12 volt battery display

I bought three 12 volt led 0 - 100 volt display for my RV. One for chassis, one for battery bank 1, and the other for battery bank 2. The displays come with three wires.
Red - positive power for the unit
Black - negative power for the unit
White - goes to positive post to measure the voltage of the battery
Each battery bank will have a red and white wire going to the positive post and a black wire going to the negative post. I want to add a on/off switch so all three displays can be turned off when the RV is not in use.
Can I take the 3 black wires from the display and connect them together and attach it to one side of the switch and then take the other 3 black wires from the 3 negative post and connect them together and attach it to the other side of the switch. Will this work or will it cause false readings because the three negative post terminals are connected. I would like to have one switch to turn off all three displays instead of three switches.

I would not. I would use a 3pST switch and switch the white/red wires,, One indicator per poll  You can get 3p or even more pole switches from any electronics supply.

Such a Switch

Assuming all 3 battery banks use a common ground connection to the chassis, and they should, you can use a single switch in the black wires to turn the displays on/off.

I don't see why not - it's a common chassis ground anyway. But if you want to play safe, you can use the triple pole, single throw, type switch that John mentioned. They sell for around $10-$12 on Ebay.

I was hoping to use a switch that was the same as the ones on my main panel. I can install the switch the way I mentioned above or I can connect the three grounds to the chassis instead of all three negative post and install the switch that away. I know it wont cause any major problems but I wasn't sure if it would cause false readings or incorrect voltage readings. Was hoping someone has done this install so I could cut down on the install/testing times.

Turning off the display seems like it has little value anyway. If you are concerned about preserving battery amps, why not install a battery disconnect switch and remove ALL 12v power from the RV? That would accomplish much more.

I have a battery disconnect switch installed and tha battery charger is pluged in while not in use. It was more for saving the led monitor so its not in use 24/7.

Don't worry about saving the LEDs, they'll outlive you, me, and your RV.

My voltage display has been running 24/7 for over 11 years now. Your mileage may vary, however.

Your displays are probably very simply voltage meters that you are installing in parallel. Voltmeters have a very high internal resistance to minimize the current flow through them for voltage testing. They do create an alternate current path around the element to be tested...in this case the battery. Note: batteries do have an internal resistance, which is why they discharge when they are not in use.

As a result, voltmeters have very little impact on the circuit. In other words, the circuit remains virtually unchanged or close to unchanged.

The reason you have two wires at the positive is that the displays don't include internal batteries of their own, so yes there is some minimal, actually de minimus draw from your batteries. That is why a multimeter can use a simple alkaline battery.
I also wouldn't worry about disconnecting them.

I would worry more about how to translate your readings under various conditions, and a good source if you need it and maybe you don't, would be the 12 volt side of life parts 1 and 2. Gaining some expertise regarding battery testing, storage, and charging is a good idea for all RVers.

Understanding ammeters, for a supplemental discussion is useful. They have a very low resistance to minimize the potential voltage drop through them. A voltage drop across a load is desirable, but not so for the conductor, leads, and contacts.

Ammeters also have minimal impact on a circuit, except you do have to break the circuit unless it has a clamp reader.  You can short an ammeter if you hook it up in parallel because of its lack of resistance.

Note: unless these meters that you are installing are continuous read, there won't be any parasitic draw from them. They are likely push button voltage readers and will draw a bit of current only when you push the button to read the voltage.

Parasitic draws serve a purpose, like the clock on a radio..the clock needs a small amount of continuous current to keep time. Nothing magical or mysterious.

Voltmeters CAN have a significant affect on a circuit depending on the circuit you are testing and the sensitivity of the meter. That's why voltmeters of different sensitivities are available.

Stay off the ammeter unless you really need to know current flow in a circuit. Since an ammeter is, in effect, a short circuit, connecting it wrong can draw excessive current and may destroy the circuit you are testing and/or the ammeter itself.

RodgerS said:

Click to expand...

???

Since the issue of a voltmeter's resistance is of major concern for some, obviously the poorer quality will be more of a concern:

Better multimeters /voltmeters in lower ranges (up to ...10..20V, using input buffers) have higher input resistance, usually >10 GOhms.

Multimeters with FET input buffers have 0.2....2TOhm input resistance (depending also on humidity, dirt, leads isolation).
Use the formulae to calculate its effect on voltage: E=IR

Be sure to consult your manual.

Rodgers: Sorry, but I didn't understand your last post. Would you expand?

Sorry, would like to, but I got to move on now. 
Unfortunately, I'm having to reduce my forum time because I have a lot on my plate starting my Mobile RV service technician part-time business.
Maybe another day.
Kindest Regards.

RodgerS said:

Since the issue of a voltmeter's resistance is of major concern for some, obviously the poorer quality will be more of a concern:

Better multimeters /voltmeters in lower ranges (up to ...10..20V, using input buffers) have higher input resistance, usually >10 GOhms.

Multimeters with FET input buffers have 0.2....2TOhm input resistance (depending also on humidity, dirt, leads isolation).
Use the formulae to calculate its effect on voltage: E=IR

Be sure to consult your manual.

Click to expand...

For the average user, these factors are of little concern. If you were dealing with sensitive electronic circuits where these factors made a difference, you would not be using these kinds of meters.

I will just put 3 separate on/off switches in. The wires are being run straight back to the battery so a battery cut off switch wont work. I also like the fact I can turn them off at night if they are to bright and annoy the people sleeping up front. 

Get a triple pole, single throw, toggle switch and you can turn all 3 on or off with one switch.