Safe battery voltage range when engine off and on?

avin83 · 09-29-2017, 04:36 PM

I had the yellow battery warning light few times earlier this year when the engine is off so I would like to figure out whether my battery needs to be replaced for the coming winter.

My car is DD (drives approx. 50km or 32 miles per day) with no iDrive and I don't use the car for music etc. when the engine is off. I had this car for a 2 years (2006, 78k miles) but I have no idea the last time the battery was replaced. I just know it is a white BMW OEM battery.

Using the hidden menu from the dashboard, I was able to get the following voltages (see youtube link below for those who are interested in):

Engine off - 12.1 ~ 12.3v
Engine idle - 14.1 ~ 14.3v

I am guessing these are safe voltage ranges? The car starts fine and no electric problems are found yet... maybe it is a bit early to change the battery?

PDX-335i · 09-29-2017, 06:11 PM

If you have a smart charger I would try to charge the battery for a 12 hr period. Wait a couple hours after disconnecting the charger and read the voltage. If it's less that 12.3 you should consider a new battery. Normal battery voltage should be close to 12.7 on a good battery. Old batteries won't hold a full charge. Voltage readings while the car is running just measures the output of the alternator, not the battery. Also, not driving the car enough will cause the battery to not fully charge.

nissubaru · 09-29-2017, 11:25 PM

Sounds like an old battery to me. I've been chasing down a battery/power issue myself since my battery died a few times and the car wouldn't start. Battery is only 2 years old though but reads between 12.1-12.3V when off. Recently replaced the alternator but not really seeing 14V unless I'm driving the car, and even then I would say 14V is uncommon to see. 13.5V is probably my average

Your alternator sounds fine and the fact that you drive 30 miles daily should mean the battery has plenty of time to charge. Since its not fully charged then I would imagine the battery is old and its way out.

However, I would be curious to hear from other members what their batteries voltage is with the car off and then with the car idling with accessories turned on. My car has only died once in the past two weeks and hasn't died since. Dunno if it's just normal or what? Is that why everyone buys trickle chargers?

09-29-2017, 04:36 PM	#1
avin83 Enlisted Member 7 Rep 49 Posts Drives: 2006 323i Join Date: Apr 2016 Location: Toronto iTrader: (0)	Safe battery voltage range when engine off and on? I had the yellow battery warning light few times earlier this year when the engine is off so I would like to figure out whether my battery needs to be replaced for the coming winter. My car is DD (drives approx. 50km or 32 miles per day) with no iDrive and I don't use the car for music etc. when the engine is off. I had this car for a 2 years (2006, 78k miles) but I have no idea the last time the battery was replaced. I just know it is a white BMW OEM battery. Using the hidden menu from the dashboard, I was able to get the following voltages (see youtube link below for those who are interested in): Engine off - 12.1 ~ 12.3v Engine idle - 14.1 ~ 14.3v I am guessing these are safe voltage ranges? The car starts fine and no electric problems are found yet... maybe it is a bit early to change the battery?
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09-29-2017, 06:11 PM	#2
PDX-335i Private First Class 22 Rep 118 Posts Drives: 2009 E90 335i Sport Join Date: Jan 2015 Location: Portland, OR iTrader: (0)	If you have a smart charger I would try to charge the battery for a 12 hr period. Wait a couple hours after disconnecting the charger and read the voltage. If it's less that 12.3 you should consider a new battery. Normal battery voltage should be close to 12.7 on a good battery. Old batteries won't hold a full charge. Voltage readings while the car is running just measures the output of the alternator, not the battery. Also, not driving the car enough will cause the battery to not fully charge.
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09-29-2017, 11:25 PM	#3
nissubaru Lieutenant 233 Rep 564 Posts Drives: 09 335i MSport + 1990 300ZX TT Join Date: Jan 2017 Location: Philadelphia iTrader: (0)	Sounds like an old battery to me. I've been chasing down a battery/power issue myself since my battery died a few times and the car wouldn't start. Battery is only 2 years old though but reads between 12.1-12.3V when off. Recently replaced the alternator but not really seeing 14V unless I'm driving the car, and even then I would say 14V is uncommon to see. 13.5V is probably my average Your alternator sounds fine and the fact that you drive 30 miles daily should mean the battery has plenty of time to charge. Since its not fully charged then I would imagine the battery is old and its way out. However, I would be curious to hear from other members what their batteries voltage is with the car off and then with the car idling with accessories turned on. My car has only died once in the past two weeks and hasn't died since. Dunno if it's just normal or what? Is that why everyone buys trickle chargers?
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