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BMS showing different Voltage than Shunt

forscience

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My BMS is showing a different voltage than my victron smart shunt. What could be the cause?
View attachment 81126
 

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A big voltage drop like that could be from undersized wire or a poor connection or crimp. Post a good picture or two showing how the shunt is connected to the battery and tell us the wire size.
 
A big voltage drop like that could be from undersized wire or a poor connection or crimp. Post a good picture or two showing how the shunt is connected to the battery and tell us the wire size.
Here is my post of the wiring diagram. The only difference is that instead of the 3x 8AWG wires connecting the BMS, it's 2x 6AWG wires. The shunt has a positive lead connected to the positive terminal of the battery.
 
You need to post a picture of the actual wiring and connections.

Also, what is the actual voltage that you measure using a voltmeter?
 
You need to post a picture of the actual wiring and connections.

Also, what is the actual voltage that you measure using a voltmeter?
This is a portable system I put together in an ammo box so I basically will need to disconnect a lot of the wirings in order to take a picture and it will be hard to get the details in the picture because there are a lot of wires in a very compact space. Are you trying to spot a loose connection? I don't have a voltmeter at the moment, but I do have a cigarret lighter usb adapter which gives the input voltage and that reads the same as the voltage from the shunt. Will there be any problems to continue to the battery as is?
 
This is a portable system I put together in an ammo box so I basically will need to disconnect a lot of the wirings in order to take a picture and it will be hard to get the details in the picture because there are a lot of wires in a very compact space. Are you trying to spot a loose connection? I don't have a voltmeter at the moment, but I do have a cigarret lighter usb adapter which gives the input voltage and that reads the same as the voltage from the shunt. Will there be any problems to continue to the battery as is?
You definitely have a serious issue in the wiring. Losing half a volt in a 12V system is a lot. Something is wrong. Do not just ignore it. Check all of your crimps. Check all of the wire connections. Check all of the bolts. Whatever is wrong and causing the voltage drop means a lot of extra resistance which means a lot of heat. You need a good voltmeter/multimeter to do any work with electricity. Verify things are what they should be and fix what isn't.
 
You definitely have a serious issue in the wiring. Losing half a volt in a 12V system is a lot. Something is wrong. Do not just ignore it. Check all of your crimps. Check all of the wire connections. Check all of the bolts. Whatever is wrong and causing the voltage drop means a lot of extra resistance which means a lot of heat. You need a good voltmeter/multimeter to do any work with electricity. Verify things are what they should be and fix what isn't.
I will do that when I get some time this week. The max load I use is 80W which I think should be ok for the time being. So the extra resistance could be from loose wiring?
 
So the extra resistance could be from loose wiring?
This will create a fair amount of heat, especially in confined in a metal box. If you feel the exterior of the box and terminals you would likely know very quickly and easily if this is the case. Your BMS shows 25.8 degrees, what is room temp?

Is there anything interesting in your history or trends screens on the shunt app?
 
This will create a fair amount of heat, especially in confined in a metal box. If you feel the exterior of the box and terminals you would likely know very quickly and easily if this is the case. Your BMS shows 25.8 degrees, what is room temp?

Is there anything interesting in your history or trends screens on the shunt app?
Room temp is about the same. This is a plastic ammo box. Could it heat up even when there's no load because of this voltage difference?
 
Could it heat up even when there's no load because of this voltage difference?
Heat is power dissipation…. P=VxI…. No current, no power. Unless you have something hidden, not showing on your design…

What puzzles me is…. How can you have a .5V drop with absolutely zero current? I would understand with a long low gage wire but with a few inches of good wire, you either have a really really bad connection somewhere or one of your meters is off… I would use my voltmeter to verify every connexion and find the culprit before putting This system to work…
 
I just had a second look at your diagram. Did you put your fuse after your main switch? If yes, you leave your first wire and your main switch without protection. You should place your fuse as close as possible to the positive pole to your b attery.
 
