To give you a better feel for the level of quality, there is NO such thing as a real owner's manual for any RV I am aware of. The one I got is generic, over 5 years old and probably identical to every other model if not brand. The factories will tell you they can not produce a wiring diagram as no two even if identical models are likely to be wired the same way. When I moved into my RV I knew to bring 150lbs of tools and I probably used them all in the 5 years I lived there. I never took my unit to a dealer for several reasons and have made major modifications to mostly the electrical/electronic circuits but also the plumbing supply side and sewer side. If you buy an RV and are NOT a handy guy/gal, you will be VERY disappointed. Remember, every time it is moved is the same as your house being in an earthquake.

I think the article nicely explains why, for this use case low side sensing is cheaper and easier to implement.

One important point that I think many have failed to take into account is the supply voltage is not from a lab-regulated power supply; it varies by close to 2V constantly. That part is improving but still nowhere close to lab quality. This is because the charger (called converter in an RV) that uses a charging algorithm suited for each battery type/chemistry controls the voltage and current over this wide range. If you are unfamiliar with 3 and 4-cycle charge algorithms, I can post some diagrams.

The key point here is that the charger (called converter in factory unmodified units) is connected to the battery at the same point that the battery loads are. It would require inserting a fairly robust and too expensive DC to DC regulator to eliminate the fluctuating voltage. 

Going by memory (which is dangerous) an unmodified conventional RV will fluctuate from 14.6VDC to at least 12.6VDC, possibly several times a day.

I just thought of a possible reason. Since we always place a 'disaster' fuse as close to the positive battery post as possible, it just seems right that a shunt that would work on either leg go on the negative to balance the placement of large parts connected to the battery. Sorry, I have no rational reason to say that, I just mention it as a possibility and to me it seems possible.

@zander, Hi Ron..Yes, the pros and cons of where to put current shunts can be perplexing at times, so a better understanding of what the differences are should help to clarify the mystery.

The I found the following link to help with this...https://www.allaboutcircuits.com/technical-articles/resistive-current-sensing-low-side-versus-high-side-sensing/

and I see it as a very simple explanation for why you would use either for certain applications, as the advantages and disadvantages are clearly apparent.

Hope you are doing well, and getting everything situated in your new digs (home),

kind regards,

LouisR

You are correct, a shunt for each battery is overkill, but since I had another identical shunt available I decided to put it to use.  I could have just tied the output from both BMSs into 1 shunt and called it good.  But now I can also more accurately measure the power going in and out of each battery.  So I guess that's an upside to having separate shunts (even though the BMSs measure it too). 

-Mike

I just thought of a possible reason. Since we always place a 'disaster' fuse as close to the positive battery post as possible, it just seems right that a shunt that would work on either leg go on the negative to balance the placement of large parts connected to the battery. Sorry, I have no rational reason to say that, I just mention it as a possibility and to me it seems possible.

https://www.analog.com/en/analog-dialogue/articles/high-side-current-sensing.html

Never to old to learn something new that may help you down the road.. lol

Regards,

  I have no disagreements with your last two messages, though I don't know any of the products you mention, but I am sure you would have chosen wisely.

Kelvin refers to Lord Kelvin, who invented and discovered a number of things in the 19th century (and a small bit of 20th Century) that is still influenetial today.

https://en.wikipedia.org/wiki/Lord_Kelvin

PS      https://en.wikipedia.org/wiki/Four-terminal_sensing

And the 4 terminal shunt was amongst them. Essentially a resistor with two wires for carrying the main current flow to be measured, and two wires for sampling the voltage drop across the resistor. This meant any voltage drop across the main current connecting wires , terminals, etc.  was excluded from the voltage drop across the resistor itself. Simple, but effective.

And the term Kelvin connections is still widely used ... e.g. In addition to the precision shunts you refer to, which would need such connections to be accurate, some high current Power Fets have additional Kelvin connection pins to sample the current flow, so that a control circuit has current flow information, without needing an external shunt.

And whilst I know nothing about RVs, shunts for measuring (say) 500A using Kelvin connections are regularly used in major transport applications, not just labs. Remember at 500A, just 1 milliOhm of resistance is 0.5 V drop, which would distinctly affect measurement of accuracy.

Best wishes, Dave

MOT the same as your lab shuts I am sure.