DC Voltmeter with Shunt - Help!

If all loads are turned off, the shunt doesn't draw anything.

Is this meter a self-powered, bench-top test instrument, or is it a panel-type meter which is externally powered?

I suspect the latter given that it specs the current draw.

If so, you can leave that meter connected indefinitely. A current draw less than 10 mA from a marine battery will take years to discharge it. That shunt, meanwhile, doesn't draw anything when no current is flowing through it, just like the rest of the conductors in that circuit.

You can hardwire that meter in your circuit without worrying about that itty bitty load. If your marine battery dies as a result either the meter is bad or the battery is, or there are problems. I suspect that meter's draw is probably less than that battery's self-discharge rate.

Thanks for the response. Is the 10 ma working current in the specs mean that the meter is using max 10ma no matter if there is a load on the circuit or not?

I also have a wire coming from the positive terminal of the truck engine battery coming to the positive terminal of the camper marine battery which is switched on and off by a solenoid in the engine compartment. While I am driving I like to charge the marine battery and when I am not driving I shut the solenoid to off and only use the marine battery in the camper. I am wondering if there will be any problem while I am charging the marine battery, while driving, with the shunt?... and can the shunt work in reverse and show how many amps are going into the marine battery while charging?

You could also put a pushbutton in the circuit through the meter, so that the tiny draw of the meter is active only when you want to read it.

What are you trying to measure (voltage or current)?

both, the meter has 2 digital readouts, one for volts and one for current.

Wiring diagram and instructions (from the link) for reference:

Instructions:
The ammeter power supply terminal and ground using common measuring terminal to design, that is, the power supply side of the ground and the testing ground is the same, so the test ammeter in series with the electrical terminal only after (ie, the device under test the anode and cathode power between the device under test)
Recommends using a separate header isolated power supply terminal
Before using shunt must first access the circuit and then draw digital ammeter head to prevent pressure damage to the meter.
Before using shunt must first access the circuit and then draw digital ammeter head to prevent pressure damage to the meter.
Lead Definition (5 lines)
Thin black line: negative power meter
The Thin Red Line: Header powered anode (thin red and thin black line between the voltage 4-30V, over shall be burnt)
Thick red line: measure the positive voltage input (this line and the thick black line is the voltage between the voltage meter display)
Thick yellow line: measure current flow leads.

Bold lines: measuring current flow leads (thick black line and the thin black line is the common ground is connected)

Looks like a pretty grim translation.

thanks for posting the wiring diagram.

When there is a load is the most the shunt using is 10ma?

When my truck is charging the battery then in the diagram the power supply becomes the load and the load becomes the power supply. Will that damage the shunt in any way.

Will the ampmeter still read the current?

The shunt won't "use" anything. It's in series with the load - when the load is not drawing anything (or, acting as a generator, charging the battery), the shunt is doing nothing. When current is flowing, the resistance of the shunt will reduce the voltage applied to the load by a little bit (the actual amount being proportional to the load current - but negligible anyway). The 10mA is what the meter circuit needs - almost all of it to supply the LED display.

The shunt doesn't care which way the current is flowing, so it won't be harmed when the battery is charging, and the meter should show the charging current.

If the meter was left switched on, the 10mA draw (nothing to do with the shunt) would be equivalent to 100 hours per amp-hour (Ah) of your battery capacity. A 100 Ah battery should drive it for 10,000 hours (with no other load). May be an idea to turn off the supply to the meter if you're leaving it parked up for a few months.

Thanks for the reply JohnDG. "When current is flowing, the resistance of the shunt will reduce the voltage applied to the load by a little bit (the actual amount being proportional to the load current - but negligible anyway)." The most the load will be is 30A so how much do you think the resistance of the shunt would use?

"The shunt doesn't care which way the current is flowing, so it won't be harmed when the battery is charging, and the meter should show the charging current."

So the yellow and black wires from the shunt to the meter (as per diagram) will be reversed when the battery is being charged..... and the meter will still read the current?

I am putting a momentary switch on the supply to the meter.

There isn't enough information given to state exactly what the voltage drop across the shunt would be, but my best guess would be that the external shunt (in parallel with the internal shunt) would drop 1V for 100A, so 0.3V (300mV) for a 30A load.

I'd expect the meter to show the charging current, but again there isn't enough information given (it states a current range of 0-100A, but doesn't explicitly mention current direction).

Unless you really can't afford to lose the 11 bucks, I'd say "give it a go".

Thank you everyone for all the help. My main concern is to allow the marine battery's charge to last as long as possible since sometimes I am spending days in the wilderness depending on it.

The red wire is power for the meter and voltage sensing. switch it to prevent parasitic drainage. The shunt does not "use" any power when no load is in use but provides a slight voltage drop proportional to current that is measured by the meter and is converted to a display of current.