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Quite simple. The BMS has shut down the charge path.

charging path.jpg

Thus you have a diode volt drop across the FETs in the BMS.

Why the BMS shut down could be determined by using the app and investigating the BMS records. Usually due to pack or cell over volts, what are the setting for these?
Could also be due to over or under temperature, have you got the temperature sensor connected?
Did you shut the charge path using the app?

If you load the battery with a few amps the BMS will fully switch on the discharge path and the voltage difference will be removed.

To conclude, you have a normal situation where the BMS shuts down the charge path, you need to find out why this occurred.

Mike
 
I just had a second look at your diagram. Did you put your fuse after your main switch? If yes, you leave your first wire and your main switch without protection. You should place your fuse as close as possible to the positive pole to your b attery.
Sorry I forgot to point out that the fuse is before the main switch so it's the closest component to the battery positive.
 
Quite simple. The BMS has shut down the charge path.

View attachment 81169

Thus you have a diode volt drop across the FETs in the BMS.

Why the BMS shut down could be determined by using the app and investigating the BMS records. Usually due to pack or cell over volts, what are the setting for these?
Could also be due to over or under temperature, have you got the temperature sensor connected?
Did you shut the charge path using the app?

If you load the battery with a few amps the BMS will fully switch on the discharge path and the voltage difference will be removed.

To conclude, you have a normal situation where the BMS shuts down the charge path, you need to find out why this occurred.

Mike
The charge button was automatically switched to off after the battery voltage hit 14.6v during charging. The screenshot was taken shortly after a full charge. The highest temperature I've seen was 32c which is well below the over temperature release and it never been below freezing.
 
Here are couple pictures I managed to take, but I don't think you can tell anything from the clutter. I did loosen the nuts on the bus bars in these pics so that's not what it's like when fully connected. I'll report back with voltage measurements when I get some time this week.
 

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The charge button was automatically switched to off after the battery voltage hit 14.6v during charging.
Are you planning on charging to 14.6V every charge cycle? I ask because this is far beyond the voltage needed for 100% charge and to the theoretical limits (if all cells are miraculously dead equal) of what the cells can withstand.

This is a good place for a BMS to stop charging and it prevents damage from overcharging (which sounds like is the case for SCC settings).

Still curious what your "History" screen looks like on the BMS.
 
I advise setting your charger to a lower voltage, 14.0 or 14.2 volts is more than enough. I expect it was cell overvolt that stopped the charge. Using the BMS to control the charge process is not a good idea.

Mike
 
Quite simple. The BMS has shut down the charge path.

View attachment 81169

Thus you have a diode volt drop across the FETs in the BMS.

Why the BMS shut down could be determined by using the app and investigating the BMS records. Usually due to pack or cell over volts, what are the setting for these?
Could also be due to over or under temperature, have you got the temperature sensor connected?
Did you shut the charge path using the app?

If you load the battery with a few amps the BMS will fully switch on the discharge path and the voltage difference will be removed.

To conclude, you have a normal situation where the BMS shuts down the charge path, you need to find out why this occurred.

Mike
I think you're right! I turned the charging button back to on and now the voltages are consistent! I just thought I should keep the charging button to off when I'm not charging. Have I done any damage by using the battery with the charge button on off? I can hear a humming noise perhaps coil whine coming from the victron smart shunt. Is that normal?
 
I advise setting your charger to a lower voltage, 14.0 or 14.2 volts is more than enough. I expect it was cell overvolt that stopped the charge. Using the BMS to control the charge process is not a good idea.

Mike
So do I stop charging when the battery reaches a non-nominal 14V or is 14V the nominal voltage that it's supposed to settle to sometime after being fully charged?
 
So do I stop charging when the battery reaches a non-nominal 14V or is 14V the nominal voltage that it's supposed to settle to sometime after being fully charged?
Assuming you have a LiFePO₄ friendly battery charger, set it up so the absorption voltage is 14.2V and the float voltage is 13.5V. Then let the charger fully charge the battery.
 
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