Battery question for when Engine on Vs Off

[Rod27]

Hello,

I wanted to know why my battery voltage is in normal range (13.7-15.1V ) when the engine is on but when the engine is off the battery voltage is at 12V or even below that like 11.7 V? I had my alternator looked at and it was performing normally. I also drive my car often, about twice a day for about 20 mins each way to work.

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[KeyLime]

How are you measuring the battery voltage when the engine is off? Measuring at the battery terminals using a volt meter? Or are you turning the car electrical system on and using the instrument cluster volt meter?

 

[ORRadtech]

Hello,

I wanted to know why my battery voltage is in normal range (13.7-15.1V ) when the engine is on but when the engine is off the battery voltage is at 12V or even below that like 11.7 V? I had my alternator looked at and it was performing normally. I also drive my car often, about twice a day for about 20 mins each way to work.

First off, any voltage measured while the car is running is measuring the charging system voltage, not the battery.
Actual battery voltage depends on a whole lot of factors. To get an exact voltage it should be disconnected from the car and measured with a good DVOM. Measuring in the car will give you a reasonable approximation but since the car is always drawing current it could be skewed lower.
Actual resting battery voltage for a 100% charged battery should be close to 12.7V. If your voltage is truly 12V or less then your battery has a pretty low state of charge and you might consider having it charged using a separate battery charger and then have it load tested.
They key to all this is starting with a good Digital Volt Ohm Meter, DVOM, and learning how to use it to interpret what it tells you.

 

[Rod27]

How are you measuring the battery voltage when the engine is off? Measuring at the battery terminals using a volt meter? Or are you turning the car electrical system on and using the instrument cluster volt meter?

I use the instrument cluster volt meter and I had Advance Auto Store use a measure my battery using the volt meter. They pretty much got the same numbers I got when they measured at the terminals.

 

[KeyLime]

I use the instrument cluster volt meter and I had Advance Auto Store use a measure my battery using the volt meter. They pretty much got the same numbers I got when they measured at the terminals.

Thanks for the reply. As ORRadtech said the resting voltage of a fully charged, good battery should be about 12.7. Much lower means a worn out or not fully charged battery.

The reason I asked about how you measured the voltage is that, depending on what comes on, using the cluster gauge with the engine off can give you a false reading. Daytime running lights, cabin fan, electronics, etc. can put a load on the battery causing the voltage to sag a little. It shouldn't be much, maybe a couple of tenths of a volt, but it introduces an error in the measurement.

But it sounds like you're on the right track to getting an accurate measure of the battery.

 

[Rod27]

I'm trying to figure out why my car isn't charging the battery itself when I am driving. Its just odd to me it keep being in the low range like that.

 

[NoVaGT]

I'm trying to figure out why my car isn't charging the battery itself when I am driving. Its just odd to me it keep being in the low range like that.

Stop obsessing.

For God's sake, stop trying to fix something that isn't broken.

And since you don't understand, the current reading on the dash isn't the battery's voltage, it's what electrical charge the alternator is putting into the battery.

Just relax, stop obsessing. Everything is fine.

 

[ZPD]

I'm trying to figure out why my car isn't charging the battery itself when I am driving. Its just odd to me it keep being in the low range like that.

The voltage you are seeing while driving is the voltage being put out by the alternator and not the voltage in the battery. The battery isn't designed to hold much past 12 V. If you see 13-15V at your battery with the car off then you will have a problem; most likely a bad meter ;)

Everything is normal. Carry on.

 

[Rod27]

Cool, thanks for the help.

 

[ORRadtech]

I'm trying to figure out why my car isn't charging the battery itself when I am driving. Its just odd to me it keep being in the low range like that.

I'm not sure what you mean. Why don't you think your battery is charging? Because of the low engine-off voltage? If the battery voltage is truly low it's not going to be the car's fault, if your engine-on voltage is what you say it is. What will be at fault is your battery.
If your engine-off voltage is 12V or less I'd recommend that you have it load tested.

 

[Rod27]

I'm not sure what you mean. Why don't you think your battery is charging? Because of the low engine-off voltage? If the battery voltage is truly low it's not going to be the car's fault, if your engine-on voltage is what you say it is. What will be at fault is your battery.
If your engine-off voltage is 12V or less I'd recommend that you have it load tested.

Well, for a while after turning the engine off the radio will cut off in about 15-20 seconds rather than staying on for about the usual 10 minutes . Then after that I can't turn my dome lights on. So that's why I was trying to figure out what was going on and looked at the voltage to make sure that the battery was good.

 

[KeyLime]

Sounds like you're going to need a new battery soon.

If the car had both a volt and amp meter it would be a little easier to diagnose these problems.

 

[Groovy]

These cars have a BMS (battery monitor system). This controls battery usage and charging. You can’t use logic from the past to troubleshoot. My F150 started acting weird. Powering down the NAV, lights, etc. I ended up disconnecting the battery and charging it overnight. Corrected all the problems. BTW… it never had a problem starting.

 

[dx2]

The voltage gauge shows 12V but that is under load after turning off the engine. At that point about 10 Amps are still drawn. To get a more accurate reading you need to wait a bit, say one hour.

I measured the current draw for one night a few months ago. It was about 15 to 17 milliAmps which is not much but over time. But when the car is not driven for a few months the battery will be depleted too far to start the engine.

If they current draw is higher, say larger than 25 to 50 mA you have a problem. Because of the amount of modern electronics in this car a battery monitoring system makes sure the battery stays healthy for a bit longer by shutting off some not essentially required components.

For A10 cars, the gear lever must be in park position. If it is not in park position the components will not be put into sleep mode and draw more power.

I have a dash cam with parking mode. It shuts off when voltage drops below 11.7 V. So far there was no problem starting the engine but the car is still quite new.

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