Digital voltmeter

[wchevron]

i was thinking of getting a voltmeter for the nav station. should i go with a digital or analog. the analog are much less expensive but would i need one for the starting battery & one for the house battery. on the digital side i was looking at a xantrex 2 bank battery monitor. it monitors volts, amps, amp-hours & time remaining for both batteries.

[sailingdog]

A xantrex Link is far more than a voltmeter. Well worth getting, as it lets you know if you need to upgrade the battery bank pretty quickly. Also, can show you if you have parasitic loads you might not be aware of, like a weak short, or bad switch, etc.

[Boasun]

I bought a digital voltmeter from Radio Shack about 15 years ago and finally broke down and changed the battery in it a couple of months ago. The meter is still ticking and gives excellent readings. I got the thing on sale... which in my opinion is the way to buy good equipment at a reasonable price.

[btrayfors]

There are advantages and disadvantages to each.

An analog meter with an expanded scale (like 10V to 16V) will tell you at a glance the approximate voltage of the battery being measured. Analog meters read instantly; you don't have to wait for them to "settle down". Also, analog meters don't create RFI problems for your radio.

You can easily wire in a switch to switch from one bank to another. If you have only two banks to measure, the simplest way is to buy a single-pole double-through (SPDT) toggle switch and run a positive wire from each bank to the end terminals on the switch, and the center terminal to the meter.

The downside of these meters is that they cannot give you the highly accurate measurement you need to really assess the state of your batteries. A change of only 0.4 volts is the difference between a fully charged (12.6 volt) battery and a battery which is 50% or more depleted (12.2 volts).

To measure voltage with some accuracy, you need a digital meter and, IMHO, one which has been calibrated against a known voltage. Once you have such a meter, you can tell with good accuracy the actual voltage of the battery bank and -- providing you remove all loads and all charging for 12 hours or more -- you can measure the voltage in a "resting state" and convert that directly to state of charge. The voltages cited above are "resting voltages", e.g., 12.6V = fully charged, 12.4V = 25% depleted, 12.2V = 50% depleted, etc.

The downside of many digital meters, including those from reputable manufacturers of digital meters for marine use, is that they often emit signals which can interfere with your AM radio reception and your SSB reception on HF frequencies as well. I have one which is very noisy, and have to turn it off when using the radio.

One alternative is to buy a good analog meter to install on the panel and also a good digital multimeter for more accurate measurements taken periodically. Every boat ought to have such a meter anyway!

Bill

[thekeip]

There is only one good reason to think about using an analog meter in your nav station...not many skippers are aware of it, either. The ABYC stipulates that if you have a permanently installed inverter or generator, you must have an AC voltmeter installed at or near your AC power panel. Further, since the output of an inverter is rarely sinusoidal, the voltmeter must be of the 'true RMS' variety. Well, it's a challenge to find a true RMS reading digital panel mount meter. Somewhat less challenging is to opt for an analog meter, but it must be the 'iron vane' style to be correct.
I have actually found and installed these for that purpose.

I don't think I've ever, ever been confronted with a noisy digital voltmeter. It should be the least of your worries in your nav station.
Howard Keiper
Sea Quest
Berkeley

[btrayfors]

Howard,

Your point about true RMS voltmeters -- and their rarity among panel meters -- is a good one. Many inverters put out "modified square waves", not sine waves. Further, to the extent that such output contains DC components as well as the "modified square wave" AC component, even many "true RMS" voltmeters won't give you an accurate reading. Some higher-end products with "AC+DC" settings do much better.

However, insofar as I know, the ABYC code for inverters does not require a voltmeter, much less an accurate RMS-reading one. Rather, it specifies "a visual means", viz:

25.5.7 A visible means (e.g., voltmeter or lamp) of determining that the inverter is “on line” and/or in “standby” mode shall be provided at the main electrical distribution panel.

The problem of true RMS doesn't come up with AC generators, since most of them have sinusoidal outputs.

Re: RFI problems, I have a Blue Sea Systems digital voltmeter which is very noisy both on AM broadcast and on HF frequencies. Many others have reported similar problems with digital meters.

Bill

[thekeip]

Bill.. Try ABYC 11.11.3.2 through 11.11.3.2.3
Howard

[btrayfors]

Howard,

Yes. Sorry, mea culpa. ABYC does require a voltmeter on the distribution panel when there is a permanent connection to a motor, a generator, or an inverter, viz:

11.11.3.2. A system voltmeter shall be installed
on the main panelboard if the system is permanently
connected to
11.11.3.2.1. motor circuits, or
11.11.3.2.2. a generator, or
11.11.3.2.3. an inverter. If the inverter does not
have a true sinusoidal output, the voltmeter shall be a
true RMS type. (See ABYC A-25, Power Inverters.)
EXCEPTION: The inverter voltmeter may be
installed in proximity to the panelboard.

Ain't this fun (and educational, too!).

Bill

[sailaway21]

And me, I was likin' the light bulb idea!

Is there anything other than battery state of charge that you guy's prefer digital over analog?

For years on hand-held meters I've used an Amprobe analog for checking out pump motors. They worked great, especially checking starting amps, but always broke after a year or so. I now have a digital version from them which does offer RMS but I otherwise do not care much for-although it's lasted almost two years. You can't pry my old analog Simpson Ohm-meter away from me!

I'll admit to an analog display bias in almost any instance.

[btrayfors]

Yeah, I kinda prefer the analog meters, too, whenever and wherever they make sense. In many applications they still do, and to me it's a joy to be able to tell at a glance -- from across the room or the cabin -- that things are XY or Z.

However, there's no dispute that the digital meters provide more accuracy, provided that they're calibrated. And, if all you're checking is voltage now present on the batteries, they can be just fine and they work in the dark! I suppose that the "13.2" I see on my digital meter on the bulkhead is a kind of comfort....I know that the Victron is maintaining the proper float voltage and compensating for any loads.

Bill