Battery voltage goes down

H

hardheaded

Joined
Nov 10, 2016
Messages
145
Location
Missouri
2011 tahoe and after the truck sits for 3 days the battery voltage drops to 11.7. I tis cold outside right now though.
New battery 12-20-23 and tests good with an Ancel tester.
This truck is new to me and the volt meter does go way down while driving sometimes, but i am told this is normal. Never a light on the dash coming on.
Wondering if i have a draw on the battery while off ?
 
How long did the old battery last? What killed it?

I'd leave the hood on the safety catch, and go sneak out there and get a reading without opening the driver's door. Don't have the key fob in your pocket, either. Lots of modules fire up when you open the door.
 
1705446005343.jpg
 
Been off the Tender for a couple of hours now and the voltage is 12.6 compared to 12.9 a couple of hours ago when i took it off the tender.
I just checked my other car and it was at 12.4 after being off the Tender since last night. Was at 12.9. It has been below 0 for a couple of days and it's now a sunny 14 out there. Hopefully it will be alittle warmer so i can get a better look at it.

I took this truck for a long ride the other day and it was charging at 13.1 when i got back home. Seems low to me, but i know it has some kind of charging system that the ECM controls. The voltmeter on the dash does go up and down but never a light. I know the guy i bought it from had the charging system checked at Walmart where he bought the battery and all was good.

I have the car fax on the truck and the last 2 batteries lasted 3 years and then the last one was 2 years old. The old guy didn't drive the truck much and most likely this is why the batteries lived a shorter life.
 
This might be one of those trucks that charges more if you turn the headlights on.... just so the headlights are bright enough.
 
hardheaded said:
Been off the Tender for a couple of hours now and the voltage is 12.6 compared to 12.9 a couple of hours ago when i took it off the tender.
I just checked my other car and it was at 12.4 after being off the Tender since last night. Was at 12.9. It has been below 0 for a couple of days and it's now a sunny 14 out there. Hopefully it will be alittle warmer so i can get a better look at it.

I took this truck for a long ride the other day and it was charging at 13.1 when i got back home. Seems low to me, but i know it has some kind of charging system that the ECM controls. The voltmeter on the dash does go up and down but never a light. I know the guy i bought it from had the charging system checked at Walmart where he bought the battery and all was good.

I have the car fax on the truck and the last 2 batteries lasted 3 years and then the last one was 2 years old. The old guy didn't drive the truck much and most likely this is why the batteries lived a shorter life.
Click to expand...
Battery charging at 13.1 by alternator seems low. I check my chevy truck twice a year, reads 14.5 from voltmeter readout on batt. terminals ,truck running.
 
I've known at least a few people who've replaced alternators needlessly because they observed their vehicle "charging" at 12.5V, low 13's, etc.

Modern vehicles regulate alternator / charging duty cycle constantly based on a number of factors: mainly electrical load, battery state of charge, and estimated battery health. Often this will result in lower than expected charging voltage. There's nothing wrong here, and those folks wish they could have gotten their money back when some shadetree mechanic sold them an alternator replacement.

11-something volts after sitting for a couple days suggests to me a suspect battery if a parasitic draw is ruled out.
 
If you're measuring the voltage with the ignition on, 11.7 V isn't abnormal if the battery is really cold. Even opening the car doors or using the key fob can put load on the battery and drop the voltage for a while. So can having the hood open if your car has a hood sensor. Be sure you're checking the voltage while the battery is at rest.
 
My 2005 Chevy 5.3 volt gauge has always done this. It bothered me for a while but I got used to it. I preemptively put a new Delco Gold 145 amp alternator on about 3 weeks ago at 244,000 miles and the new alternator acts the same way. Never had a problem with the charging system.
hardheaded said:
This truck is new to me and the volt meter does go way down while driving sometimes, but i am told this is normal. Never a light on the dash coming on.
Click to expand...
 
Last edited:
I read somewhere that you can put your multimeter in AC mode and check the battery for AC voltage. If it shows AC voltage at the battery the Diodes are bad in the alternator. Is this a creditable test ?
 
OK, I just did the amp draw test and it's drawing 0.003 on the ground cable. So i don't think i have any parasitic draw on the truck.
No voltage drop from the battery to the alternator. Battery still at 12.7.
Voltage seems to be lower after i drive the truck.

Maybe i am just to old school and expect the thing to charge like the older trucks.
 
hardheaded said:
Been off the Tender for a couple of hours now and the voltage is 12.6 compared to 12.9 a couple of hours ago when i took it off the tender.
I just checked my other car and it was at 12.4 after being off the Tender since last night. Was at 12.9. It has been below 0 for a couple of days and it's now a sunny 14 out there. Hopefully it will be alittle warmer so i can get a better look at it.

I took this truck for a long ride the other day and it was charging at 13.1 when i got back home. Seems low to me, but i know it has some kind of charging system that the ECM controls. The voltmeter on the dash does go up and down but never a light. I know the guy i bought it from had the charging system checked at Walmart where he bought the battery and all was good.

I have the car fax on the truck and the last 2 batteries lasted 3 years and then the last one was 2 years old. The old guy didn't drive the truck much and most likely this is why the batteries lived a shorter life.
Click to expand...
While running the voltage should be around 14.4v
 
The above post #11 seems to be what is going on. I talked to a tech at a dealer and he seems to think it's normal behavior of these new charging systems. My owner manual has the exact same thing in it.
I carry a jump pack with me so i won't be stuck it if would go dead on me, so we will see how long it can set without going dead. Gonna let it set for the next 4 or 5 days.
 
If the alternator diodes were bad wouldn't you get a amp draw in the test ?
Would you get some kind of OHM reading between the alternator case and hot post of the alternator ?
This would be with the battery disconnected .
 
jhellwig said:
Normal in this cold weather. IF the vehicle still starts don't worry about it.
Click to expand...
Not normal. OCV should not be lower in cold weather. Actually the opposite, it should be higher because the solubility of sulfuric acid is decreased
 
jeffy876 said:
Not normal. OCV should not be lower in cold weather. Actually the opposite, it should be higher because the solubility of sulfuric acid is decreased
Click to expand...
He did not say he disconnected the battery to test it. These vehicles will power up half the systems if you open a door and drop the voltage down that low or lower.

Open circuit voltage is a poor way to test any battery and does not give you an indication of health or a very accurate state of charge.
 
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