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How does Inverter meassure Battery state of charge ?


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Posted

Hi guys.

I'm struggling with communication between my stupid Easun MKS IV and the Pylontech and I am trying to understand how does the Inverter measures the Battery state of charge.

So even if the Pylontech tells me SOC is 100% , the Inverter will always see Battery state of charge 67% even when Battery voltage is 52.5 V.
Is it supid and looks at the Flooded bulk charge voltage or how does it see 67% ?

So how does it calculate the SOC ?
 

Thanks in advance.

Posted (edited)
2 hours ago, Jaxone said:

So how does it calculate the SOC ?

If Battery type (Setting 05) is PYL, and the BMS connects properly to the inverter (no warning 61) then it comes from the BMS. So if the BMS says 100%, it should display 100%.

If Battery Type is USE, then it assumes lead acid, and uses this formula for 48 V models:

SOC = (battery voltage - (low DC cutoff setting voltage + 0.2V) * 10%. So if your cutoff voltage happened to be 45.6 V, then it would display (52.5 - 45.8) x 10% = 67%.

Well, that's how all the 5 kVA models that I'm familiar with calculate it. It's possible that the very latest firmwares might change this. That's in battery mode; there are a few modifications to this in other circumstances. For example, there is a special case where it will display 95%. But I'm not aware of any that display 67%.

Edit: with USE battery type, there is no internal resistance compensation, no coulomb counting, no temperature adjustments. Just the above straight line formula based on battery measurement. It's very rough for lead acid batteries, and almost completely useless for LFP chemistry.

Edited by Coulomb
Posted (edited)

Ok , now I see it shows 64% ...

Battery state of charge 63 -64 %
Battery power 0 W
Battery voltage 52.5 V
Battery current 0.0 A
 
Looking at the explanation you gave me , means I could try to cheat it to show correct values by changing the values of Flooded battery settings ... this UNLESS it calculates  after the DEFAULT settings and not after the custom settings for the Flooded batteries.
If it calculates after the custom settings of Flooded batteries then I can switch to flooded , modify the parameters of flooded , then switch back to USE and all should be good.
 
For my flooded I set cut off at 46V if I remember correctly.
 
This until I figure out why the BMS does not wanna talk RS485.
 
Thanks for the information.
 
Edited by Jaxone
Posted
3 hours ago, Jaxone said:

Just realized I can't cheat it ... it just uses the FORMULA and not the voltages :)

 

You perhaps try by using the USR setting and see if you can adjust some values to get to 100% in line with a BMS.

Posted

I would rather let it SHOW wrong information than mess my battery pack :) but yes, I was thinking at this also.

Strange enough is that when I run in "Solar First" mode , it shows 100% , only when I switch to SBU it goes down like crazy.

Posted

Found something rather INTERESTING on this manual (updated on 15.1.2021) from MPP Solar in collaboration with Pylontech.

http://www.mppsolar.com/manual/Pylontech Lithium/Pylontech inverter setup SOP.pdf

Quote :

Battery Type US2000B/US2000BPlus/Phantom-S/US3000

As there is no communication between inverter and battery, for a better using experience, it`s also
acceptable to introduce monitoring device to visually display the real-time information from battery
management system via the communication channel, such as Inverter Control Center(ICC) from
centurionsolar. Same as the inverter compatibility condition, such a monitoring system needs get
authorization from Pylontech in advance for the compatibility before using with the products from
Pylontech mentioned above, otherwise the products from Pylontech will be exclusive of warranty.

Change the ‘maximum charging current’, ‘battery type’, ‘back to grid voltage’, ‘back to
discharge voltage’, ‘CV voltage’, ‘floating charging voltage’ in the setting page, all these
setting must be set to the value listed in below table. Select the right value, then click ‘Apply’
for the changes to take effect.


Item Setting                     Value
Maximum charging current       Set to N*25A, N=battery amount
Battery type Set to      USE
Back to grid voltage Set to     48V
Back to discharge voltage Set to     51V
Bulk charging voltage Set to     53.2V
Float charging voltage Set to     47.5V "

End quote.

Float charging voltage Set to 47.5V - Would this be a mistake ?

 

Posted
20 hours ago, Jaxone said:

I could try to cheat it to show correct values by changing the values of Flooded battery settings ...

The flooded and AGM settings are fixed. If you choose one of these battery types, the voltages are fixed.

But when you are using  battery type USE, it's pointless changing the voltage settings also, since the BMS is supposed to set them, and it will ignore any settings you entered with battery type USE.

13 hours ago, Jaxone said:

Bulk charging voltage Set to     53.2V

That's way too high. Sigh.

13 hours ago, Jaxone said:

Float charging voltage Set to     47.5V

That's way too low! It doesn't give you much confidence in that document, sadly.

When you see the bad SoC, are you using battery type PYL or USE? Or even FLD?

Posted (edited)
4 hours ago, Jaxone said:

If you look into the Pylontech BMS ... they recommend 53.3 ... so 53.2 seems to be viable ?

Pylontech are a battery company; it's in their interests to make their batteries seem to have higher capacity, and longevity is not a priority. (A short sighted outlook for sure). But apart from reducing battery life unnecessarily, it would make sense except that Axperts in particular tend to overshoot the voltage targets that they are given. You could argue that Voltronic should take this into account and actually aim for a lower voltage to prevent overshoots, but they don't do this (at least so far). So whenever you have the choice, it's wise (in my opinion) to use a lower absorb/bulk voltage target. You won't lose significant battery runtime, you'll increase battery longevity, and you'll reduce the rist of disconnections due to over-voltage of the battery.

Edited by Coulomb

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