How much should a batterys voltage drop once you turn off the engine?

[bob8619]

How much, if any, of a drop in voltage should you see in a battery once you turn the engine off? My 3 month old battery keeps dying and I can't find a draw anywhere so I disconnected the battery and within an hour it went from 12.50 volts to 12.35 volts. I don't know if theres an acceptable range or if it should hold steady.

[Shinysideup]

Have it load tested at the auto parts store. A new battery should stay up to at least the 12.5v
Curious what brand etc?

[bob8619]

Originally Posted by Shinysideup

Have it load tested at the auto parts store. A new battery should stay up to at least the 12.5v
Curious what brand etc?

Its a cheap Walmart battery. An aside question: Once a battery reaches a state of being undercharged as mine is, does that somehow cause the battery to want to just continue dying even without any load on it?

[Mr_Rich]

Originally Posted by bob8619

Its a cheap Walmart battery. An aside question: Once a battery reaches a state of being undercharged as mine is, does that somehow cause the battery to want to just continue dying even without any load on it?

It's called deep cycling and some batteries won't survive a lot of that practice. I think some of the car stereo guys like deep-cycle batteries so they can run their music with the engine off and can run their batteries down and recharge with little problem.

[Steeveedee]

A fully charged battery is 12.6 Volts. If your battery drops below this voltage even when disconnected, it is defective. It should not continue to drop voltage when disconnected. You may see minor drop because of the trickle draw for a radio to keep it alive, if it has a memory and a clock, if it is connected.

[davepl]

Originally Posted by Steeveedee

A fully charged battery is 12.6 Volts. If your battery drops below this voltage even when disconnected, it is defective. It should not continue to drop voltage when disconnected. You may see minor drop because of the trickle draw for a radio to keep it alive, if it has a memory and a clock, if it is connected.

Actually, each cell is 2.2V and you have six of them for a total potential voltage of 13.2 (that's lead acid, anyway). But the rest holds.

[B. W.]

Originally Posted by Steeveedee

A fully charged battery is 12.6 Volts. If your battery drops below this voltage even when disconnected, it is defective. It should not continue to drop voltage when disconnected. You may see minor drop because of the trickle draw for a radio to keep it alive, if it has a memory and a clock, if it is connected.

Backing up Steve, as per ASE standards a fully charged battery is 12.6 volts. The proper way to load test a battery is 1/2 the CCA for 15 seconds, voltage should not drop below 9.5 volts. The industry standard for many years was the VAT 40 it uses a large carbon pile to simulate starter draw. I still test with a VAT 40.

[Shinysideup]

You probably have a dead cell and the reason it looks ok for a bit is simply residual energy. The second you apply a load it will base out to the true charge state. Try returning it and get a quality battery.

[Mr_Rich]

It really depends on whether you have an always hot lead hooked up to a radio/ clock or something like that. Normal voltage in my '89 is between 12 and 12.2 VDC. The real test is not having a voltage drop below 9 VDC when you try to start your engine.

[Mr_Rich]

I dealt with wet cell and glass mat batteries in commercial use for thirty years. There are a lot of reasons batteries can lose voltage and not be defective. Extreme heat for one; it kills batteries. Ask anyone in the know in Arizona. Two years tops on batteries. Batteries have the longest life if kept at seventy degrees. Is that possible for the life of your battery? Probably not. This is one piece of equipment that tends to be forgotten about; including me, and all of a sudden there's a problem to deal with.

[Andy4639]

Car batteries are for cranking the car. They are not for running accessories. They don't like being discharged and they don't take well to being left discharged for long times. The alternator is for running all the accessories in the car to keep the battery fully charged.
The chart I posted is a good starting point for new batteries.
So looking at the chart a 12 volt battery is fully charged at 12.73 this is at least 6hrs off the charger. Take and charge the battery as fully as possible. Take it off the charge and let it sit for at least 6 hrs with nothing hooked up to it. After 6 hrs take a reading if it's not close to the 12.73 it's on the bad side. The older the battery the less charge it will take.
This is for your 3 month old battery.
I use this chart all the time for golf cart batteries which I have been doing now for 30 years.

[davepl]

It's also POSSIBLE but unlikely that you have a shorted diode in the alternator. If so they'll run hot (the alternator I mean), not charge well, and burn power once turned off. But I'd suspect at a faster rate than what you describe.

[truckster]

I'll just tell you, Walmart is the worst place in the world to buy automotive batteries. They don't honor their warranties very well. If you take it in, they'll tell you it needs a charge (even if it's been on a charger overnight). Then they'll want to put it on their charger and load test it immediately after they pull the charger off. Most batteries will pass under those conditions.

I took the Walmart battery that was in my wife's Highlander Hybrid to Walmart; they tried to tell me the alternator was bad (hybrid cars don't have an alternator). I ended up taking the battery home and killing it chemically and then returned it 2 months out of the free replacement warranty. They offered me about $30 back on a $120 battery. So I bought a new battery, then returned the new battery and went to Costco to buy a much better Interstate battery for $78.

Bottom line: Don't buy batteries from Walmart.

[davepl]

My BMW was at the dealership and needed a new battery, which was like $350 and was some giant Interstate unit. When it died, 1 year in and still under warranty, I called Firestone (the local Interstate dealer) to see what to do.

They said to bring it in. I said it was a warranty claim so I didn't have the receipt, but the little circles you pop out marked the date. He said that was fine, come on down.

Of course when I got there the manager wanted a full documentary paper trail. I made them call the BMW dealer and sort it out, because he said if I had run up enough MILEAGE (the car has 10,000 miles total) on that battery, it might be void. So they had to get in contact with BMW in order to ensure that, I guess, I didn't start with -90,000 when I bought it.

All in all, good battery, but annoying resolution process.