How do you test a DC shunt?

Started by Trukinbear, October 16, 2020, 01:46:38 PM

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Trukinbear

Hi guys, me again - sorry, and thanks.

I've been trying to get to the bottom of my widely divergent SOC calculations from my WhizBang Jr equipped Classic 250 (rev 4) and my MNSHUNT equipped SMA Sunny Island SI6048.

Looking at historical readings from the WhizBang Jr, and the SMA the numbers are all over the place. This morning I was standing in front of the console when I noticed something weird, the Si6048 was showing a 60+ A draw from the battery, but the WhizBang Jr showed around 8 A, about what I expect early AM before everything wakes up. This continued long enough for the Si6058 to adjust it's SOC down from 70% SOC (WBjr said 89%) to 20% (it goes to 20 in an attempt to recalibrate SOC after confusing data) SOC - WBjr still says 89%.

So this causes me to look at everything I can find as to a record of shunt readings. The biggest thing that jumps out at me is the Maximum reading from the SI6048 - 198A out 177A in - According to my MyMidnite WhizBang Jr data I've never come close to either. The charge rate is set limit at 70A, and my array is only 6kw with the two never at the same time. IF I turned on every piece of equipment we have all at the same time I'd never hit 198A, we rarely exceed 50A out and even then not for long.

I guess what I'm trying to ask is, could my shunt be bad? Is there a way to test it?
SMA Solar Sunny Island SI6048
JA Solar 330W 72 cell panels (18)
Classic 250 (2)
MODBUS/Canbus comm adaptor for Sunny Island
SMA-OG E-panel
WhizBang Jr
MNSPD300V (3)
Atkinson GSCM mini-i
Onan Quiet Diesel QD-8000
Monterey 986Ah 'Big Sur' 24-125-11 48V battery
Off the grid on the PNW coast

qrper

I'm going to stick my head out on this one, so here goes,

Shunts don't go bad!

Unless one took a lightening strike, there's nothing to go bad.

Basically, a very, very, very low Ω resistor that the whizbang reads the resutling voltage drop across the shunt. If I remember, the shunt with the whizbang is 500 A at 50mV. Which means when 500 A of current is flowing, there will be 50 mV produced across the shunt.

To test one, you can use a good VOM, set it for 1x Ohms range.
Now a good meter will read it's own meter leads resistance, so you'll see .3 Ω or something close when the tips are connected to each other. Take a lead from the meter at one end of the shunt, and the other at the opposite end. The reading should go up a tiny bit, that's the resistance of the shunt. Most digital meters will have an issue trying to resolve such a low value.

To really get down and dirty, you'll need to use a 4 wire resistance measuring device of which I can't recall the proper name.

Bottom line is if you have continuity between the ends, the shunt is good.

Mike, wb8vge
System one: 7kWp w/ Trina 250 W panels @90 Vdc. Classic 150 to 16-6 V U.S batteries. Trace 5548 sine wave inverter.
System two: 6kWp grid tie with solaredge inverter.
System three: Midnite Brat, two 120 W Astropower modules, 100 Ah battery. Runs the LED streetlight in the back yard.

Trukinbear

#2
Thank you, since it does not agree with the WhizBang Jr, how do you test the WBjr?

And yes I have a trouble ticket at SMA in the chance that the SI6048 itself might be the problem.
SMA Solar Sunny Island SI6048
JA Solar 330W 72 cell panels (18)
Classic 250 (2)
MODBUS/Canbus comm adaptor for Sunny Island
SMA-OG E-panel
WhizBang Jr
MNSPD300V (3)
Atkinson GSCM mini-i
Onan Quiet Diesel QD-8000
Monterey 986Ah 'Big Sur' 24-125-11 48V battery
Off the grid on the PNW coast

ClassicCrazy

#3
Where exactly are you reading the Whizbang from ?
If you are reading the system amps then it has already compensated for any loads and is only showing exactly what is going into the batteries.
So if your Classic was putting out 60 amps but the loads like inverter was taking 55 amps - the system amps is only going to show 5 amps which is what is going into the battery .
One way to test is to get a DC clamp on amp meter and put it on the wire that has the shunt and compare it to the other readings. Depending on the clamp on meter it might not be exactly as accurate as the whizbang but it would be close enough to tell you it is working correctly.


Larry
system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable

Vic

#4
In addition to what Mike and Larry mentioned,  several things:

Do the SIs use the same MNShunt,  as the WbJr?

Really seems that the Si SOC readings would be most suspect,  compared to the WbJr.

