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I have a boat with a dual battery system and have purchased two 0-30V voltmeters similar to this one, it has three wires which I understand is negative, positive and a wire for the voltage to be measured.

Three-Wire voltmeter

I am wanting to put one on each of the battery's and have them operated with a push button switch.

Am I correct in that the switch would go on the positive wire, and would it be better to have the voltmeter for battery A be powered off battery B, and vice-versa, or would it not matter much?

My intention is for this to be an easier and safer means of testing that the batteries have juice and a means of diagnosing faults at sea or seeing how much drain the house electrics are putting on the batteries so it doesn't need to be super accurate.

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closed as off-topic by Michael Karas, Olin Lathrop, Ricardo, PeterJ, Daniel Grillo Oct 28 '15 at 10:47

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Michael Karas, Olin Lathrop, Ricardo, PeterJ, Daniel Grillo
If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Without a datasheet for your device, any answer would be just guessing. \$\endgroup\$ – Olin Lathrop Oct 27 '15 at 11:18
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    \$\begingroup\$ @OlinLathrop it's a cheap Chinese device, the datasheet is the sticker on the side ;-) As long as the voltage you want to measure is more than 5 V or thereabouts, these devices work just fine if you connect both red and white wire through a switch to the voltage you want to measure and the black permanently connected to ground (minus side of the battery). \$\endgroup\$ – Bimpelrekkie Oct 27 '15 at 12:05
  • \$\begingroup\$ Thanks, I know its cheap but it will do, it will probably get used a heap in the first few weeks once we get it back in the water to make sure the electrical systems work correctly and there are no unexpected drains, and after a few weeks when we are confident all issues have been fixed it probably wont get used much \$\endgroup\$ – Lima Oct 27 '15 at 12:10
  • \$\begingroup\$ The close reason doesn't seem solid to me. The said voltmeter is not a self-sufficient device, it must be connected properly in order to function. \$\endgroup\$ – Dmitry Grigoryev Oct 29 '15 at 9:15
  • \$\begingroup\$ Thank you, I agree but of course I am biaised :) \$\endgroup\$ – Lima Oct 29 '15 at 10:05
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Since you intend to operate the volt meters through a push button activate switch I would operate these cheap volt meters from a separate power supply such as a local alkaline battery pack. Momentary operation means that these batteries would last a long time. Use in this manner eliminates any concern about the state of the monitored batteries with respect to the meter operation. Such setup would be all similar to if you had a regular DVM that you turned on from time to time and probed your batteries with its leads.

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  • \$\begingroup\$ Cool, hadn't considered this option, this sounds like a plan. Thanks for your response \$\endgroup\$ – Lima Oct 27 '15 at 11:32
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The only reason there are three wires is that the circuit needs 4-30V to operate and it may be configured for measuring voltages outside this range.

To measure a battery that is always below 30V you would just connect the power and sense wires together. If it does not light up when you press the test button you know it is below 4V and the battery is pretty much scrap from very deep discharge.

Not only that you are going to have to get the oars out and row home.

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    \$\begingroup\$ Very true, hopefully the new battery management system that I've installed prevents the oars every coming into contact with the water :) \$\endgroup\$ – Lima Oct 27 '15 at 23:58
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Connecting signal lines before power lines are connected is a recipe for disaster with some devices. For example, take a close look at a standard USB connector:

enter image description here

Notice the ground and power pads (on the sides) are longer than signal pads? This is to make sure the power and ground are properly connected before any signal lines receive juice. If you don't have a USB cable, examine an SD card, a SATA connector, etc., pretty much any hotplug interface.

So NO, you should not put a switch on the positive wire while permanently connecting signal and ground. It may work, or it may damage the voltmeter, and there is no way to know which one it is without a datasheet which explicitly says you can do that.

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  • \$\begingroup\$ That makes sense. Given the lack of a datasheet, what other way could I put a switch in place? If I put a switch on the signal wire then the LCD is going to remain lit, is there a component that I could put on the contact side of the switch so when it is turned on then the signal wire is active too? \$\endgroup\$ – Lima Oct 28 '15 at 14:09
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    \$\begingroup\$ If you can, connect the power and signal wires together, like @KalleMP suggested. If not, you will need a two-pole swhich which cuts the power and the signal together. At the very least, you can try to apply voltage to signal line while power is not connected while not a sea, and check if your voltmeter tolerates that for long enough to feel confident. \$\endgroup\$ – Dmitry Grigoryev Oct 28 '15 at 14:37

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