car battery woes revisited - buick century

I thought i had the problem solved...
battery in 96 buick century would go dead after a day or two. I swapped batteries from my pontiac.. problem seemed solved.
after a while i decided to swap the batteries back... the problem still seemed solved.. I let the buick go untouched for probably two or three weeks i guess.. so then i decided to start it up... very low voltage, not enough to crank.
so i jumpstarted it.. let the battery charge up a couple of minutes. turned it off.. it started back up fine. Put the battery under a load tester for 10 seconds. The battery held the charge fine and stayed in good range.
So now my questions are:
Should I buy a new battery? (the battery seems fine when used on a daily basis, and the load tester said it was ok)
Isn't it unnacceptable that a car sit only a few weeks and not start?
Could it be a small drain on the battery that drops the battery voltage over time?
Is a charging system of 16 volts while running an indication of a bad altenator? and if so could that be draining the battery when the car is off?
Related link to original thread:
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&th 4f8779e1998ff5&rnum=2
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16 volts is too high. It is an indication of a bad regulator. Voltage on a charged battery and little load should be about 14.5 volts. There is always a load on the battery and could discharge after 3 or 4 weeks if the battery isn't quite up to snuff. Put a current meter on the battery and check it's current with the key off...

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&th 4f8779e1998ff5&rnum=2
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My voltage goes to 15.5 all the time under heavy load. (large stereo amplifier).
Could it be 16 because the battery is drained?
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Supernoma wrote:

Is that is 16 on your dash meter? I wouldn't belived it. Only a measurment at the battery with a good meter is valid.
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Has anyone ever considered the crappy grounds on modern cars ? For long alternator, battery, and delicate electronics life, beef up your grounds. Shine up alternator bracket where alternator contacts bracket, scrape away paint from mounting bracket where cable bolts to bracket, and where alternator cradles.
I like to run a Ford starter cable from block to frame rail, rather than trust that goofy wire from the negative terminal to a sheet metal screw in fender.
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the battery will not fully charge by running the car you should use a battery charger.yes you could have a small draw but the other battery should have also went dead when in the car any auto shop or parts store should be able to test the battery.you might want to diconect the negative cable if the problems is a draw and the vehicle will be sitting try charging the battery fully good luck
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A loose (side terminal) and/or dirty side terminals and/or partial connection can cause an undesirable voltage drop which can cause a high voltage like you described.
A parasitic or static current draw of more than 50 milliamperes d. c. on a computer controlled ignition car measured by a multimeter on the ammeter scale with more than 10 meg ohms per volt input sensitivity is considered excessive.

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