Yes,  it is difficult for the average person to measure the voltage output of the Shunt,  or its Resistance.  100A flowing either direction,  yields only 5 mV.  Even a 4.5 digit mulitmeter has too little resolution,  and,  usually too many counts of uncertainty,  to measure this accurately.   And,  most of us do not hate 5+ digit DMMs,  MicroVoltmeters,  or MilliOhmmeters,  which could measure either the shunt voltage,  or resistance. (current flowing through the shunt would affect resistance readings).

Would suggest two things:
1. On an early morning after a full charge on the previous day,  compare  the Net Ah,  with the SOC,  either on the Classic MNGP,  or using the LocalApp.   Is the WbJr SOC vs Net Ah believable? Is the Si's SOC believable?

2.  AND,  SOC readings from any battery monitoring device are just approximations.  Fairly often,  these are rough approximations.   Fooded Lead Acid(FLA)   batteries seem to be one of the most difficult battery types for these monitors to read,  due to Peukert  effects on discharge, (and probably for charge,  as well,  difficulty in guessing battery efficiencies,  temperature effects,  and many other variables that we cannot know,  or measure very accurately.

For FLA batteries that are cycled daily,  would just look at the Net Ah in the AM,   to see close to the DOD of the battery,  overnight,  and if the  battery discharge and charge are fairly repeatable,  day-to-day,  you can come up with settings for the Wb,  that could give general ideas of SOCs throughout the daily usage.

MyMidnite2,  the free MidNite site that records snapshots of many Classic CC operating parameters,  battery voltage,  temps,  etc,  will help you get a very accurate graphic display of this date,  recorded at about 10 minute intervals 24/7/365.

Have never seen a WbJr report erroneous data.   We did have one very early WbJr,  that stopped working after one month,  or so.   MN replaced it very quickly,  and all of them here,  have worked perfectly for more than 5 years  (forget when the Wb was introduced).   But,  from what other company,  could you get the WbJr functions,  at such a LOW cost??   Amazin',  to me  ...   Thanks NidNite !!

IMO,  Vic
Off Grid - Sys 1: 2ea SW+ 5548, Surrette 4KS25 1280 AH, 5.25 KW PV, Classic 150,WB, Beta Barcelona, Beta KID
Sys 2: SW+ 5548s, 4KS25s, 5.88 KW PV, 2 ea. Classic 150, WB, HB CC-needs remote Monitoring/Control, site=remote.
 MN Bkrs/Bxs/Combiners. Thanks MN for Great Products/Svc/Support&This Forum!!

Trukinbear

The WBjr is attached to the MNSHUNT.
They should both show the same thing - they both only measure what goes into or out of the battery. I can read this number directly from the SI - they do agree when I calibrate the shunt at zero, but a most loads the numbers appear to be off by about .4-.8 Amps on most of the small loads. When the generator is running the WBjr and the SI are spot on, but appear to differ greatly when the Classics supply solar energy to the battery. The SI is aware of the readings from the Classics, but the absolute shunt values do not jive with the WBjr. The shunt is between the loads/classics/SI and the battery - the shunt readings couldn't possibly compensate for what the inverter is doing, now can I access that, yes... but that isn't what is going on here.
I monitor them with the Local Ap, the Android app (not at the same time as the local app) and with MyMidNite. As far as I know the charge the Classics report to the SI via the MODBUS/Canbus adaptor are NOT from the WBjr, right? They couldn't be with only one Classic being able to read the WBjr and that data not being shared between them.
SMA Solar Sunny Island SI6048
JA Solar 330W 72 cell panels (18)
Classic 250 (2)
MODBUS/Canbus comm adaptor for Sunny Island
SMA-OG E-panel
WhizBang Jr
MNSPD300V (3)
Atkinson GSCM mini-i
Onan Quiet Diesel QD-8000
Monterey 986Ah 'Big Sur' 24-125-11 48V battery
Off the grid on the PNW coast

Trukinbear

And I really wouldn't give a flip about the SI's SOC readings except when it drops to 20% to recalibrate SOC it starts up all the battery protection stuff, and cranks the generator. It's trying to literally fry the battery that it believes is 20% when it is almost fully charged.
SMA Solar Sunny Island SI6048
JA Solar 330W 72 cell panels (18)
Classic 250 (2)
MODBUS/Canbus comm adaptor for Sunny Island
SMA-OG E-panel
WhizBang Jr
MNSPD300V (3)
Atkinson GSCM mini-i
Onan Quiet Diesel QD-8000
Monterey 986Ah 'Big Sur' 24-125-11 48V battery
Off the grid on the PNW coast

