volt gauge

Queen of Africa

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Jan 16, 2012
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I am in process of a complete rewrite of a 1977 ranger bass boat.

when I purchased it, it had two volt meters.

when complete, I will have three batteries-1 for ignition system to include accessories and 2 batteries for minn Kota 24 volt trolling motor.

can I use one gauge to read two 12 volt systems (12 volt start battery and 2-12 volt/24volt in series=12 volt)?

If so, how would I wire it for toggle switch?

Also, as this is my first full size boat...is this a good to have read...pragmatic?

This would free up a hole for tachometer which I am planning to add.

Any suggestions are appreciated as I am planning on using boat to fish bass tounaments.

I have gained much from this site. Thanks in advance!
 

NMShooter

Petty Officer 2nd Class
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Mar 17, 2010
Messages
196
Re: volt gauge

Yes... you can use one voltmeter for both batteries. It would need to be capable of reading ~25V full scale for the two batteries in series.

Basically, you do the following :

1) Attach the ground of the meter to ship ground.
2) Get a single pole, double throw switch. Attach the "pole" (center position of the switch per the schematic symbol) to the positive side of the voltmeter.
3) Attach one input side of the double throw switch to one battery, and the other input of the switch to the second battery. This allows the switch to toggle back and forth between the two battery banks.

Most single pole double throw (spdt) switches are break-before-make, but see if you can confirm this before installation.

Always, always, always, fuse your batteries using an inline fuse or fuse link back at the battery.

Good luck!
 

NMShooter

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Mar 17, 2010
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196
Re: volt gauge

One more thing... this method would have one of your batteries always connected to the meter, which would put a very small load on the battery. Ok for use, but not so good if you leave your batteries attached during storage. You could also put one more switch in-line between the battery select switch and the voltmeter. This would allow you to turn on/off the voltage as applied to the meter and would allow you to completely isolate the meter from the batteries when you want.
 

Silvertip

Supreme Mariner
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Sep 22, 2003
Messages
28,758
Re: volt gauge

That will work ONLY if the ground for the 24 volt system and the ground for the engine starting battery are connected together. You could connect the 12 volt input to the switch from the "I" (ignition) terminal on any of the gauges. This would kill power to the voltmeter when the ignition is off. You would also need to set the switch to the 12 volt system for this to work.

WARNING: DO NOT connect the engine start battery ground to the ground terminal on the troller battery that also has the POS 24 volt line going to the trolling motor. Connect the start battery ground to the ground terminal on the troller battery that has the NEG line from the troller.
 

Queen of Africa

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Jan 16, 2012
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Re: volt gauge

okay...from your input I have made myself a diagram but will have to verbalise it for your assessment to confirm workability

1. connect wire from start batt ground to wire from neg terminal ground (24 volt system that feeds directly to trolling motor) which then connects to the neg lead on back of the volt gauge

2. a. using a spdt switch, connect wire from the center input pole to the "I" lead on back of the volt gauge fed off ignition power ( my case- purple wire)
b. connect wire from 2nd input (top or bottom) to the pos terminal on start battery
c. connect wire from 3rd input (remaining input) to the pos terminal on troller battery (fused wire going directly to trolling motor)

so if correct, when ignition key is on position then I can flip switch up to gain volt reading on one system (start battery) and flip switch down to gain volt reading on 24 volt system (which in series will read as one 12 volt battery system)


Will this work as I understand it?

Also, based on the volt meter I have as original equipment... I assume I will need to purchase a new volt meter that has upward of 25 volt capacity based on input by NMShooter.
 

NMShooter

Petty Officer 2nd Class
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Mar 17, 2010
Messages
196
Re: volt gauge

Here you go... make it look something like this. There are a lot of ways to skin this cat. You can pick up 12V ship power under your console from the main line or the switched accessory power. You'll probably need to route a wire from your trolling batteries to the console. This can be a small gauge wire, as it will not be carrying much current. However, do put a small inline fuse for this line.

