Installing two voltmeters

Reg M

.
May 21, 2016
131
Hunter 31 Montague
I want to install two small digital voltmeters in my 1984 H31, one for each battery. Rather than have them on switches, I would like to have them both come on whenever the battery selector switch is on. I want both meters on whenever the selector switch is on, regardless of which battery is selected and I want meter #1 to monitor Batt #1 and meter #2 to monitor batt#2. I just can't seem to get my head around it so would appreciate any insight anyone can provide. Thanks. Reg
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
This is one possibility. The meters should have been labeled Meter 1 & Meter 2, rather than both being Meter 1. Sorry for the typo in the quick sketch
 

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SG

.
Feb 11, 2017
1,670
J/Boat J/160 Annapolis
Why do you want the meters to only be on when the batteries are in the "on" position?
Using shunts, you could set it up so you actually got a measure of each battery.

This shows how to install a specific manufacturer's equipment, but the principles are the same: https://theboatgalley.com/monitoring-the-batteries/
 
Nov 6, 2006
9,884
Hunter 34 Mandeville Louisiana
Easy to do with one double pole, single throw switch. Switch "ON" and ya see both meters, each reading its batery, switch "OFF" and both meters are out of the circuit. The little "V" in the circle is the voltmeter.. Not automatic, but if mounted by the batt selector you could turn it on when you turn the batt on if you wanted to see the volts that day. You could sub a Double pole, Single Throw relay for the switch and hook the relay coil to the "load" terminal. That way both meters would come on as soon as the big batt switch is turned "on".
 

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Last edited:
Feb 26, 2004
22,759
Catalina 34 224 Maple Bay, BC, Canada
RegM,

Have you considered buying or downloading a BOOK about boat or DC electrical systems?

None of us was born to be an electrician.

All of us who are passing this stuff on to you were babes in the woods, just like you.

Before the internet, we went to libraries.

With the internet, there is simply to excuse to say: "
I just can't seem to get my head around it...
I know your ? was rather specific, but this is really Electrical 101.

Good luck, but luck should have nothing to do with it. Kloudie "alluded" to it: momentary for always on. If always on, why? Even with LEDs, why waste the energy needed to run meters all the time when you only need to see them momentarily?

Your boat, your choice. :)
 
Feb 6, 1998
11,665
Canadian Sailcraft 36T Casco Bay, ME
I want to install two small digital voltmeters in my 1984 H31, one for each battery. Rather than have them on switches, I would like to have them both come on whenever the battery selector switch is on. I want both meters on whenever the selector switch is on, regardless of which battery is selected and I want meter #1 to monitor Batt #1 and meter #2 to monitor batt#2. I just can't seem to get my head around it so would appreciate any insight anyone can provide. Thanks. Reg
#1 For any reasonable accuracy of a volt meter, the meter should directly sense / be physically attached to the battery banks + & - terminals not at the battery switch. Otherwise you're not reading battery voltage, just random bus voltage, with all the voltage drop associated with the wiring & terminations and this is most often not an accurate representation of the actual battery terminal voltage.

#2 If you want both volt meter to come on when you fire up a 1/2/B switch then you will want a DPST 12V relay that is driven by the "C" post of the battery switch. The relay will increase the "on" consumption of the digital volt meters and you'll find average coil currents in excess of .075A.

#3 Beware of cheap Chinese voltmeters as the accuracy is all over the map. Remember every 10% of battery capacity is represented by roughly 0.1V change in voltage. For the best accuracy and "trend monitoring" you'll want a voltmeter that reads to the hundredths position (12.65) not just the tenths (12.6). Also, using volt meters to track battery bank SOC is difficult unless using it as a "stop discharge" guidance point. In other words, do not discharge your 12V bank below 12.2V at your average house loads.

#4 One of my favorite inexpensive volt meters is the Blue Sea 1733 DC Voltmeter. It draws just 15mA or 0.015A. If you want two of them this would be 0.03A. If you want two of them, plus a relay to drive them, then you're looking at 0.105A +.

#5 If you use one voltmeter, as in the diagram below, and you forgot and left it on, consumption looks like this:
0.015A X 24 Hours = 0.36 Ah per day or 2.52 Ah per week

#6 You are better off with one digital volt meter and a SPDT switch (momentary or fixed your choice). There's no dire need to know the voltage of a bank you're not actively using.

#7 Don't forget that each battery lead to the volt meter needs a fuse within 7" of each banks positive post.

The below diagram keeps it simple.

