Battery effects

[Tony]

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.

 

[mulder]

If the battery was really weak it would have been placing a heavier than
normal load on the charging system, which then takes some HP from the
engine. If the load was high enough you would be able to feel it.

 

[johninKY]

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.

 

[Tony]

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.

 

[Jack]

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.

 

[WisedUp]

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.

 

[Mickey]

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.

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

 

[Tony]

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