SmartShunt Aux port to monitor starter battery voltage - wiring

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adphotoman asked Jul 10, '20

SmartShunt Aux port to monitor starter battery voltage - wiring

Please bear with me. I'm brand new to this forum and a novice at electrical circuitry. Two days ago I installed a SmartShunt 500A on my Sprinter platform RV. The install was pretty straight forward and only required the additional purchase of an 18" 1/0 negative cable. Since the install I've determined that I would like to monitor the starter battery via the Aux input. The wiring schematic in the manual illustrates the negative connection leading from the starter battery in the same bold black line that the current monitoring house batteries are illustrated with. My starter battery is located 6 feet away from my house battery bank and would probably require a cable of 8 or 9 feet to properly route the cable. Since this leg is only monitoring voltage and not current, could this leg be wired with something like 18ga rather than 1/0 or some similar gauge? Also, again since the new leg is only monitoring voltage, would this negative leg need to initiate from the battery post or could it be wired to the location where it terminates at the chassis ground? It all seems rather redundant as both circuits terminate at the same chassis ground point. One more thing, does the positive wire actually need to attach to the battery or could I just tap in to any positive feed on that circuit? It seems as though voltage would be consistent on any of the legs fed by that battery. Any help here would be greatly appreciated.

John

BMV Battery Monitor

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Answer 1 · 2020-07-10T10:48:17Z

The General preference is to run off Bus Bars where possible - it helps to distribute cleanly. A recent install I have done in a VW Crafter Per this image:
The install is behind the drivers seat, around 9-10ft away from the Starter battery. I belive it's 12awg cable I have back to the battery (mainly because I will eventually run a charge current back to it as well.)
You will notice in the bottom right the Main Bus bar from right to left - Battery > BUS1 > Shunt > BUS 2 > EasySolar & then also the Negative cable going up to another Bus Bar (BUS3) up the top. This Negative Bar up the top is the Negative for all DC loads in the van and also where the Vehicle Negative comes to.

from ther over the Left you will see a small 2 Post Bus Bar - this has 2 connections on it only. One is the 12Awg back directly back to the Starter battery, whilst you will notice the small gauge cable going back down to the SmartShunt.

So the Starter Battery is ONLY connected to the Negative Bus Bar, and the Aux input on the Shunt and it's Positive is completely seperate.

This setup of course means my Inverter system is Negative Rail tied to Vehicle negative, and of course I need to be mindful of this when it comes to any chance of the Positive bus bar or cables touching the Vehicle chassis - However there is also an inline fuse on the negative line for this very reason (normally of course it would be on positive, but since the Negative is the common element it is the only way to achieve that "fault seperation"

Although on such a short run you probably won't experience too much Voltage loss, and as you said it won't carry significant current (unlike mine for charging.) - however you would definitely get a more accurate reading, as well as having a more durable cable if you were to similarly terminate the Cable provided with the Smart shunt against a Small Bus bar and get a cable more like 14ga or 12ga.

Your other query about whether you attach to Battery or another positive point (assuming you mean within the House circuit - i.e. the Main battery you are monitoring...) - No it does not need to go direct to the Battery, any point that is appropriate - again in my image you will see the Main Power for the Shunt going across to a 10mm post on the Large vertically oriented Bus Bar to the centre of the image.

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