# DC volt/ammeter w/shunt questions

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#### ryancousins

Joined Jul 15, 2012
14
I have a DC volt/amp meter with shunt that I'm using in my camper to monitor battery usage. I have included a couple pics.

https://drive.google.com/file/d/1TwFQHoE7Hz7T0DxQT1_ijRVLqmm7W7rx/view?usp=sharing
https://drive.google.com/file/d/1DjPK1GVXiT2miAdbxiFXPi3FStKi91IC/view?usp=sharing

I have three questions regarding the wiring of the shunt:

1. It states in the instructions that the shunt must be put in series between the negative side of the battery and the load. Why couldn't it be put in series on the positive side? What difference would it make electrically? In both cases the current through the circuit and the voltage drop (what it's using to make a current measurement) across the shunt would be the same.

2. It shows in the wiring diagram (note the red arrow I have added in the 2nd picture) and mentions in the instructions that the negative voltage lead (#3) must be tied to the shunt and not home-run back to the negative terminal of the battery. Why? The only difference would be the voltage drop across the wire that goes from the negative terminal of the battery and the shunt, which I would imagine is negligible. Essentially the negative terminal of the battery and that bottom side of the shunt are at the same potential so I don't know why it would matter.

3. The negative voltage lead on the meter (#3) and the "negative" current lead (#2) are both supposed to be wired onto the bottom block of the shunt. If this is the case then they're electrically the same thing. Why wouldn't the meter just have one negative lead that goes to the bottom block of the shunt to both power the meter and read the current?

#### CharlesWMcDonald

Joined May 16, 2019
233
1. Most systems apply ground to the negative lead. Placing the shunt in the negative lead keeps the meter leads near ground potential. Internally the measurement terminals (1 & 2) of the meter need to be properly referenced to the power terminals (3 & 4).
2. That (terminal 3) is main reference point for the meter. The voltage drop across the lead to the battery may adversely affect the meter.
3. Terminals 3 and 4 are the power leads to the meter and have current flow and as a result a voltage drop across the wire from terminal 3 to the shunt. The voltage drop would make the reading across the shunt inaccurate. The current through the wires to terminal 1 and 2 is extremely low and the voltage drop across the wires has negligible effect on the measurement.(i.e. highest accuracy)