75mv or 100mv shunts?

[Justin]

Does the SBMS0 preform best with 75mv or 100mv shunts? The manual isn't very clear on what is best to use. Just a warning I don't know what I'm talking about but I'm trying to learn! The manual says the SBMS0 has a range of 0-90mv. Does that mean a 75mv shunt is better as it's output won't be more then 75mv. Doesn't a 100 mv shunt offer better resolution? What would be best to use. Thanks

[Dacian Todea]

Any shunt will work with SBMS0.  Shunts are nothing more than a power resistor.

So say you have a 300A 100mV shunt that is basically a resistor 100mV/300A = 0.3333mOhm and that is what you will set in the SBMS0 ADC menu as shunt resistance value.

Shunts are rated to work at 66% of their rating for continues use so this example shunt can only handle 200A continues and 300A for a few seconds as peaks even 500A or 600A is not problem as peaks and yes at 600A shunt voltage drop will be double so 200mV and it will not be a problem for the SBMS0 even if SBMS0 input is limited to 90mV as that is the limit for measurement.

That means that with this shunt the SBMS0 can measure plus minus 270A if current is higher than 270A you will still just see 270A.

Say you can find a 225A 75mV rated shunt that will have the same exact resistance 75mV/225A = 0.3333mOhm so you may think there is no difference.

The difference is that one shunt can measure 200A continues while the other just  150A continues so one resistor has higher power rating.

Both of this resistors will have the same plus minus 270A measurable range and same resolution as far as SBMS0 is concerned there is no difference between them is just that you can use one up to 200A continues and the other up to 150A continues.

So in this case if say your max continues current is just 130A say you have a 3000W inverter and 24V battery then any of the shunts will work just fine with the 75mV one likely being the lower cost option as it is a lower power shunt.

[Justin]

I didn't completely understand how a shunt worked when I asked this question. I probably still don't. So when 300 amp current goes through a 300A 75 mv shunt the voltage drops 75 mv. So a 75mv shunt would be 0.25mOhm? Is that better then 0.3333mOhm? So a 100 mv shunt would never read 100mv at 2/3 if used at compacity. Would a 75 mv or 100 mv 300A shunt be most accurate and precise? Should I use a victron smart shunt so I also have a battery monitor? I could check the values of the SBMS0 and smart shunt against each other. You said the PV shunt is optional. In the SBMS0 manual it seemed like the you were saying the DSSR20 has a shunt built in for up to 20 amps? It said you could use that or an external shunt of 10-750 amps. Thanks so much for your help.

[Jhon]

From the perspective of minimizing losses you want the lowest value resistance, from the perspective of accuracy, you want the highest value resistance.  Accuracy for me is a little less important as the SBMS0 will stop charging when one of the cells of  the battery is full and it reset itself at 100%  For most people it doesn't really matter if they are down to 10% charge or 9% charge as the SBMS rounds that number anyway and the capacity of the battery changes as well depending on temperature, age, (dis)charge rate.  However, to answer your question, the 100 mv 300A shunt would be most accurate and precise, I doubt you would notice the difference though.

The way a shunt works is that when you have a current flowing through the shunt there will be a voltage difference between the two sides of the shunt and the SBMS0 will measure that voltage difference. 

The PV shunt is used to measure the current from the solar panels and with that the SBMS0 can calculate how much goes into the battery (if any) and how much goes towards the load.  You can do the same manually or in a custom program.

It took me a long time to wrap my head around it as well and the good news is that becomes easier when you have spend some time "playing with it".  Just be careful when working with these cells, the currents they are capable of producing in short circuits is outright scary, you will want to wear safety glasses and insulate all the tools you plan to use, for when you accidentally drop them.

[Oberon Robinson]

Justin, did you see my comment to check out the Beginner's Guide?  It will likely answer a lot of your questions. https://diysolarforum.com/resources/beginners-guide-to-electrodacus.174/

[Dacian Todea]

Justin,

Shunt as I mentioned is just a power resistor so at a certain current there will be a voltage drop across so yes a 300A 75mV shunt will have 75mV voltage drop when 300A flow trough that shunt.  But you can not continually use 300A if you have a 300A shunt as shunts are all rated for 66% continues. So for 5 or 10 seconds you can have 300A trough the shunt even 400A no problem for either the shunt or the SBMS0 but you can not use more than 200A (that is 66% of 300A rating) for long therm meaning minutes or hours as shunt will overheat and can get damaged.

So you need to know what will be the max continues current you will be using in your system and then get a shunt based on that.

Both a 300A 75mV shunt and a 300A 100mV shunt can do continues 200A max the 100mV shunt with the SBMS0 will just have slightly better resolution so good when measuring super small currents and max current SBMS0 can display with the 100mV 300A sunt is 270A but nothing bad will happen if that current is exceeded for a few seconds.   The 300A 75mV shunt allows SBMS0 to measure up to 360A but maybe not very helpful.

The diffrence betwen the two shunts is that the 100mV needs to be a bit larger in order to have the higher power rating required at 66% since 66mV * 200A = 13.2W so it needs to be able to dissipate the 13.2W as heat when used at 200A while the 75mV 300A can be less expensive as it will be a bit smaller and need to dissipate 200A * 50mV = 10W

There is no shunt inside the DSSR20 and shunts have noting to do with current limiting is just for current measurement.

The PV shunt is optional but if you do not install that then all you can see is battery current and so battery power can be calculated and SOC (State of charge).

If you install a PV shunt then you have way more info not just battery power but also the PV power and Load power thus 3 things instead of just 1.

[Justin]

Your guide. Is supper helpful. I was wishing for a guide like that from the beginning! I missed you comment somehow.