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E90/E91/E92/E93 (2006 - 2013)
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  #1  
Old 03-07-2013, 02:59 PM
Ilovemycar Ilovemycar is offline
wooohooo
Location: CA
 
Join Date: Feb 2010
Posts: 2,742
Mein Auto: '08 335i coupe 6MT sport
My battery voltage readings

Anyone want to do a palm reading on my battery? (Some readings might be abnormally high; is my multimeter worthless?) It's the original to the best of my knowledge, over 5 y.o., I believe AGM.

With all of the recent battery threads, I thought I should get my own. I'm just kidding, I was actually hesitant at first, but I didn't know which thread to hijack. So my multimeter is a RS cheapo that I got a few years ago for a different hobby. It looks just like this, and I'm pretty sure this is it:

http://www.radioshack.com/product/in...ductId=2103176

I can't remember the last time I charged the battery, but I'll just guess a few weeks ago.

Car awake: 12.36V
Car asleep, trunk light asleep: 12.58V
Maybe a couple of hours into the ongoing charge, asleep: 12.93V
Fully charged, CTEK immediately disconnected, asleep: 13.04V

That last one, that can't be right?

The truck read 12.8V ("asleep", which makes me chuckle), and 12.4V when I turned on the parking lights. I may do a rereading after a charging at some point.
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  #2  
Old 03-07-2013, 03:56 PM
greyE92coupe greyE92coupe is offline
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Location: Ireland
 
Join Date: Feb 2013
Posts: 20
Mein Auto: E92
Those readings look normal. A Lead Acid Battery should measure 12.6v when fully charged. Not immediately after taking it off charge however. The Voltage is higher during charge and takes some time to return to normal after being taken off charge.
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  #3  
Old 03-07-2013, 06:37 PM
Ilovemycar Ilovemycar is offline
wooohooo
Location: CA
 
Join Date: Feb 2010
Posts: 2,742
Mein Auto: '08 335i coupe 6MT sport
Thanks for your input. I wonder how quickly the battery will fall back to the normal 12.6V range.

Question: Why do you think the truck reads at 12.8? I haven't charged it in at least a few weeks as well, I think. It had been driven just a modest amount of time before taking the measurement; do you think the alternator just gets it up for a bit, and that it just takes a bit of time for it to fall back again? Sorry for all the dumb questions.
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  #4  
Old 03-07-2013, 08:33 PM
DSXMachina's Avatar
DSXMachina DSXMachina is offline
Non compos Mentos
Location: New Hampshire
 
Join Date: Dec 2007
Posts: 10,648
Mein Auto: 335i E92 TiAg 6MT ED
When a battery is freshly recharged from a plug-in source it gets what is called 'surface charge'*. It is normal and common to read 12.8 to 13V when the charger is first disconnected. This excess charge is quickly dissipated if you turn your headlights on for 15 seconds. Check again and the voltage will read very close to 12.6/7V.
No need to worry as everything is OK, the readings you are getting indicate your CTEK is working normally.

*You have 6 cells in your battery, each cell provides 2.1V when fully charged. That's how you end up with a 12.6V 'fresh' battery. Now assume that the CTEK charges each cell to 2.15V your reading will be 12.9V. Five hundredths of a volt extra in each cell isn't much of an overrun.

Last edited by DSXMachina; 03-08-2013 at 06:37 AM.
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Old 03-07-2013, 11:42 PM
Ilovemycar Ilovemycar is offline
wooohooo
Location: CA
 
Join Date: Feb 2010
Posts: 2,742
Mein Auto: '08 335i coupe 6MT sport
Thanks DSX. Next time I run this little test, I'll think to do the 15 sec count on the headlights just to see.
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