Trukinbear

Last night for example, the battery was fully charged - we used 168Ah. The WBjr said our SOC was 100% before and 89% after - the SI said 90% before and 70% after then suddenly dropped to 20% and cranked the generator. Of course it see's it's trying to push 70A into a pretty full battery so that starts it's own cascading failure events. I just need to make sure it isn't the WBjr, or the MNSHUNT before I demand a replacement SI6048 from SMA.
SMA Solar Sunny Island SI6048
JA Solar 330W 72 cell panels (18)
Classic 250 (2)
MODBUS/Canbus comm adaptor for Sunny Island
SMA-OG E-panel
WhizBang Jr
MNSPD300V (3)
Atkinson GSCM mini-i
Onan Quiet Diesel QD-8000
Monterey 986Ah 'Big Sur' 24-125-11 48V battery
Off the grid on the PNW coast

Vic

Quote from: Trukinbear on October 16, 2020, 05:56:40 PM
The WBjr is attached to the MNSHUNT.
They should both show the same thing - they both only measure what goes into or out of the battery. I can read this number directly from the SI - they do agree when I calibrate the shunt at zero, but a most loads the numbers appear to be off by about .4-.8 Amps on most of the small loads. When the generator is running the WBjr and the SI are spot on, but appear to differ greatly when the Classics supply solar energy to the battery. The SI is aware of the readings from the Classics, but the absolute shunt values do not jive with the WBjr. The shunt is between the loads/classics/SI and the battery - the shunt readings couldn't possibly compensate for what the inverter is doing, now can I access that, yes... but that isn't what is going on here.
I monitor them with the Local Ap, the Android app (not at the same time as the local app) and with MyMidNite. As far as I know the charge the Classics report to the SI via the MODBUS/Canbus adaptor are NOT from the WBjr, right? They couldn't be with only one Classic being able to read the WBjr and that data not being shared between them.

So,   both the Wb AND the Sis attach to the MNShunt?,  or,  only the Wb is attached to the shunt,   and the Classic/s report charge/discharge currents to the Si,  via the Commbox?

And,  the MNShunt is attached to only one Classic,  via the WbJr,   BUT,  it sees all of the charge/discharge currents (from all charge sources,  and all loads,  when correctly installed),  as the shunt is in the negative battery lead,   where all currents appear,  and can be measured.

Realize,  that I know nothing of Si inverter/chargers,  at all.

Possibly,  a stand-alone Generator Start Module (GSM)?

Vic
Off Grid - Sys 1: 2ea SW+ 5548, Surrette 4KS25 1280 AH, 5.25 KW PV, Classic 150,WB, Beta Barcelona, Beta KID
Sys 2: SW+ 5548s, 4KS25s, 5.88 KW PV, 2 ea. Classic 150, WB, HB CC-needs remote Monitoring/Control, site=remote.
 MN Bkrs/Bxs/Combiners. Thanks MN for Great Products/Svc/Support&This Forum!!

ClassicCrazy

Quote from: Trukinbear on October 16, 2020, 06:04:25 PM
Last night for example, the battery was fully charged - we used 168Ah. The WBjr said our SOC was 100% before and 89% after - the SI said 90% before and 70% after then suddenly dropped to 20% and cranked the generator. Of course it see's it's trying to push 70A into a pretty full battery so that starts it's own cascading failure events. I just need to make sure it isn't the WBjr, or the MNSHUNT before I demand a replacement SI6048 from SMA.

Keep in mind that the Midnite SOC is a calculated amount where the Whizbang amp reading is actual.
The SOC is determined by efficiency setting and temperature of batteries .
I have no idea how the SMA does it's SOC calculations.

Larry
system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable

Vic

#10
Quote from: Trukinbear on October 16, 2020, 06:04:25 PM
Last night for example, the battery was fully charged - we used 168Ah. The WBjr said our SOC was 100% before and 89% after - the SI said 90% before and 70% after then suddenly dropped to 20% and cranked the generator   ...   

But,  looking at the Wb data or Net Ah, (and your knowledge of what were the actual total demands on the battery),  you could probably infer weather the Si,  or the Wb are incorrect.

For the battery to actually go from 70% SOC,  to 20%,  what load would be required in that time interval for the Si-reported 20% to be accurate?

Also wonder if there is a firmware update avail for the Sis that might have addressed this situation.

IMO,   Vic
Off Grid - Sys 1: 2ea SW+ 5548, Surrette 4KS25 1280 AH, 5.25 KW PV, Classic 150,WB, Beta Barcelona, Beta KID
Sys 2: SW+ 5548s, 4KS25s, 5.88 KW PV, 2 ea. Classic 150, WB, HB CC-needs remote Monitoring/Control, site=remote.
 MN Bkrs/Bxs/Combiners. Thanks MN for Great Products/Svc/Support&This Forum!!

