battery dead

[tubz]

97 grand prix gt. Battery goes dead. Alternator is charging fine.The car will run fine for several days, no indication of a problem. Then one morning we will go to start it and it is completely dead, can't even get the key out of the ignition. Charge the battery, then it is OK again for a while. Nothing is being left on by us. Is their something in the car that could be faulty and killing the battery.

Puzzled

[BNaylor]

It sounds like you may be getting parasitic drain/draw of the battery. But the battery should be tested first to make sure it is not the cause. After a period of time the battery loses efficiency/power and the reserve capacity goes to pot.

Check to make sure no interior, trunk lighting or glovebox lighting is staying on. Parasitic draw can be troubleshot with a digital multimeter by checking current draw at the battery negative terminal to ground. Alot of components can cause the problem.

[richtazz]

Welcome to AF. If you have a digital multimeter, set it to milliamps, and disconnect the negative battery cable. Hook your multimeter in-line between the end of the battery cable and the battery and record the reading. Next, check your battery and see if it has an amp/hour (a/H) rating on it. IF it does, divide the a/H rating by 4, if your multimeter reading is higher than the a/H divided by 4, then you have an excessive draw that is killing the battery. If it's below, then your battery may have an internal short. Typically, with everything off and no added electronics like a remote start/alarm/stereo/etc..., the draw shouldn't be higher that 5-10 mA, but the above formula is to figure the battery's draw capacity.

[BNaylor]

It will probably be higher than 5 - 10 milliamps not millivolts volts Rich. I get 16 ma on my GTP and it carries me through a whole month before I see any effects of the battery draining. FYI.

[richtazz]

oops, I was making corrections to my post when I saw your response Bob.

[MT-2500]

Originally Posted by tubz

97 grand prix gt. Battery goes dead. Alternator is charging fine.The car will run fine for several days, no indication of a problem. Then one morning we will go to start it and it is completely dead, can't even get the key out of the ignition. Charge the battery, then it is OK again for a while. Nothing is being left on by us. Is their something in the car that could be faulty and killing the battery.

Puzzled

As said check the battery for amp draw with key off.
Hook up volt/amp meter and let it sit foe 20 - 30 minutes for everything to power down and read amp draw.
But have you had the battery tested and all cables cleaned and checked.
Another test would to be disconnect the battery and let sit a couple of days and see if battery goes down disconnected.
What is the voltage reading on battery or has it been load tested.
Most parts places will run a battery test for you.
MT

[tubz]

Originally Posted by richtazz

Welcome to AF. If you have a digital multimeter, set it to milliamps, and disconnect the negative battery cable. Hook your multimeter in-line between the end of the battery cable and the battery and record the reading. Next, check your battery and see if it has an amp/hour (a/H) rating on it. IF it does, divide the a/H rating by 4, if your multimeter reading is higher than the a/H divided by 4, then you have an excessive draw that is killing the battery. If it's below, then your battery may have an internal short. Typically, with everything off and no added electronics like a remote start/alarm/stereo/etc..., the draw shouldn't be higher that 5-10 mA, but the above formula is to figure the battery's draw capacity.

Thanks for the info. I tested 11.2ma, so i don,t think there is excessive draw with the car shutoff. The battery is 12.8 volts off/14.4 volts running. I will get the battery load tested, maybe an intermmitant internal short in the battery.

[richtazz]

Your reading is within the acceptable range (my original post was a little on the conservative side considering all the electronics our cars have), so I'd suspect the battery has a short. I'd try leaving the car set with the battery disconnected, and periodically check voltage. If it drops, the battery is bad. A fully charged 12v battery will have 12.8-13.2 volts at rest (1.1-1.2 v per cell). If the battery has removable caps, a hydrometer could pin-point a low cell.