On Sat, 13 Dec 2014 05:24:17 -0600, wrote:

My Chevy pickup truck was getting hard to start on cold days. I took it
to a service garage and had the guy put a battery tester on it. The
results showed it to be borderline between Fair and Good. The
alternator was fine. The guy said the battery should last till Spring.
WRONG! The other day I parked at a local restaurant to use their WIFI.
and plugged my laptop computer into my inverter. About 40 minutes went
by, when the computer switched to it's internal battery and my truck's
dashboard clock went to 12:00. That little 300W inverter actually
drained the car battery so much the dome light was barely visible.

I got a jump and drove home. The next day I bought a new battery.

Anyhow, my reason to post this is because the truck has a built in volt
meter on the dash. All my older vehicles had either an idiot light, or
ammeter gauge. The ammeter would always indicate charging or discharge,
but the volt meter is a little harder to understand.

But I'm a little puzzled by the erratic readings on that volt meter when
I had that failing battery. Normally (and with my new battery), it's
almost always reading about 14V (normal), and slightly less after
starting the engine or if the lights were left on for awhile after the
engine was turned off.

When I had that failing battery, I'd be driving down the road, and the
volt meter would suddenly drop from 14v to around 11v. Then it would
slowly creep back up. This was under a constant load, (not when I was
braking and using brake lights, etc). This seemed to be happening more
and more, and I would often notice the headlights would get a little
dimmer when that meter dropped. That's why I had the service station
test my battery and alternator.

Oddly enough, while the battery tested borderline between Fair and Good,
it apparently was much worse than the test showed. What I dont
understand is why the volt meter would fluctuate, drop from 14v to 11v
and then go back up, and do so repeatedly...... My thinking is that one
(or more) of the cells were shorting out randomly. But I'm just
guessing.

Can anyone give a better explanation?

Note: With my new battery, the meter is staying at 14v all the time and
everything is working well, so that shows the problem was the battery
itself, not charging components or bad wiring or a bad volt meter.


I don't know about Chevy but I know that FORD runs their voltmeters
from the computer and they ALWAYS read at the 14v mark until something
is SERIOUSLY wrong and then they drop down a lot lower. It's not
really a voltmeter, it's just a go-no go indicator just like an idiot
light. It tells you you have "good voltage" OR you have "bad
voltage". That is probably why you saw it fluctuating like you did,
because the battery was bad, but not because it was intermittently
bad, just because it was running right at the computers switch point
between bad and good. The way to verify if your voltmeter is a REAL
meter or just an idiot-meter is to run the car at idle, see where the
voltmeter needle points, and then turn on the headlights on high beam.
If the needle doesn't drop at least a volt or two it's just an idiot
gauge. A lot of cars temperature gauges are similarly set up although
not as go-no go but still computer controlled so the computer tries to
hides bad news till its really bad by restricting the needle so it's
not linear with temperature rise.. They are better then just an idiot
light but not as good as a true gauge.

The average driver thinks he wants gauges but he really doesn't, the
fluctuations make him nervous and result in extra warranty work when
someone complains "my oil pressure keeps dropping". Most oil pressure
gauges are also just a go-no go nowadays.