Trukinbear

To actually go from 70% to 20% would have required a discharge of 493Ah in a couple of minutes. I believe the WBjr/Classic to be correct, but the SI has control of everything even the Classics.
SMA Solar Sunny Island SI6048
JA Solar 330W 72 cell panels (18)
Classic 250 (2)
MODBUS/Canbus comm adaptor for Sunny Island
SMA-OG E-panel
WhizBang Jr
MNSPD300V (3)
Atkinson GSCM mini-i
Onan Quiet Diesel QD-8000
Monterey 986Ah 'Big Sur' 24-125-11 48V battery
Off the grid on the PNW coast

qrper

Here's a bit of FYI

I an old analog guy, and I like to watch analog meters more than a digital display.
So i connected a analog meter to the same sense wires on the Whizbang jr so I could see the current flow on a analog meter. Apparently the load from the meter caused issues with the Whizbang, so I ended up placing a 100 A 75 mV shunt in the high side of the battery lead to the inverter.

Once the meter was removed from the Whizbang's shunt, everyone was happy.
I did add a 200mA fuse in the shunt's sense wires that fed the analog meter, since I was measuring current in the high side.

There was an old dead guy named Kirchoff that said current in and current out of a node had to be the same, so a shunt in the high side will read the same as one in the negative lead like the Whizbang.
It's possible the other equipment doesn't like the shunt the Whizbang is married to. I'd add another shunt to the battery neg lead just for the external equipment and see what happens.

Mike, wb8vge
System one: 7kWp w/ Trina 250 W panels @90 Vdc. Classic 150 to 16-6 V U.S batteries. Trace 5548 sine wave inverter.
System two: 6kWp grid tie with solaredge inverter.
System three: Midnite Brat, two 120 W Astropower modules, 100 Ah battery. Runs the LED streetlight in the back yard.

Vic

Quote from: Trukinbear on October 16, 2020, 06:04:25 PM
Last night for example, the battery was fully charged - we used 168Ah. The WBjr said our SOC was 100% before and 89% after - the SI said 90% before and 70% after then suddenly dropped to 20% and cranked the generator. Of course it see's it's trying to push 70A into a pretty full battery so that starts it's own cascading failure events. I just need to make sure it isn't the WBjr, or the MNSHUNT before I demand a replacement SI6048 from SMA.

OK,  so trying to poke around in the fairly obvious,   and suggesting you try things that you must have already tried;

SO,  the Si system seems to not do SOC calcs that well,  and therefore, SOC-driven Genstart does not seem to work well,  but you have an Atkinson GenStart Mini,   which you must have tried.  Obviously,  this module has preset voltages for Start and Stop.   How did/does that work with the Si inverters?

Voltage-driven Genstart has been around for ages,   and can work fairly well,   depending  ...

Does it create other issues,  'cause the Si s  "know"  that they are in exclusive,  overriding  CONTROL?

Just wondering,   Vic
Off Grid - Sys 1: 2ea SW+ 5548, Surrette 4KS25 1280 AH, 5.25 KW PV, Classic 150,WB, Beta Barcelona, Beta KID
Sys 2: SW+ 5548s, 4KS25s, 5.88 KW PV, 2 ea. Classic 150, WB, HB CC-needs remote Monitoring/Control, site=remote.
 MN Bkrs/Bxs/Combiners. Thanks MN for Great Products/Svc/Support&This Forum!!

Trukinbear

Oh yeah, it'll shut off and have us in the dark if the GSCM mini-i cranks the generator without a signal from the SI. I can't manually crank the generator either other than commanding the SI to send an order to the GSCM mini-i. I'm pretty sure this is an SMA issue, I just wanted to make sure nobody had ever had a shunt fail other than totally fail before I proceeded. It looks like they are going to send me another unit and I don't relish breaking my system down and hefting a 143# inverter down and back up on the wall. When the SI drops the SOC to 20% to recalibrate that triggers all the battery protection schemes - the SMA will not let me draw any more juice out of the battery than it thinks is OK. Sometimes that is a damn near fully charged battery it won't let me use...
SMA Solar Sunny Island SI6048
JA Solar 330W 72 cell panels (18)
Classic 250 (2)
MODBUS/Canbus comm adaptor for Sunny Island
SMA-OG E-panel
WhizBang Jr
MNSPD300V (3)
Atkinson GSCM mini-i
Onan Quiet Diesel QD-8000
Monterey 986Ah 'Big Sur' 24-125-11 48V battery
Off the grid on the PNW coast