So, the vehicle in question is a 94 f250 xlt 4x4

The battery just dies when the vehicle is off, sometimes in as little as 20 min or so. I left the car off overnight, and this morning it started. Battery reads 12.4v and the current that its drawing while off is a mere .01A. Any ideas?

 

Does the battery also read 12.4v from post to post when it's completely unhooked? Then it's low. Should be about 13.5 v. So first thing I would do is give it a good charge before you check the voltage again and also do a load test on the battery. Most parts stores like AutoZone or Advance will do this free of charge. You'll need to see if the battery is just weak or dead. Once you've determined that the battery is ok next thing to check is the alternator. As an example a bad diode rectifier will cause the alterantor to drain on the battery. Again most parts stores can check it out on or off the truck. Let us know how you make out. Good Luck!

 

I would think if the battery was good, and you drained it in 20 minutes, the problem would be obvious because something would be smoking from the tremendous heat. A good fully charged battery just can't be drained that fast without serious damage to something.
Since you didn't mention any smoke, I would think either the battery is not good, or you have a bad connection, or something else wrong.
A starter that is dying will act like a very low battery when the starter is hot. I bad battery cable or any number of connections in the starting circuit can cause the problem.
Or the obvious one is a bad battery, that really doesn' hold a charge.
If you have a battery charger, you can do a simple test at home. I have charged the battery and then removed the coil wire. Then tried to start the truck. It should crank strong and for several minutes without slowing down. If battery doesn't keep it cranking it is probably bad. If it does, recharge it before starting the engine, or you may damage the alternator.
If you are going to take it somewhere for a load test, it must be charged up pretty good before they test it or the results are no good.
Good Luck Frank

 

Should be 12.75 volts

A properly charged battery measured from post to post with nothing hooked to it should read around 12.75 Volts.

With the battery fully charged and connected with the engine running, it should read about 13.4 volts.

 

I would check and clean all connections well.

Check the wire and connection from the negative battery terminal and the fender well.

 

13.5v is not what the voltage should be. Too high.

A fully charged good battery will produce about 12.4-12.6v with no load. Under a load test, it should be about 11 or so depending on the level of load.

Saying 12.75 is way too specific. There is variance there. I've tested hundreds of batteries (new and used) and I'd say less than 5% were over 12.5 volts. The actual appropriate voltage should be between 12.4-12.6ish. In reality, it doesn't matter all that much because voltage is not an indication of how much power the battery actually has. A battery can be dead and still produce 12+ v without a load.

Also, a vehicle that is running should produce 14-14.5v at the battery if the alternator and cables are functioning properly. Below 14 and there is a charging problem there.



I also agree you should not be able to kill a good battery in 20 minutes unless the load is extremely high.

Have the battery charged and load tested and then go from there.

 

Of course I didn't mean it to be exactly specific, but the values given above mine were dead wrong, and mine were pretty good ballpark values.

I don't know what you use to measure with or how accurate it is, but using high quality extremely accurate DVM's, when checking fresh good fully charged batteries for the past 30 years or more I have always gotten 12.7 - 12.8 readings. If you are reading older marginal batteries, even though they may have a few years life still in them you may read lower.

Some batteries even when new are defective and don't have the same specific gravity on each cell when fullly charged ect.

If you are only getting 5% of them reading over 12.5 you have a problem somewhere.



When a good battery is fully charged, most charging systems charge below 14 volts at idle.

If the battery is one that is marginal, 12.5 ect, or soft plates, charging rates will run higher. The worst thing you can do to shorten the life of a battery is to constantlly overcharge them.

And of course the only way to properly reliablely test a battery is a load test, and their voltages will vary under load, because a load test is current specific.

The charging system however is very voltage specific.


EDIT ________________ As an afterthought, the measurements and readings I stated above were taken in most cases very soon after the vehicles were shut down,.............not after the batteries were sitting around for a while.

 

If you completely drain the battery or drain it below half the alternator won't be able to charge it fully. when it is down that far you should put a battery charger on it to bring it back up to full.

 

Didn't mean to start a voltage war there guys . But really, that was some good information. However, the problem just went away! I think that there is some loose wiring somewhere, and its probably a starter cable or something, because when the thing died that quick, there must have been ALOT of current draining (Ive heard of these arc welding themselves to the starter.) So anyways, Im gonna wait till the problem pops up again and then see whats going on...

......and carry a set of jumper cables in the bed...

 

WOOPS..................................

I checked my battery this morning before starting and it only read 12.44 volts.
It was only 20 degrees out.

It's probably the first time I've ever measured an unused battery that was good that had set overnight for no reason at all.

When I came home this evening, a 3 mile trip, I measured it right after shutting it off and it read 12.71.

Looks like there is a difference in the unloaded static voltage potential of a freshly charged vs sitting overnight battery.

I probably should have measured it every 15 minutes or so this evening to see how fast it comes down to the 12.44v. And I have to recheck the overnight settling in this summer to see if the temp affected it much.

Sorry If I mislead anybody.

 

I think I'm just going to extend the lines on my voltmeter and run them through a hole in the firewall and to the battery termanal and tape the thing on the dash so I can see what its doing ALL THE TIME. Actually though... Would the same thing work if I just pluged the multimeter into the cigarette lighter?

 

Might not work if your problem is a bad battery cable connection on the fender relay.

 

Don't worry about voltages being 12.4 or 12.6 or whatever so much...

An easy way to load test the battery and starting system is to have someone crank the engine while you observe the battery voltage. If it goes below 10 volts...the battery is weak. A strong battery will hold about 10.5-11v under starting power.


Like I've said, the voltage of the battery without a load does not tell you very much at all...so don't get hung up on what the actual voltage is w/o a load.