 
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Likes: Dan_Y

Reg M

.
May 21, 2016
131
Hunter 31 Montague
Easy to do with one double pole, single throw switch. Switch "ON" and ya see both meters, each reading its batery, switch "OFF" and both meters are out of the circuit. The little "V" in the circle is the voltmeter.. Not automatic, but if mounted by the batt selector you could turn it on when you turn the batt on if you wanted to see the volts that day. You could sub a Double pole, Single Throw relay for the switch and hook the relay coil to the "load" terminal. That way both meters would come on as soon as the big batt switch is turned "on".
Kloudie1 - thanks for your reply but unfortunately the attachment won't open for me. In hindsight I realize I should have only installed one meter but when I rebuilt my electrical panel I installed two meters so I thought I may as well use them both. It sure is nice to have a forum where folks like yourself are willing to share knowledge without feeling a need to lecture the person seeking the knowledge. This is stuff I have done in the past on previous boats but electrical was something I always had to read up on, I just did not seem to retain it. I had an excellent book written by Nigel Calder but did not expect to own another boat in my old age so I gave it to a friend. Regarding your attachment, maybe you can just give me the source and I can try to find it directly. I guess I could just install two SPST switches and turn on whichever one I want to monitor.Thanks again. Reg
 

Panera

.
Jan 18, 2014
57
Hunter 280 Portland, ME
Just remember when you set the battery selector to BOTH, the reading will be the same since both sets of batteries are now tied together. To read each bank you will need to set the Battery Selector to either OFF or 1 or 2.
 
Nov 6, 2006
9,884
Hunter 34 Mandeville Louisiana
Reg, that file is a "PDF" so it should open with ADOBE Reader .. I agree with Maine Sail that if you want absolute accuracy, the meters should be hooked directly to the battery + instead of the 1-2-Both-OFF master switch, but I figured you could have a nice installation with one meter on either side of the master switch on the H-31.. You'd not get absolute accuracy but the volts drop in either leg should be about the same since the legs are symmetrical.. I would not use a relay in order to avoid the extra draw, I'd put the switch under or above the master so you could turn the meters on if you want to or leave them off..
 

Reg M

.
May 21, 2016
131
Hunter 31 Montague
Reg, that file is a "PDF" so it should open with ADOBE Reader .. I agree with Maine Sail that if you want absolute accuracy, the meters should be hooked directly to the battery + instead of the 1-2-Both-OFF master switch, but I figured you could have a nice installation with one meter on either side of the master switch on the H-31.. You'd not get absolute accuracy but the volts drop in either leg should be about the same since the legs are symmetrical.. I would not use a relay in order to avoid the extra draw, I'd put the switch under or above the master so you could turn the meters on if you want to or leave them off..
I agree with the idea of not using relays. I already installed the meters last fall when I built a whole new panel, using the existing switch panel. I found that the panel was set too far back under the main companionway and the switches were hard to see, combined with the fact that years of the panel being taken out and put back caused the screw holes to be enlarged and all the excess wire for my electronics is behind there causing it to be a tight squeeze. When I made the new panel I brought it out (further forward) about 2 and a half inches giving more room for wire and more visible. Anyway, when doing the job I cut the hole in the plywood big enough to accomodate a piece of black plastic which is to port of the switch panel, and installed the meters there. I bought two spst switches today and will put them either beside or under the meters and just wire them directly to the batteries.. I want to get my new hour meter in there too. I'll post a photo when its all done.
 

Reg M

.
May 21, 2016
131
Hunter 31 Montague
Well, I finally got around to taking a photo of the new panel setup. The two spst switches are working out just fine for the two voltmeters, they are wired directly to the respective batteries.
IMG_0299.JPG
 
Oct 24, 2010
2,405
Hunter 30 Everett, WA
#1 For any reasonable accuracy of a volt meter, the meter should directly sense / be physically attached to the battery banks + & - terminals not at the battery switch. Otherwise you're not reading battery voltage, just random bus voltage, with all the voltage drop associated with the wiring & terminations and this is most often not an accurate representation of the actual battery terminal voltage.

#2 If you want both volt meter to come on when you fire up a 1/2/B switch then you will want a DPST 12V relay that is driven by the "C" post of the battery switch. The relay will increase the "on" consumption of the digital volt meters and you'll find average coil currents in excess of .075A.

#3 Beware of cheap Chinese voltmeters as the accuracy is all over the map. Remember every 10% of battery capacity is represented by roughly 0.1V change in voltage. For the best accuracy and "trend monitoring" you'll want a voltmeter that reads to the hundredths position (12.65) not just the tenths (12.6). Also, using volt meters to track battery bank SOC is difficult unless using it as a "stop discharge" guidance point. In other words, do not discharge your 12V bank below 12.2V at your average house loads.

#4 One of my favorite inexpensive volt meters is the Blue Sea 1733 DC Voltmeter. It draws just 15mA or 0.015A. If you want two of them this would be 0.03A. If you want two of them, plus a relay to drive them, then you're looking at 0.105A +.

#5 If you use one voltmeter, as in the diagram below, and you forgot and left it on, consumption looks like this:
0.015A X 24 Hours = 0.36 Ah per day or 2.52 Ah per week

#6 You are better off with one digital volt meter and a SPDT switch (momentary or fixed your choice). There's no dire need to know the voltage of a bank you're not actively using.

#7 Don't forget that each battery lead to the volt meter needs a fuse within 7" of each banks positive post.

The below diagram keeps it simple.

On ours, I used momentary contact switches so I just press #1 or #2 when I'm looking at the meter. I have no parasitic current when I don't need to look at the meter. I installed a small fuse in case someone pushed both switches at once, but it's never been a problem. A small resistor (1 K or so) would also work to isolate the 2 button problem. I also verified meter accuracy against my calibrated Fluke.
Ken