On Tue, 14 Nov 2000 21:24:21 -0500, Roger Turk wrote:
> Sam Heywood wrote:
>>> A voltmeter will not tell you whether a battery is good or bad. If I go
> out to my car right now and use a voltmeter to read the voltage on my car
> battery I will get a reading of 12 volts. Then I can turn the headlight
> switch on and leave it on all night long. By morning my headlights will
> no longer be shining and I will know that I have a bad battery. If I then
> turn off the headlight switch and use my voltmeter again to read the battery's
> voltage, I will still get a reading of 12 volts. The battery passes the
> voltage test, but it will not pass a load test because it won't provide even
> enough power anymore to turn on my headlights. If it can't even turn on the
> headlights, then of course I know that the starter motor won't run either.
> My point is simply this: A good voltage reading does not necessarily mean
> that a battery is good. You have to perform a load test. A voltmeter does
> not do this.
<snip>
> However, I decided to take a different tack. I had some batteries (AA and
> AAA) that I knew were going bad and some brand new AAA batteries that I just
> bought this weekend. As I understand Sam's hypothesis, the old, weak
> batteries and the new batteries should each measure 1.5 volts.
This depends on how old or how weak the batteries are. If you turn on your
car's headlights and leave them turned on until the point when they just
won't shine anymore, and then you turn off the switch and measure the battery
voltage, you will still get a nominal reading of 12 volts. If you leave
the switch on for a couple of days, then this will drain the battery
completely and you will probably get a voltage reading close to zero.
> Using the
> same voltmeter and the same setting, the old weak batteries measured 1.35
> to 1.42 volts, while the new batteries measured 1.52 to 1.54 volts.
The old weak ones are batteries that have been discharged over too long a
period to continue to conserve a nominal voltage of 1.5 volts. In this case
the voltmeter reading indicates that the batteries are no good. Even if you
were reading 1.5 volts on the old batteries, this would not necessarily
indicate that they were still good enough to use them for the purpose for
which they were designed. A low reading on a voltmeter can tell you that a
battery is bad, but a normal reading does not necessarily indicate that the
battery is good.
> A couple
> of the old batteries were Duracell with the integral battery condition tester
> (push the two dots and watch the yellow mark) and the integral tester did not
> even start to move.
The integral battery condition tester applies a load on the battery to
determine how much current (i.e amperes) may still be drawn from it. Some
batteries can pass the voltage test, but fail the load test. If you turn on
your car's headlights and leave the switch on just until the lights shine no
more, the battery will still pass the voltage test, but it won't pass a load
test.
> Hence, by this experiment, without angering my neighbors, the conclusion is
> that bad/weak batteries will have lower voltages than new batteries.
This is just a defining characteristic for "bad" or "weak". Of course we
know that bad/weak batteries will have lower voltages than new batteries.
Of course we know that a brand new battery is going to be better than an old
one of the same construction and design. My only point is that a voltmeter
will not tell you whether a battery is good or bad. A load test will.
All the best,
Sam Heywood
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