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2004 DeVille Battery Dead After Two Days

1759 Views 10 Replies 3 Participants Last post by  Submariner409

On Wednesday, we drove the car and parked it in the garage. My wife went out tonight to open the trunk and found that none of the interior lights turned on so it looked like the battery was completely dead. However, I did not check the battery voltage with a voltmeter to see what it actually was. I connected my 10 amp battery charger and turned it on. The battery started charging at about 12 amps (on the meter). After about an hour and a half, I disconnected the charger and checked to see if the lights worked, and they did. When I reconnected the charger, it continued at about 12 amps.

Any ideas on what could have discharged the battery after only two days? It's only 2 years and 9 months old, but I guess it could still be defective. I have a Schumacher digital battery tester that I can try when the battery is fully charged, but it looks like that will be quite some time...maybe tomorrow at this rate. In order to use the tester, I have to input the specified Cold Cranking Amps. Since the battery is under the back seat and my back muscles are currently incapable of doing any work does anyone know what the CCA rating of a Delco battery for this car is supposed to be?
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Cadillac wants at least 800 CCA for the Group 79 vented battery under the back seat.

A dead cell will kill a battery overnight - NOT uncommon. - battery age makes no difference.

A dead/nearly dead battery will accept a pretty high charge amperage until it nears full charge; internal resistance then signals the charger to ramp down the charge rate, both voltage and amperage. At full charge a good smart charger will produce literally NO amperage and will hold the battery at 13.1 - 13.3 volts.

A fully charged battery, disconnected from anything for a half hour should read 12.6 volts.

EDIT: Saw your latest post. Yes, the highly unlikely switch in ACC would kill the battery quickly as well as set a slew of U-codes. (U = Power and data)
U codes are ALL related to power and data glitches. Entirely possible, due to power loss, to see a XXX no communication or no data.

With the ignition switch in ACC or ON the car draws 3 to 6 amps, depending. If all's normal the parasitic draw should be about 15 - 20 mA but you have to jump through many time and power cable hoops to accurately measure that. One of our cars will kill the battery in 3 weeks to a month of storage.

Good idea to use a Battery Minder. My garage queen STS sits there, Battery minder plugged in, never a battery problem - and a Battery Minder greatly extends battery life if used regularly. It DOES NOT need to be any larger than 0.5 - 1 Amp capacity - it's NOT a charger - it keeps a good battery charged.

Our cars also have RAP - Retained Accessory Power - that keeps some accessories hot for 10 minutes after Key: OFF. Sometimes one of the accessories stays hot and that drains the battery. Do you have anything plugged into one of the 12V power ports? They can be configured to turn off with RAP or stay hot at all times, so a phone or charger will kill the battery in that case.

The power port fuses in the underhood fuse box have two possible positions - RAP or always hot.

Here's my setup - yours may differ slightly. Use the locator legend inside the fuse box top.

Line art Text Diagram Plan Drawing
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That USB charger may have a small transformer to drop the 12.6 - 14 system voltage to 5 or 6 volts for phone charging. It may be a bit more of a constant drain than you think.
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