Two switches shown... one for selecting between the starter or trolling batteries, and another to allow you to not drain the batteries when you are not monitoring the voltage. This second switch is not so critical... especially if you disconnect your trolling batteries from the system when you are storing it. You can use either a toggle switch or a momentary, "normally open" pushbutton switch as the isolation switch. Note that I didn't show the illumination connection for the meter which you might run off of your console light switch too.

You probably should use a 30V DC meter. A 12V battery is really a 12.6V battery, and during charging or shortly after charging it is not uncommon for that battery to be close to 14V. So for two of those stacked up, 28V or slightly more is not out of the question. Once you start putting a load on the batteries, the float charge comes off the top pretty quick and the 12.6 to 12.4 Volt range is more common.

Don't skimp on the inline fuse, and it is probably a bad idea to try and tap into the wires from the trolling batteries to the trolling motor. Just run a small 16 ga automotive wire from the small fuse (1A to 5A) on the trolling battery directly to your console.

dualbattmeter1.jpg
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: volt gauge

Absolutely WRONG. You cannot use a voltmeter that registers to a maximum of 18 volts on a 24 volt system. Two batteries in series makes 24 volts -- not 12 volts. That is why you were told early on in this thread that you need a voltmeter that reads higher than 24 volts. There are many 0 - 30 volt digital and analog gauges on E-bay. You will not likely find one to match the gauges you currently have.
 

NMShooter

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Mar 17, 2010
Messages
196
Re: volt gauge

There is one way to use your existing "12V" gauge that matches your instrument cluster, but it is a bit a baling-wire approach. It will work, but will have a few drawbacks :

1) The below circuit uses a voltage divider to divide your trolling motor bank voltage in half. This will introduce a small amount of error depending on the values of the resistors being matched, and the amount of current that your voltmeter uses. If you have a digital meter, the effects will be somewhat minimized.

2) This divides the trolling motor battery voltage in half. So you will need to double whatever the gauge reads.

3) The resistors can be placed under the console. Insulate and secure them well, and protect them from the environment.

4) You'll need a double pole, double throw switch for this.... wiring it up will be a bit of a pain.

5) Notice that when the selection switch is in the trolling battery position, that you are sucking 25mA off of the battery. Not a lot of current, but still a waste.

I think this is a non-optimum solution for what you are trying to acheive, but it is an example of how you might work around saving or matching your gauges.

dualbatt12vmeter.jpg



Note... schematic edited to correct error that UncleWillie caught....
 
Last edited:

UncleWillie

Captain
Joined
Oct 18, 2011
Messages
3,995
Re: volt gauge

There is one way to use your existing "12V" gauge that matches your instrument cluster, but it is a bit a baling-wire approach. It will work, but will have a few drawbacks :

1) The below circuit uses a voltage divider to divide your trolling motor bank voltage in half. This will introduce a small amount of error depending on the values of the resistors being matched, and the amount of current that your voltmeter uses. If you have a digital meter, the effects will be somewhat minimized.

2) This divides the trolling motor battery voltage in half. So you will need to double whatever the gauge reads.

3) The resistors can be placed under the console. Insulate and secure them well, and protect them from the environment.

4) You'll need a double pole, double throw switch for this.... wiring it up will be a bit of a pain.

5) Notice that when the selection switch is in the trolling battery position, that you are sucking 25mA off of the battery. Not a lot of current, but still a waste.

I think this is a non-optimum solution for what you are trying to acheive, but it is an example of how you might work around saving or matching your gauges.

dualbatt12vmeter.jpg

Correction:
The Left side of the Resistor Pair should go to the TOP Terminal of the DPDT Switch.


If the Votage for the Starter battery is not behind the Ignition switch, without a switch in the Ground lead of the Meter, The meter will always be ON and slowly draining the Battery.

Do Not leave the Switch in the 24 volt when not operating.
 

