Which side of battery cutoff switch to install BMV 712 shunt on series 12v system

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mcanfield asked Jun 25, '23

Which side of battery cutoff switch to install BMV 712 shunt on series 12v system

I have two questions that I'm hoping y'all can help me out with. First some background. I also attached a diagram.

I have two 6v batteries in series for my travel trailer 12 volt system. I installed a marine kill switch on the NEG side for storage when the trailer isn't in use (vampires on board eg CO monitor, etc.). I was initially going to install the shunt on the ground side of the cutoff (Fig. 1) so that the 712 monitor wouldn't draw current, but in looking through the forums, it sounds like I would lose history and synchronization on the 712 monitor.

Question 1: Is it better to install as Fig. 2. with the shunt directly off the battery, or can it go after the cutoff switch as in Fig.1?

Question 2: How much will the monitor draw if it's not included in the cutoff, as in Fig 2?

Thanks in advance for the help

SmartShunt switch

wiring-diagram.jpg (5.1 MiB)

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Answer 1 · 2023-06-26T01:59:16Z

Kevin Windrem answered · Jun 26, 2023 at 01:59 AM

Figure 2 is better for the reasons you mention plus the negative connection will be lower resistance so will provide a more accurate battery voltage.

The BMV-712 and SmartShunt draw less than 1 mA. That computes to about 0.7 AH per month. This is usually less than a battery's self discharge.

Your diagram shows an incorrect positive connection. The battery positive connection on the shunt should go to the other battery positive (same connection as the loads).

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Answer 2 · 2023-06-26T05:36:13Z

mvas answered · Jun 26, 2023 at 05:36 AM

I thought the cut-off switch was typically installed in the battery positive wire?

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Answer 3 · 2023-06-26T07:46:14Z

seb71 answered · Jun 26, 2023 at 07:46 AM

Also do not short the battery (as in your drawings).

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Answer 4 · 2023-06-30T07:03:24Z

mcanfield answered · Jun 30, 2023 at 07:03 AM

Thanks Kevin, that was what I was thinking on the draw, just wanted some confirmation. Thanks for the heads up on the drawing too, I banged it out pretty quick and that could have been a problem if I didn't catch it on install.

As for the battery cutoff side, Pos. or Neg., there's quite a bit of debate. I landed on the Neg side for the trailer; there's no possibility of short to ground.

Pulled the UTP cable for the display this week (love these long summer days). Didn't get it to the exact spot I want it in, but found a good alternative that will work nicely and got the cable pulled. Finishing the install this weekend.

Thanks for the feedback.

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