[alberg30] Re: Voltmeter vs. AMP hr meter

[Robert E Johns]

From: Robert E Johns <bobjns at nais.com>

Lee and Chris,

I apparently missed Chris' comment, but since I use both voltmeters and an
amp/hr meter, here is my 2 cents worth:

	The voltmeter (it has to be an expanded-scale voltmeter that can
read to at least a tenth of a volt) gives you inaccurate readings
immediately after the batteries have been charged. It is reading  the
voltage accurately,  but after charging, the battery will read near the
charging voltage for quite a while unless you put a load on it. (Turn on a
25 watt bulb for at least a minute or two - wait until the voltage steadys
out before relying on the reading.)

The voltmeter is most valuable for determining battery condition when you
first come aboard. I use golf cart batteries for the main  battery and they
normally read lower than the normally accepted of 12.6 volts (about 12.45)
when fully charged and having been sitting for days. Gel batteries also
read a different voltage at full charge. If the engine has been charging
the battery, the meter will confirm that the battery is being charged, but
will not tell the battery's state of charge except that you can see the
charge voltage decrease if the regulator is a "smart" regulator that
switches back to a trickle charge after it thinks the battery is fully
charged. The voltmeter will give you an indication that you are running out
of charge when you are reading at anchor late into the night. The dimming
lights do the same thing.  :-)  The voltmeter will probably show an
increase after you have turned out the lights and the battery recovers
slightly, but the reading under load was probably the more accurate one.

The Amp-Hr meter is expensive, but has gotten cheaper since I installed
mine a few years ago. It also is somewhat difficult to  install as it
involves placing a shunt in the negative leg of each battery being
monitored. This leg is the heavy cable from the battery that has to carry
the full starter current so you are working with the big cables. I monitor
only the golf cart batteries with the A-H meter so I only had to install
one shunt. The other battery is used only for backup and starting. (The
separate battery for starting eliminates having electronics go offline when
the engine is started.)

Now for the pros of the A-H meter. It reads all of the current entering or
leaving the battery by measuring the voltage drop across the shunt. It
multiples the current by the time it is flowing and the meter reads
directly the number of ampere-hours added by charging or subtracted by
loads. If you have a 200 A-H battery, and the A-H meter reads 150 A-H, then
you are 1/4 discharged. (For best battery life you should not discharge
below 50%, so in practice I consider this 1/2 discharged.)

My A-H meter also reads voltage to a precision of one hundredth of a volt,
which is better than any analog expanded-scale voltmeter that I am aware
of. I do use it because I am used to monitoring battery voltage from the
time when I didn't have the A-H readings. It also reads current to a
precision  of one tenth of an ampere. This lets me read the actual current
drain of any device on the boat. It also shows me that I don't have any
significant leakage currents when I read it with all loads off. (The meter
is theoreticaly capable of reading the starter current, although I use the
separate starting battery which is not in the A-H meter path. I tried it
once before I separated the starting  circuit, but the engine starts so
quickly that the meter didn't have time to read.) I use the current reading
when charging the batteries because I can see the current decrease as the
battery comes up to full charge. This is a clue to the aging of the battery.

As you can see I like the A-H meter!

Bob Johns
Wind Call #397


>From: FINNUS505 at aol.com
>
>Hi Chris,
>Why are you installing a voltmeter vs. an amp-hr meter. The way it was
>explained to me, the voltmeter gives much more variable readings than an amp
>hr meter.
Skip
>Lee



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