NMShooter

Petty Officer 2nd Class
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Mar 17, 2010
Messages
196
Re: volt gauge

Fixed! Thank you for checking.

BTW, I called Faria gauges just for grins to see what the input impedance was for some of their gauges. I was not surprised to find out that they did not know.

I talked to a friend, and he indicated that he has seen cheap meters with impedance as low as 1k to 50k Ohm/Volt. So using that voltage divider could be as incorrect as 5%.

If I were to do this, I would probably measure the actual trolling motor battery with an accurate meter, and compare it to the console meter to determine the amount of error, and then just compensate.

I would still recommend finding a 30V digital gauge and be done with it. But I have an old boat and mis-matched gauges is not the ugliest thing on my boat :)
 

UncleWillie

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Joined
Oct 18, 2011
Messages
3,995
Re: volt gauge

Fixed! Thank you for checking.

BTW, I called Faria gauges just for grins to see what the input impedance was for some of their gauges. I was not surprised to find out that they did not know.

I talked to a friend, and he indicated that he has seen cheap meters with impedance as low as 1k to 50k Ohm/Volt. So using that voltage divider could be as incorrect as 5%.

If I were to do this, I would probably measure the actual trolling motor battery with an accurate meter, and compare it to the console meter to determine the amount of error, and then just compensate.

I would still recommend finding a 30V digital gauge and be done with it. But I have an old boat and mis-matched gauges is not the ugliest thing on my boat :)


Your welcome.

A DPDT Center OFF switch would be the cleanest solution.
But I doubt that anyone that is electrically challenged is even going to take us up on the circuit.

The digital meter is the prefered option.

When are the Marine gauge makers just going to produce a nice, all glass, engine display.
Heck even UltaLight Aircraft instruments are years ahead of the marine guys!
 

NMShooter

Petty Officer 2nd Class
Joined
Mar 17, 2010
Messages
196
Re: volt gauge

Occurred to me that doing something like the DPDT solution above and picking the starter battery voltage off of the ignition switch would also do away with the isolation switch. But I am far too lazy to redraw that schematic again.

Hmmmmm.... I wonder if there would be a market for an inexpensive "all in one" LCD gauge for older boats.
 

Queen of Africa

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Jan 16, 2012
Messages
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Re: volt gauge

I appreciate all the effort...I'm trying to find a 24+ gauge that I'm also able to see accurate 12 volt reading for the start battery... So far no luck

I have abt decided to buy a separate 24 v gauge and just use two separate gauges for the set-up

Thanks for the diagrams!!!
 

UncleWillie

Captain
Joined
Oct 18, 2011
Messages
3,995
Re: volt gauge

Try.... This. Or This.


Meter.jpg


99 cents and free shipping!
And it will fit in a 1.9 inch circle.
Mount it in a 2" dummy meter!! :D
 

sschefer

Rear Admiral
Joined
Nov 13, 2008
Messages
4,530
Re: volt gauge

Just hook the first volt gauge up to your starting battery normally and the second to the first battery in your 24v system. That will be 12v. its 24 volts on the terminals of the second battery in the series not the first. Both batteries in the 24 v system will equalize and should drain equally so your when your second gauge hits 11.9v you're done fishing for the day.
 

endoracing

Cadet
Joined
Sep 29, 2011
Messages
10
Re: volt gauge

Your welcome.

A DPDT Center OFF switch would be the cleanest solution.
But I doubt that anyone that is electrically challenged is even going to take us up on the circuit.

The digital meter is the prefered option.

When are the Marine gauge makers just going to produce a nice, all glass, engine display.
Heck even UltaLight Aircraft instruments are years ahead of the marine guys!

Couldn't help but post one of my projects...

IMG_0500.jpg

No reason to wait for the gauge manufacturers. It is still in its early stages so that may not be how it ends up looking. Downside is it needs a control board to interface with the boat itself, but that is being made right now :)
 
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