I have a Forester S Turbo. Could a dying battery cause it to run badly? I just had to replace the battery because it wasn't holding it's charge and the car seems to be running a lot smoother since then. I don't understand why this would be because the charging circuit was OK, so whilst driving the voltage would have been normal.
Think about all the electronics onboard. All require a minimum voltage. Your dying battery caused things like the ECU to receive less than the 12 volts it likes.
OK I can understand what you are saying, but when the engine was running the electronics were running on 14.4v. Overnight, last night the battery volts dropped to 11.96v which would account for the poor starting I had this morning. Once the engine was running though, the voltage was restored, so surely the ECU and everything else were getting the correct voltage?
Could it have been that ECU was reset whilst the battery was being changed? If so then I need to do this more often as it has made such a difference.
I experienced the same thing on my 1995 BMW 328i a while ago. Can't really explain it, but the car was misbehaving badly for a week or so before the battery died. Replaced battery and everything returned to 'normal'. Most electronics runs on +5v and/or +12v so I would have expected a running alternator to provide adequate power... but who knows.
I had a 99 Chrysler T&C for 3 years. In that time I had two batteries lose a cell, so that the voltage was down to 11 volts. One of the symptoms was that, on starting, the speedometer, tachometer and the other gauges wouldn't work. They still refused to work until I had run the car for a while, stopped the engine and restarted. So it seems that in some cars adequate voltage must be present on startup -- the alternator can't make them begin working later even though the voltage is up to snuff at that time.
Sound like you know something about auto electrics. I wouldn't consider high load on alt as even if alt could put out 100A that is about 2HP not enough to make a noticeable impact.
The 11.9V is a concern. A fully charged 12V LA battery should read at rest 12.7V. I would check and see what kind of load there is on the battery when engine is not running. At the sub 12V level a large percentage of capacity is gone. Why, is the first thing I would be checking on. There could be a relationship between what is discharging the battery and the poor performance you first mentioned.
Mickey
There is negligible load on the battery when everything is switched off.
Using a Specific Gravity tester on the electrolyte, the battery was showing an *almost* full charge on all cells after a short run. The voltage was 14.7 when the engine was running and everything was switched on so I know it wasn't a charging problem. The high current discharge of the battery was OK and it tested at about 12.7v as you suggest but it was loosing charge overnight, down to 11.9v over 12 hours.
I have already replaced the battery and the car runs a lot better now.
My original question was why this would make the car run so much better. Logic says it can't be the voltage drop from the apparently faulty battery because as long as the engine will start and is running there is no voltage drop.
Merry Christmas to all
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