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Depends - how's your alternator? And other issues... Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 17 Oct 2007 09:42:59
In Reply to: How much driving needed to keep battery charged, SaabEli, Wed, 17 Oct 2007 08:18:13 Members do not see ads below this line. - Help Keep This Site Online - Signup |
It's not how far you drive, but how long (time). If the alternator is in good shape, you should be able to keep a charge on the battery with 5 or 10 minutes worth of engine running. Faster is better, but even 2000 rpm has the alternator at full output.
What's missing here is the alternator voltage. The alternator kicks in as soon as the engine gets to idle. But if the alternator is tired (actually, the brushes on the voltage regulator are worn), the alternator won't output much voltage - maybe only about 13 volts at speed. Now, anything over 12.6 charges the battery, but with not much current. The higher the voltage, the more energy you'll cram into the battery in a given period of time.
So first, make sure the BAT light works - turn the key to ON but don't start the car. The BAT (or ALT) light should come on. If not - fix it - the alternator won't work without it.
Next, start the car, and measure the voltage on the battery with a meter. At idle I'd want to see at least 13.0 volts, preferably higher, like at least 13.4. Rev the engine to about 2000 rpm - you should see 13.4 volts at least. If not, think about replacing the Voltage Regulator. It's easy - right on the back of the alternator. A new VR gets you a new set of brushes.
If you are getting good voltage readings - 13.4 at least while the engine is at 2000 rpm or more - then you may not be driving enough to keep it charged. Throw a trickle charge on. But since you did that and the battery keeps running down, I'd suspect tired brushes.
There is a different issue to worry about - you're killing you engine with evil oil. What? The engine never gets to temperature in a mile drive. You suck in warm, wet air, and the engine produces warm wet air as part of the combustion process. This gets sucked up by the oil. Normally, a decent drive (15-20 minutes) warms up the oil enough and long enough that the water bakes out. In your case, it doesn't. So the oil contains lots of water, which combines with the combustion by-products and produces acid. So instead of nice lubricating oil, you're pushing water and a weak acid solution around your engine. Not good at all.
To keep from destroying the engine, you should take the car out for a drive at least once, preferably twice, a week. Drive for 20 minutes - make sure the temp gauge gets up to normal and stays there for 15 minutes or so. And even with that, you should be changing the oil at least every 3 months, even if it has only a couple of hundred miles on it. Believe it or not, driving very short distances counts as severe service - the engine never reaches operating temp, the engine is running rich most of the time (leans out when it warms up), and you don't bake water out of the oil. So you need to replace than odious stew in the crankcase often.
Hey - it's a Saab - drive it!
posted by 192.249....
, Wed, 17 Oct 2007 09:42:59 <-- Viewing This Message, Wed, 17 Oct 2007 09:49:38
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