Dual voltmeter wiring

Submitted: Thursday, Nov 16, 2017 at 15:55
ThreadID: 135874 Views:2660 Replies:9 FollowUps:5
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Hi Folks, I have a small dual voltmeter that I want to install in my Landcruiser, to display the voltages of my main and auxiliary batteries.
Obviously I don't want them connected when the engine is not running, even though it appears that the current drain is very low.
Is there a way, using an appropriate single switch or relay, that I can have this meter turn off and on, given that I need to keep the 2 circuits separated?
Thanks in advance,
GRandMA

GReg and MArgaret
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Reply By: Member - Trouper (NSW) - Thursday, Nov 16, 2017 at 16:55

Thursday, Nov 16, 2017 at 16:55
Greg. I have 2 volt metres in my Troopy and just wired them directly from the batterys. Yes they are 'on' all the time but the current draw is miniscule and have had no issues these last 4 years. Adding a switch is just extra wiring to stuff up down the track. KISS principle mate
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Reply By: Member - bbuzz (NSW) - Thursday, Nov 16, 2017 at 16:58

Thursday, Nov 16, 2017 at 16:58
Hi Greg,
I have this one fitted -
Voltmeter

Runs all the time and the lecko says it draws bugger all current.

bill
Bill B

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Follow Up By: TTTSA - Monday, Nov 20, 2017 at 09:54

Monday, Nov 20, 2017 at 09:54
Have one of these also, great unit.
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Reply By: Warren B - Thursday, Nov 16, 2017 at 17:07

Thursday, Nov 16, 2017 at 17:07
Hi greg, have led voltage gauges in the 200 and they are hard wired like the other replies have said and current draw is bugger all.
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Reply By: Notso - Thursday, Nov 16, 2017 at 17:12

Thursday, Nov 16, 2017 at 17:12
If you are worried about it, pair up the two negative wires and put a switch on the negative wire.
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Reply By: Allan B (Sunshine Coast) - Thursday, Nov 16, 2017 at 17:35

Thursday, Nov 16, 2017 at 17:35
,
As said, they probably draw only a few milliamps and could be left on at all times.

However, in my vehicle, the two meters are combination volt/amp meters using induction ammeters which draw a bit more current so I have them controlled by a 2-pole relay from the ignition circuit.

Incidentally, it is very interesting to observe the relatively large charging current (50A) to the cranking battery after an engine start, then how quickly it falls to zero charge.... less than a minute.
Cheers
Allan

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Follow Up By: HKB Electronics - Thursday, Nov 16, 2017 at 18:57

Thursday, Nov 16, 2017 at 18:57
Zero charge Allan, even a fully charged battery will have some current flow into it?

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Follow Up By: Allan B (Sunshine Coast) - Thursday, Nov 16, 2017 at 21:01

Thursday, Nov 16, 2017 at 21:01
.
But less current than the last digit of my voltmeter Leigh. Picky, picky. lol
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Allan

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Reply By: Greg A6 - Thursday, Nov 16, 2017 at 17:57

Thursday, Nov 16, 2017 at 17:57
Thanks very much for the advice Folks. Looks like no switch needed. That's the way I'll go.
Cheers All..
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Reply By: RMD - Thursday, Nov 16, 2017 at 18:48

Thursday, Nov 16, 2017 at 18:48
I use two digital voltmeters and use a single pole switch to earth their negative lines to neg line. To be able to switch it off is a good thing because sometimes at night the brilliant LED numbers are bright and can reflect in the side mirror and side views, depending on meter position. Also if they are off you are not advertising additional gear you have fitted by attracting attention.
When making the connection of the neg side from the switch, make sure it goes to as near as possible to, or at, the battery negative/s. That way you get the actual voltage indication and not a, slightly higher than negative point, which will reduce the indicated display and make you think the system is less than it really is.
PS. Cranking amps of 100 amps for 3 seconds is not much battery discharge at all and will be replaced very soon by the alternator which is only a couple of volts higher than the discharge difference of voltage.
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Reply By: HKB Electronics - Thursday, Nov 16, 2017 at 18:56

Thursday, Nov 16, 2017 at 18:56
Personally I would avoid the use of switches or relays etc in the circuit as dirty worn contacts etc can cause the voltage display to be inaccurate, but then I guess it will depend on how accurate you want the reading to be.

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Follow Up By: RMD - Thursday, Nov 16, 2017 at 23:04

Thursday, Nov 16, 2017 at 23:04
A sealed switch which is rarely used and carries almost no current flow is going to be reliable for a long time and will serve the purpose well.
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Follow Up By: HKB Electronics - Friday, Nov 17, 2017 at 18:51

Friday, Nov 17, 2017 at 18:51
Actually I would disagree, after having spent many years fault finding on different equipment it is not uncommon for contacts carrying little current and operating rarely to cause problems. Contacts that are used often are self cleaning due to the wiping motion of the contacts. When not operated for long periods of time or with little current flowing through them they can tarnish and become resistive.

Regular operation and sufficient current to clean the contacts is better than little operation and very low currents.

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Reply By: Zippo - Sunday, Nov 19, 2017 at 23:05

Sunday, Nov 19, 2017 at 23:05
I use a DPDT (double pole double throw) centre-off switch with a LCD voltmeter. Image Both negative and positive lines selected by the switch. The OFF position is required because the backlight operates whenever voltage is presented, and can be a distraction as others have mentioned.
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