Multiple buck converters using a shared ground

@davee I have never heard a debate about high or low side shunts. I have heard debates about low side vs normal high side cutoffs. I have had to rethink my thoughts on that one.

The shunts we use are normally rated for 500Amps. I stress tested my RV at 400A, but it rarely goes above a few amps. The only reason it got higher was to test out my ability to run an AC on battery.

It worked fine, but the batteries would not have lasted more than a few hours.

The shunt is just a very precise very low value Resistor. We measure the voltage drop in millivolts and direction as well as temperature. The sum of all the measurements if we start at 100% is the current State of Charge. It is extremely accurate.

An RV is not a lab instrument, it is more akin to a blunt stone axe in terms of accuracy and sophistication. Unmodified RV's historically have what is called a hybrid battery (in NA, the EU does it better). It is NOT a deep discharge battery so is easily damaged if overcharged or if SOC drops below 50% more than a few times. The reason they get away with ut here is that the vast majority of RVers are always plugged in. Those that go off grid regularly either waste a lot of money, or do as I did and educate themselves and install the right equipment. The traditional battery charger/converter is really what we call here a trickle charger although they have improved a little bit in the last 5ish years. I removed mine and installed a Victron Inverter/Charger. They are a dutch company well known in the boating segment for a long time. My Victron is a 120A charger with a full 3 or 4 cycle (depends on your definition) charge curve. Since I have LiFePO4 batteries, the charge curve is basically 'give em hell' or a straight line charge.

There is a big difference between the shunt systems we use and the various sensing systems you mention. We are literally counting coulambs of charge.

What I did not mention, is that the shunt is backed with a controller that the user programs to their battery spec to give the best charging performance. Only recently has there been a solar add on to the forerunner of these produced by Bogart Engineering. That is what I started with. I actually ordered the battery monitor BEFORE I got the RV and of course scrapped the factory hybrid batteries in favour of Trojan T105's. Some years later I bought a bigger RV and rewired the electrical system to install Lithium batteries and Victron equipment. It is now state of the art.

The payoff first came when we had to leave the RV with a residential fridge full of food for 3 days where there was no power. We came back to 100% batteries since the location was unshaded and it had been sunny all 3 days in early May just outside Calgary. I have 6 100AH 12V LiFePO4 with 6 x 180 Watt solar panels wired as 2 parallel strings of 3 for 57V at 20A which gets converted to 12V at 70A (yes I waste a little some days).

@davee All the RV shunts I have seen work very well. I don't know about Kelvin, I just know that there are 100,000 of my model of RV alone in NA, let alone all the other models from my manufacturer plus all the other manufacturers. I would guess in the millions of them and a lot use similar shunts. As far as I can tell they are all working just fine.

MOT the same as your lab shuts I am sure.

@davee Now I see the problem. YES we the OP and I are ONLY talking about RV power circuits, you are talking about electronic circuits. Totally different. What is a concern for you is of no interest to us.

@davee I may be mistaken, but I am sure at one point you mentioned voltage differences between one end of the wire and the other. In a RV circuit with as much as a 2VDC swing in high value happening regularly, millivolt differences are of no interest. For your experience with lab conditions, that is different but not relevant to the OP. Like I said, an RV is a stone axe, it can handle huge fluctuations in power, they are built that way.

Hi Ron @zander,

  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

@davee Thanks, Dave, I didn't know that was what they were called, but two big bolts for the main current and 2 smaller ones for measuring the voltage drop. Simple ohms law after that and we even have temperature compensation on the better more expensive shunts.

@zander , Hi Ron, just for your edification, I'm including a link to high side shunts, and how they are very common in commercial & industrial equipment. 

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,

@inst-tech TBH, I never understood why they couldn't go on the high side but I also never understood why we drive on the right side and others drive on the left. I see no obvious advantage to either in either case. 

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.

Posted by: @zander

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@iwannastout Wow, some adventure. I am unsure are the two shunts. Is that needed? I am not sure what benefit it has and how to 'manage' with two.

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

Posted by: @zander

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@inst-tech TBH, I never understood why they couldn't go on the high side but I also never understood why we drive on the right side and others drive on the left. I see no obvious advantage to either in either case. 

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

@inst-tech Thank you for the research. As I mentioned earlier, RV power fluctuates a lot; this is not a lab. Also, they do not spend $ if they don't need to. The RV industry maintains a powerful lobby to get what you and I would call electrical safety regulations that are too loose. However, remember it's just 12VDC, so the odds of a death are extremely remote. I should add that there have been major changes in this area in the last 3 or 4 years to increase safety, but the cost has gone up several 10's of thousands of dollars. The installation and wiring is done by uneducated farmers who work from 4 a.m. to noon before returning to their farms. You can look up where most of the factories are and what kind of farmers surround them.

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.