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ContinuousWave: Small Boat Electrical
Is a Voltmeter switch necessary?
|Author||Topic: Is a Voltmeter switch necessary?|
posted 04-29-2009 08:56 AM ET (US)
I heard the voltmeter can put a drain on the battery if it is on all the time, anyone have an opnion?
Repowered my 40 Yamaha with a 70 (2-stroke) and the trim gauge is built in the new tach, so I'm thinking of putting a voltmeter in the hole where my old trim gauge is now.
Electronics are not my forte so I thought that might be simplest, but don't want to cause problems. Any thoughts or advice would be appreciated.
posted 04-29-2009 10:16 AM ET (US)
Unless you leave the boat unattended for months, the drain of a voltmeter is insignificant.
However, The more common wiring would be to hook it to the ignition circuit (purple wire) which is hot when the key is in the on position (and the engine is usually running).
Myself, I've always wired voltmeters to the battery buss, as the reading with engine off is also useful information on the state of the battery.
But then again, I've always had a master batter switch, so I never quite had the situation where the voltmeter was always on.
posted 04-30-2009 11:03 AM ET (US)
I'm currently planning a complete rewire and this is something I was wondering about also. I have a two battery system. My current plan is to install two volt meters. One will monitor the distribution block when the ignition is on, and the other will us a momentary two position toggle to check either battery 1 or 2, therefore never causing a drain. This 2nd gauge/toggle will be wired directly to each battery (not to a distribution bar). I don't know much about wiring either, so comments please.
posted 05-01-2009 09:31 AM ET (US)
A voltmeter is actually a current meter or ammeter with an input limiting resistor. The typical ammeter in a voltmeter is a 1-milliampere meter, or a 0.001-A meter. However, this is not a guarantee that the particular meter being discussed is a 1-milliampere meter. If the nominal voltage is about mid-scale on the meter, the current drain is then about 0.0005-Ampere
A current drain of 0.0005-A on a battery is not very much, but why have any current drain on a battery when it is not being used? Why have a meter indicating the voltage when no one is around to watch it?
The typical installation of a voltmeter on a small boat will place the voltmeter in the IGNITION circuit on the engine key switch. When the switch is in the RUN position the meter will monitor the voltage. This monitors the voltage in the branch of the circuit associated with the engine.
If you wish to monitor the voltage directly at the battery, which is not a bad idea, then a switch with a momentary contact is a good idea.
posted 05-01-2009 09:51 AM ET (US)
When the dial pointer of a meter is moved upscale, a small amount of work is done. The energy to do that work is provided by the electrical current that has flowed through the meter. When a battery is providing the electrical current to do that work, the electrical energy is being drained from the battery. This is not really a matter of dispute or opinion, nor should it come as a surprise.
When I mentioned above that a typical voltmeter would be built using a 1-milliampere meter, that may not be true of inexpensive voltmeters sold for recreational boats. In general, a meter becomes more expensive as it becomes more sensitive, so a boat voltmeter might really be based on a 10-milliampere meter which is probably less expensive than a 1-milliampere meter would be.
Also, if the meter is any kind of meter other than a D'Arsonval or Weston type meter, for example, if it were a digital meter, it will draw much more current than I suggest above. A digital meter needs power to illuminate its display and to run the electronics which are performing the measurement. Thus a digital meter could consume 100-milliamperes or more of current.
For a voltmeter that is being used to monitor a battery to determine its state of charge, the accuracy should be in the 3-percent range. The terminal voltage on a battery varies only a small amount as its state of charge changes. If the state of charge is to be determined with any accuracy, the meter used to make the measurement has to be properly calibrated and have a known accuracy.
In many cases the voltmeter installed on the helm panel of a small boat is not particularly accurate or well calibrated, but it does provide a rough indication of the battery condition. Such a meter is useful to see if the battery is
--being charged, if the voltage is greater than 13-volts, or
--being overcharged, if the voltage is greater than 14.5-volts, or
--not being charged, if the voltage is less than 13-volts when the engine is running.
Many voltmeters do not contain any kind of scale markings that provide an indication of less than 1-volt of resolution, or they just contain fields of green and red colors to indicate good and bad voltage levels. This does not mean they are of no value, but they are just not accurate enough to determine the battery state of charge with much resolution.
I prefer to carry with me a portable voltmeter with good calibration and accuracy. If I have worries about the battery condition I can use the portable meter to measure the battery directly on its terminals. I can also use the portable meter as diagnostic tool for investigating other electrical problems in the boat or in the engine.
posted 05-01-2009 10:56 AM ET (US)
The CruzPro digital voltmeter I recently installed is speced
at 18 milliamps.
posted 05-10-2009 10:52 AM ET (US)
Thank you everyone for your in depth replies. I need to read them more carefully, but it sounds like putting it on the ignition is the way to go. I don't like the idea of anything draing the battery no matter how insignificant the drain.
posted 05-10-2009 10:54 AM ET (US)
Thank you everyone for your in depth replies. I need to read them more carefully, but it sounds like wiring through the ignition is the way to go. I don't like the idea of anything draining the battery no matter how insignificant the drain.
posted 05-10-2009 05:21 PM ET (US)
Good decision. Simple and functional. And cheap.
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