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#1
04-04-2003, 11:59 PM
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alternator, battery relationship
My wife's 89 300E died while driving. She said radio
first died and subsequently car lost power and died. I went to the rescue and jumpstarted the car but likewise died. Finally towed to my indy, who diagnosed it to be a shot alternator. He had my alternator re-built. For two hours and 400 bucks later, car is running (rough, but maybe just my imagination). My mechanic also recommended for me to change my battery, to prolong the life of the alternator. Can someone please explain the relationship between alternator and battery? I know a bad alternator could kill a good battery, but could it also be the other way around? TIA joel
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joel Prayers bring forth enlightenment. 
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#2
04-05-2003, 12:20 AM
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Joel,
A bad battery will not hurt an alternator, once the car is running it is relying on the alternator for "juice". However I wouldn't recommend leaving a bad battery in place as it certainly can't be good. The battery just enables you to get the car running. But, a bad alternator will most definitely kill a good battery as it seeks for a source of energy. Subsequently the alternator doesn't charge the battery contrary to popular belief other than a minuscule volt here or there. The alternator would surely blow a battery to high heaven if that were the case. Hope this helps a little. I'm sure someone will chime in if I have gotten anything twisted. Happy motoring  P.S. I like your signature/quote
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Brandon 2008 S550 1957 Dodge D100 1967 VW Microbus 21 Window 2001 Suburban 2004 Beach cruiser bicycle -----------------GO DUKE!----------------- "It cannot be emphasized too strongly or too often that this great nation was founded not by religionists, but by Christians; not on religion, but on the Gospel of Jesus Christ. For that reason alone, people of other faiths have been afforded freedom of worship here." Patrick Henry 1776
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#3
04-05-2003, 01:42 AM
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Batteries are sized based on the cranking load and alternators are sized for the total electrical load. There is no direct relationship between one and the other.
When the alternator dies the car will run off the battery until voltage drops to the point were systems won't function anymore, which is what you experienced. Usually if the alternator quits the charge light on the dash will remain on, but a lot of people ignore warning lights. If the battery was charged while the alternator was being worked on then it will probably work okay. If the battery was significantly weakened by the discharge, it will probably show up as a slow cranking rate or an inability to hold a charge, which will result in low cranking speed or no-cranking if the car sits for a few days, so you should watch for these symptoms and replace the battery if it appears to weaken. A shorted battery can cause excess current generation by the alternator, but that should not harm a healthy alternator. Shorted batteries can happen suddenly. You can start the car and drive somewhere, then come back in an hour and it's completely dead. Jump starting a car with a shorted battery will sometimes not work because the shorted battery will not allow the alternator to maintain sufficient voltage because of the large current draw. In this case, disconnect the positive cable from the shorted battery, jump start it, and drive it home on the alternator. Leave all electrical accessories off and don't stall the engine! Duke
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#4
04-05-2003, 10:08 AM
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Hi joel,
I'm curious as to what type of alternator was put into your car. Was it a MB rebuilt? $400 sounds a bit high. Also, for those who read this thread in the future, check the regulator brushes first. Many times, this problem is the fault of worn brushes and, at least on a 300E, it's about a $25 part. Check out Fastlane. joel, it may well be that your alternator was fried and needed replacing. However, in the future, replace the brushes first. I had my alternator replaced last year but suspect that brushes would have worked. Anyway, glad your wife is back on the road. Regards,
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#6
04-05-2003, 10:31 AM
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hi chicago,
the alternator was my own. it was re-built. it probably could have been the brushes but i was not in a position to examine it any further. it was a difficult situation. my wife called me from work stuck on the freeway with my daughter. In Canadian Dolllars cost breakdown: 250.00 for alternator 97.50 1.5 labor 6.50 shop supplies -------- 354.00 26.55 provincial tax 7.5% 24.78 federal tax 7% -------- 405.33 total or 279.53 USD Thanks for asking and concern. Cheers! joel
__________________
joel Prayers bring forth enlightenment.
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#7
04-05-2003, 10:35 AM
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joel,
That sounds like the going rate of a bosch alt. So I think you did ok. Duke is right the usual fail from the electrical system in addtion to the battery is the Brushes 20-25$ part and 10 minute replacement!
__________________
Brandon 2008 S550 1957 Dodge D100 1967 VW Microbus 21 Window 2001 Suburban 2004 Beach cruiser bicycle -----------------GO DUKE!----------------- "It cannot be emphasized too strongly or too often that this great nation was founded not by religionists, but by Christians; not on religion, but on the Gospel of Jesus Christ. For that reason alone, people of other faiths have been afforded freedom of worship here." Patrick Henry 1776
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#8
04-05-2003, 02:57 PM
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Hi joel,
Ah, Can. dollars. That's about right then. Regards,
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