How to measure dual battery voltages? (1 Viewer)

[CBdeJ]

Is there and easily tapped switched ground (on/off with ignition) in the dash area? I'm adding one of these guys into the panel next to the radio (one of BenCC's 3D prints) and I need a switched ground otherwise it will be on all the time.

Thanks in advance
Chris

 

[Ayune]

You can certainly switch a ground wire but you won’t find one in most vehicles. Typical order is: Battery->Fuse->Relay/Switch->Load->Ground.

For monitoring a spare battery you may have to use switched power to operate a relay which controls the ground connection from the meter. But others who have done this may have better insight than I.

 

[CBdeJ]

I'm a bit naive, electrically speaking, so bare with me. The display has two red wires (one for each display) that go directly to the two batteries and two black ground wires. I would attach the relay to the ground wires coming from the display (lets call them input) and an output wire to ground. The relay would draw power from a switched source (say the 12v/cigarette lighter) and be grounded to the same point as above. When the switched source is on, key to on or truck running, the relay would close the circuit and the display would work. Turn the truck off and the circuit is broken. Correct. If that's the case, where can I get such a relay. Do you have a recommendation/part that won't confuse the hell out of me/burn my truck to the ground.

Cheers,
Chris

 

[Ayune]

I'm a bit naive, electrically speaking, so bare with me. The display has two red wires (one for each display) that go directly to the two batteries and two black ground wires. I would attach the relay to the ground wires coming from the display (lets call them input) and an output wire to ground. The relay would draw power from a switched source (say the 12v/cigarette lighter) and be grounded to the same point as above. When the switched source is on, key to on or truck running, the relay would close the circuit and the display would work. Turn the truck off and the circuit is broken. Correct. If that's the case, where can I get such a relay. Do you have a recommendation/part that won't confuse the hell out of me/burn my truck to the ground.

Cheers,
Chris

That’s the idea, although the relay must be between the batteries and the meter for safety.

The reasoning is this: If you turn off your vehicle and meter goes blank because its ground was disconnected it looks ‘safe’ to work on. In fact, the batteries are still directly connected to the meter and if you slip with a screwdriver you’ll be welding and burning your truck to the ground in no time flat. Having the switch/relay between the power and load eliminates this possibility: When the vehicle is off most things are not energized in any way, and truly safe to work on.

You will need a “double pole” relay to switch each battery on its own circuit. And the circuits can’t be commoned together either or else you’ll connect the batteries together. I don’t think you’ll find a Bosch equivalent at AutoZone that’ll work, you need something like below. But again. Someone else with a dual battery setup may have a better solution. Perhaps you should change the name of this thread to be “How to measure dual battery voltages?” and someone may chime in.

Amazon product ASIN B07KMC8MW7

 

[CBdeJ]

Title changed, and Thanks.

 

[Ayune]

Bump - Anyone out there wired up a dual battery meter?

 

[jerryb]

I'm waiting also. I now have a car with two batteries. But.. I can't understand why I would want or need a relay for them.
What I would like is an on off, or only on when pushed readout for both.

 

[Ayune]

I'm waiting also. I now have a car with two batteries. But.. I can't understand why I would want or need a relay for them.
What I would like is an on off, or only on when pushed readout for both.

The relay is for automatically connecting the batteries to the meter when the ignition is on, and disconnecting when ignition is off. Otherwise meters would be on all the time.

The relay is kinda like an electric 'finger' pushing a pushbutton or moving a switch to 'ON' when you want to view the voltages.

 

[yukon]

Could you do the ground on a switch? May be easier than a whole relay setup, a little ghetto though, but would let you check voltages when vehicle off. Also could be left on... Maybe a timed switch? Do those exist?

Just some thoughts.

 

[Ayune]

Could you do the ground on a switch? May be easier than a whole relay setup, a little ghetto though, but would let you check voltages when vehicle off. Also could be left on... Maybe a timed switch? Do those exist?

Just some thoughts.

Yes, disconnecting meter ground via a switch/ignition was the original question, and may be the ultimate solution, but be sure to fuse the wires (1A or less) and label them as "HOT" behind the installation.

 

[dogdaysindurham]

For mine dual battery set up, I used almost the exact same voltage display that the OP is referencing. I got it from Ebay through an Australian vendor (SolidKit), it should come with inline fuses on the positive wires. Dual Battery Volt Meter for Toyota Prado 120, FJ, Landcruiser 100, 79, Hlux | eBay

I used a DPDT (double pole, double throw) switch.

Battery 1's ground goes to #3 leg, Battery 2's ground goes to #4 leg.

The meter's ground for battery 1 you connect at #1 leg, and the meter's ground for battery 2 one would connect at #2.

So when you toggle the switch into position 1, your ground is completed for both circuits (#3 to #1 and #4 to #2) and your voltage display turns on. But when you want it off, just toggle the switch into position 2 and the circuit should be broken (#3 to #5 and #4 to #6), these legs aren't connected to anything and would be floating.

 

[hoser]

An On/Off/Momentary switch on the ground would be fine too. Current draw on that panel is only 14mA.

 

[yukon]

An On/Off/Momentary switch on the ground would be fine too. Current draw on that panel is only 14mA.

Right, and couldn't the grounds be combined?

So run the grounds to the switch, then the output of the switch to the body?

 

[uHu]

Yes, disconnecting meter ground via a switch/ignition was the original question, and may be the ultimate solution, but be sure to fuse the wires (1A or less) and label them as "HOT" behind the installation.

That's the way. You will anyhow need a small fuse next to the battery for the meter supply, so there's no problem with switching the ground instead of the +12v.

 

[hoser]

Right, and couldn't the grounds be combined?

So run the grounds to the switch, then the output of the switch to the body?

Yes.

 

[CBdeJ]

It is a solid kit meter. I believe that if I use the relay pictured I should be able to tap the switched 12V source for the cigarette lighter and run the grounds as shown in the solidkit diagram. Should be pretty easy given the same diagram is on the relay (added in edit. that's the five prong version shown, it comes in a four prong -- which I would use).