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Old 05-16-2008, 02:22 PM   #1
bstyle74
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Default Battery or alternator problem?

So about 6 weeks ago, my car wouldn't start. After cleaning the clamps and battery posts, checking water levels and hooking up battery charger for couple hours, the car started and ran fine. I had been checking my voltage at the battery and alternator about every 5th day in the morning and after the car has warmed up, reading consistent 12.21 in the mornings and about 13.75 at warm idle.

Yesterday it had a VERY hard time starting.... just couple slow turns but then it started. Car runs fine once going, so though alt is under 14 i think the alt is ok. After I got it home, I let it sit overnight and checked voltage again... 12.15. I turn the key to "on" without starting the car, and the voltage went way low about 10.8. No accessories were running, just the seatbelt beeper.

Any thoughts on what might be the problem? (completely stock car, with a 2 year old aftermarket radio and bazooka tube)
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Old 05-16-2008, 02:43 PM   #2
Mulder
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The battery may have one or more weak cells, if it's the original battery it's well past its typical life expectancy. Many of them have crapped out much earlier.
If you have a battery hydrometer (looks like an eyedropper with balls in it, about $5 at parts stores) you can check the state of charge in each individual cell to see if there are any bad ones. If any cells use more water than the rest that's a good sign they are bad.
Also most parts places will test the battery for you, or if you know a local shop they may do so.
I would not be at all surprised if your battery is shot.
Your voltage reading with the engine running is normal so most likely your alternator is ok, although it is possible to have some bad diodes (which will reduce charging output) and still see normal voltage with a meter. This type of problem can be diagnosed with a scope at most well-equipped shops.
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Old 05-16-2008, 02:52 PM   #3
bstyle74
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The battery was replaced with a Napa Legend in December 2005, so if it is bad then it will still have a little warranty left. I will check with hydrometer and see what comes of that.

I'm charging the battery now and will take it to couple free battery/alternator test shops and see what they say, but from my understanding if one cell is completely dead and the other cells ok, wouldn't the battery read no more than 10v (2v per good cell)?
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Old 05-16-2008, 02:55 PM   #4
Mulder
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If a cell is bad and not producing voltage, yes. But they can also be weak and show a normal no-load voltage which will drop when loaded.
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Old 05-16-2008, 05:14 PM   #5
Cougar4
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It does sound like one of the cells may be bad in the battery. A good charged cell should be around 2.15 volts I think. This would mean that the 10.8 volts you read is about right for having 5 good cells.
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Old 05-16-2008, 07:15 PM   #6
silver2004impreza
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Something else to keep in mind, having a constantly dead battery will eventually kill your alternator, by causing it to have to work overtime to recharge the battery.
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Old 05-18-2008, 02:31 AM   #7
bstyle74
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So I bought the $5 hydrometer, and tested the acid/water. One of the six cells didn't even float the indicator, so I took that as bad cell.

Then I took it to a couple places and here are the different free test results:

battery is good, alternator is good.
battery is bad, alternator is good.
battery is good, alternator is bad.

wtf? I'm pretty sure the alternator is good because voltage is ok and even the shop that said the alt was bad said the diodes are ok (he insisted anything under 14v is bad). So I just decided to buy a new battery and keep an eye on it. If I have problems again in a couple weeks, I'll look to replace the alternator.... car has 120k on it so I can't complain too much about durability if that is the case. Thanks everyone for tips and explanations.
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Old 05-18-2008, 10:38 AM   #8
Cougar4
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Quote:
Originally Posted by bstyle74 View Post
So I bought the $5 hydrometer, and tested the acid/water. One of the six cells didn't even float the indicator, so I took that as bad cell.
Very good testing on your part. A bad cell it is alright.

It is pretty amazing to see the different testing results the shops gave you. If the diodes in the alternator are really good then you most likely will have no more trouble.
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Old 05-18-2008, 02:24 PM   #9
dougb76
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This thread could help me out a lot. I've had similar problems on my 06 wrx and had the alternator replaced (under warranty) about 6 months ago, and it didn't really fix the problem. The shop insists the battery is ok, but really have my doubts, especially since problems tend to pop up when the temperature quickly drops. I'm going to try on the those hydrometers.
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Old 05-21-2008, 11:51 AM   #10
dougb76
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Did the hydrometer thing and 4/6 were right on the line between good and needs attention, and the other 2 are good. I'm guessing this means the battery is still ok and it's back to the drawing board.
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Old 05-24-2008, 10:48 AM   #11
Cougar4
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The 4 cells that were marginal my be that way because the battery wasn't charged up properly, to its full capacity.

I would check the charging voltage at the battery while the engine is at idle and also at around 2,500 rpm. The voltages should be around 13 volts at idle and 14.5 volts when reved up.
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