to 14.4v as the DC-DC turns itself on. So that means the 12v battery is OK since some of the car's electrics are loading it. If the car has been unused for a few weeks the reading is like 12.2v which is not surprising with a 5 year old battery. But with this aging 12v battery my plan is to disconnect the 12v when I plan to be away for more than several days.

VR. PNW USA
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