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.