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Jk BMS on inverter current issue from the maker

Farmgirl0527

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A pure sine wave inverter does not draw a constant current. The current from the battery follows a sine wave shape. It ramps from zero current up to peak current in 1/4 of a cycle. The peak current is actually about 40% more than the average current. Then the current will ramp back down to zero at the 1/2 cycle time. At this point, the inverter reverses polarity, and ramps the current back up again, hitting the peak at the 3/4 cycle time. The battery current is positive again, and the inversion happens in the inverter circuit. The the current ramps down back to zero again at the end of the cycle. The process them repeats for every cycle.

If you measure with an averaging meter, you will just see a fairly steady current that should be close to the true RMS current. I use a True RMS Fluke meter, and it is able to give a vey accurate reading on this odd waveform. The current reading in the BMS just measures the voltage across a shunt resistor. This reading is only taken periodically, and it is not synced the the changing current from the inverter. It might take a reading at zero current, or at the peak current, but most likely, it will fall somewhere in between. Due to the shape of the wave, the reading tends to bounce a bit above and below the true RMS current. It is not perfect, but over the long term, it is "good enough" for the BMS to calculate the amp
hours charged in or discharged out of the battery. This works because if you average the readings over a full hour, the high and low readings will average out.

There is not a simple fix for the short term reading fluctuations. Ideally, reading it much faster and doing a running RMS type average would be best, but that takes more memory and computation. That is basically how my Fluke meter does it. An easier and cheaper way to do it is to just put an analog low pass filter on the signal from the current shunt. This will not be perfectly accurate, but by putting in an average to RMS correction factor for a sine wave, it will be very close, as long as the current wave follows a sine wave shape. Many AC voltage and current meters do this. When you measure 120 volts AC, it is actually averaging the sine wave and doing a small correction. It is accurate for a clean sine wave, but if it is a square wave, or triangle wav etc., then the reading will be a little off. A true rms meter is still accurate, no matter the wave shape.

In the case of the BMS random reads, it is actually very accurate over the long term because it is adding up the amp hours of each reading on the waveform. If you just watch the reading for a few seconds, you can get a pretty good
idea of what the average reading is. If you need more accurate, use a clamp on amp meter or install an external shunt with averaging or true rms reading.
 
Interesting!

Never really thought about it, but it makes sense that a battery hooked to an inverter would have a 120 Hz current frequency in the U.S. The impact of the ripple must be proportional to the capacity of the capacitors in the inverter and their drain.

Made me curious enough and I found a paper to see if that current ripple had an adverse effect on LFP. Their conclusion was:

The test results indicate that the current ripple causes a slight but noticeable increase in the heat generated within the storage system; but the increase is not significant enough to justify any changes to the grid interface circuit.

For the hyper-milers thinking "heat" means wasted energy and battery degradation, they did also present two circuits in the paper to minimize the effect and warned their study didn't include studying any impact towards battery longevity.
 
Even without an inverter connected I found that the JK BMS readings bounced around more than it really should on the APP even with a CC bench supply. They could slow down or apply averaging to the voltage readings on the overall plus the individual balance lines as well.

On edit: What were these BMS designed for in the first place if not for inverters?
 
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I have a JK 200A 8S BMS. The current is jumping all over the place and the SOC is useless. I don't see this in the videos with the 24S BMS. My inverter is a Victron Phoenix 1200/24.
My supplier says this is normal if an inverter is connected to it. But imho using a inverter is the hole idee?
Do all BMS manufacturers have this problem?
 
I have a JK 200A 8S BMS. The current is jumping all over the place and the SOC is useless.

It's normal for the JK - it just doesn't do averaging (inverters don't have a constant draw). This however should not impact the state of charge - you need to calibrate this by making sure the capacity is set correctly in settings and then cycling the battery. This is similar to how a smart shunt operates.
 
Ok, thanks.
I did calibrate, set the capacity settings and cycled the battery.
My Victron shunt is stable.
 
The BMS is jumping between 9A and 42A.
Victron shunt a steady 15.4A
Clamb meter steady 15.7A
Victron SOC 87%
JK BMS SOC 75%
 
Hi,

have the same problem here.
SoC SmartShunt 64%
SoC jk-bms 99% and If I look at the app I see 65Ah Remain Capacity near by the smartshunt

is there any suggestion pls?

regards
 
Credit @GXMnow for writing that text:


I am not sure he is perfectly right :)

MPP_Solar_MGX_schema.png

Usually 48V battery is connected into an isolated two way DC/DC buck-boost converter (20-40kHz) in the inverter.
And it is working into the DC Bus (about 370-410Vdc)
This DC Bus has 5-600V capacitors to store the power.
And from there the DC/AC inverter (again 10+ kHz) generates the sine wave.

So I would really like to see an oscilloscope capture (Volt and Ampere probe) from the battery side :)
 
I think he is probably right, but the BMS is not a scope, I just want to know the (average) current.
Why is the SOC than not more accurat?
The SOC should be an average.
Are all BMS-es act like this?
 
No. I use electrodacus SBMS and they usually match what my shunt says within a few percent. Current readings are a close match too.

That said, the SBMS0 has been a good product; however, I disagree with the creator's assertion of certain verifiably incorrect information and his unwillingness to acknowledge it.
 
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I'll add it to my todo list...

If I have to guess I would say you will see the 10-40kHz PWM.
The duty cycle will not be the same as the Amps go out from the inverter.
It will be much much smoother (and shorter bigger bursts)

The DC/DC converter has capacitor and inductor and transformer. All store power.
The DC bus has capacitor ... etc

I am thinking right now about an Owon SDS1102 :) Maybe I will order it.
 
I decided to forget about using the JK for anything, current measuring or SOC measuring. It's ridiculously inaccurate at both when used with my victron multiplus. My BMV on the other hand seems extremely accurate and the JK is usually way way out with its SOC in comparison. In terms of its balancing and protection though, it seems absolutely fine.
 
I think he is probably right, but the BMS is not a scope, I just want to know the (average) current.
Why is the SOC than not more accurat?
The SOC should be an average.
Are all BMS-es act like this?
I've had the same issue with the Daly.
 
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