SOC and voltage not agreeing, then pack shuts down.

I've recently built an 8S pack and been using it with relatively few problems. But a couple times now I've looked at the BMS status (JKBMS JK-B2A24S15P) and while the SOC might be at 80%, the voltage is down around 24.2V, then soon the pack shuts down and reports 0% SOC.

What's going on here?

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There seems to be little consensus as to how various BMSs count cycles or coulombs or amp*hours, etc. Little consensus also on where (voltage wise) the BMS resets its state of charge calculation. In a perfect world, the BMS would count amp*hours and accurately present state of charge, but there are inaccuracies associated with measuring current.

As a result, the SOC gets out of whack and needs to be reset. My batteries are typically cycled from 95% to 75% daily. I intentionally have set the 95% limit. Over the course of a week or so, I notice the SOC starting to drift and have to increase the charging voltage sufficiently so that the BMS will reset the SOC to 100%.

A problem occurs when the BMS has the protective features of disconnecting on low voltage and low SOC, as you experienced, if the SOC is wrong-and it is always wrong!

Try increasing the bulk and float charging voltages until you reach the point of the SOC being reset to 100%, but only do this when you notice a large discrepancy between voltage and SOC. You probably will need to do this every couple of weeks.

Also, LFP batteries have a very flat voltage curve, so using voltage to determine SOC can be error prone. If you use voltage as an indicator, do it when the batteries have had as little charging/discharging current as possible for the last couple of hours. In other words, the batteries need to be resting.

A better solution is to use a shunt and actually measure the energy in and out of the batteries to determine SOC. Problem is that it doesn't solve the issue of the BMS disconnecting based on bogus SOC, but at least you'll more accurately know what the actual SOC is.

You can adjust parameters in the BMS to modify the protective features, but I wouldn't do that until I had a solid understanding of how the overall system performed. All systems have their own unique idiosyncrasies.

ikidd said:

the voltage is down around 24.2V, then soon the pack shuts down and reports 0% SOC.

Click to expand...

Have you checked the actual voltages with a meter?

It sounds like the BMS might be cutting out, perhaps there is a single cell that triggers low voltage protection. If you can identify which cell, check the connection and battery sense wire. Similarly you can test the voltages on the connector that plugs into the BMS; that will tell quite a bit of the story i suspect.

ikidd said:

I've recently built an 8S pack and been using it with relatively few problems. But a couple times now I've looked at the BMS status (JKBMS JK-B2A24S15P) and while the SOC might be at 80%, the voltage is down around 24.2V, then soon the pack shuts down and reports 0% SOC.

What's going on here?

​

Click to expand...

JK bms seem to have worse SOC calculations vs JBD. My cousin is using the silver JK 8s BMS and always has SOC settings that are looney toons, jumps all over the pace.

The solution for him was to use a smart shunt.

A few possibilities.
The BMS takes values from the settings and the algorithm calculates SOC from that and from full cycles between the values.
The default settings are not optimal by any means.
Cutting off @ 24.2 / 3.025 Volts per cell is a bit high actually, 21.6(2.700) - 22.4(2.800) is more appropriate. When the BMS cuts off, what are the cell voltages ?
Have you changed the settings from the Default Lifepo setup ?
If you have, capture the screens and post them for people to look at.
Also, have a look at this link to compare with.

Hope it helps, Good Luck

Alkaline said:

JK bms seem to have worse SOC calculations vs JBD. My cousin is using the silver JK 8s BMS and always has SOC settings that are looney toons, jumps all over the pace.

The solution for him was to use a smart shunt.

Click to expand...

My Overkill (JBD) BMS’s SOC tracks the Victron Smart Shunt with very good accuracy.

JBD is certainly more accurate, If you look at there app it has 10 points of SOC calculation so I think it is self correcting as it goes.
Only thing with JBD is active balancing which is a shock to me on why they don't have it already in their BMS.

Alkaline said:

JK bms seem to have worse SOC calculations vs JBD. My cousin is using the silver JK 8s BMS and always has SOC settings that are looney toons, jumps all over the pace.

The solution for him was to use a smart shunt.

Click to expand...

Does the Smart Shunt do a SOC calculation on the management page? I can't say I've used one but I see them recommended a lot.

If you want to see the Smartshunt screens and settings (or almost any Victron item), download the VictronConnect app from your App Store.

There is a “Demo Library” you can select the item you are interested and see all the screens and settings for that device.

Yes a Smartshunt will be much more accurate.

Steve_S said:

A few possibilities.
The BMS takes values from the settings and the algorithm calculates SOC from that and from full cycles between the values.
The default settings are not optimal by any means.
Cutting off @ 24.2 / 3.025 Volts per cell is a bit high actually, 21.6(2.700) - 22.4(2.800) is more appropriate. When the BMS cuts off, what are the cell voltages ?
Have you changed the settings from the Default Lifepo setup ?
If you have, capture the screens and post them for people to look at.
Also, have a look at this link to compare with.

Steve_S-Tech: My Final Config for Midnite Classic SCC & JKBMS' w/ Large Bank (works amazingly well) Includes Charge Profile & BMS Config

INTRO: I have debated posting this for a while until I got everything tweaked in and working as best as possible. The End Result is Gobsmackingly Good ! I cannot speak for other SCC and such as each is different & unique in their own ways BUT this may likely be a Good Reference. Civility &...

diysolarforum.com

Hope it helps, Good Luck

Click to expand...

It's actually dropping out at the correct voltage, 22V as I've set it. It's the pack voltage that's shutting it down, not a low cell. When fully charged, the cells are well balanced and they seem to stay in sync through the curve. I've bumped the balance point to be fairly close to a full charge before it kicks in, like 3.60V

I'll check out that link.

Rocketman said:

If you want to see the Smartshunt screens and settings (or almost any Victron item), download the VictronConnect app from your App Store.

There is a “Demo Library” you can select the item you are interested and see all the screens and settings for that device.

Yes a Smartshunt will be much more accurate.

Click to expand...

That's cool. I've installed the app for use on a solar pumpbox I set up, but never noticed the dummy devices.

On the main screen on the top left (on iPhone) there are three bars - the demo library is under that.

Rocketman said:

f you want to see the Smartshunt screens and settings (or almost any Victron item), download the VictronConnect app from your App Store.

There is a “Demo Library” you can select the item you are interested and see all the screens and settings for that device.

Click to expand...

AND you can save those settings for the demo model and upload them to the actual device. I just did this 3000 miles away where I’m here and my wife in the camp trailer is there. She doesn’t want to learn to mess with the Victron settings so I just do it for her and email her the file.