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Discussion Starter · #1 ·
Maybe I have too much time on my hands, but I wanted to do some comparisons among my 3 Smart EV's and get some questions answered. I have one with the original battery in it and it is over seven years old - is it still good? We think the test unit JimSokoloff built seems to read a bit low and I wanted to confirm that. The volt meter that plugs into the "cigarette' lighter gives me a very quick 12v reading when I turn the car on - how close is that reading to the actual multimeter reading? And generally I wanted to see if anything bad pops up.


So I did a test/reading at about a 20% charge. Then did one right after I charged to 100%. Then did another test/reading after letting the car sit for two days at 100%. I did this on all three cars to see if anything stood out.


Answers - I don't see anything of real concern which is good news. The seven year old battery still reads okay. Do I live dangerously or do I change it out? The test unit does read about 0.2v low - why I don't know. The volt meter is somewhat questionable on the initial 12v reading. If any of you see anything of concern or have comments, please feel free to share them. See attached test data.

Len
2014 Coupe - 21,939 miles
2014 Cabriolet - 23,038 miles
2015 Coupe - 20,518 miles
 

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Wouldn't the car's main battery begin charging the 12V battery as soon as you "turn the car on"? But in that case I would expect to see a charging voltage like 13+ volts, so maybe I'm confused.
 

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With our test unit, we can get a reading from only the 12v battery or from the HV battery. To get a reading from the 12v battery, we don't turn the car on. We only click the door lock or unlock button. That lets the test unit read the 12v battery voltage with the HV battery out of the process. This is done with the key not in. If we put the key in and turn the car on, then "yes," we immediately get the HV battery start charging the 12v battery and it will typically read close to 14v.

And using the volt meter in the "cigarette" lighter, the HV battery is in play and will start charging the 12v battery as soon as the key is turned on. Thus the reading will again be near 14v for the 12v battery. However, I believe that I get a reading of the 12v battery if I'm looking right at the gauge when I turn the car on, because it will give me one or more readings at 12+. If you look at my attachment above, you will see two volt meter readings. The first one is what I believe the 12v battery is before the HV battery starts charging it. The second reading is what the 12v battery is once the HV battery has kicked in and starts charging.

Is that clear?

Len
 

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Len -

My battery initially shows a reading of 12.93 volts, before any actions, measured directly across the terminals.
As soon as I clicked on the unlock button on the key fob, the voltage dropped to 12.55 volts.
The only thing to seem to come on were the courtesy lights. Well, once a fan or pump kicked on for a few seconds. Eventually, in less than 3 or 4 minutes, the voltage had climbed back up to 12.89 volts.

This was repeated using the trunk unlock button, or just opening a door or the trunk. Same results, about .5 to .75 v drop before climbing back to the 12.8-12.9 v range.

Is this what you see too?
 

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Discussion Starter · #5 ·
Thanks for the reply Jeb. Maybe the lock/unlock button turning on a few things is why the 12v battery reading is about 0.2v low on the test unit. Makes sense. I've never had a 12v battery read as high as your 12.93v. Even with nothing on and going directly from the battery terminals, you can see I did get a 12.8v reading, but the other two were lower no matter if the car was at a 20% charge or right after it was fully charged.

Len
 
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