JK-B2A20S 20 P - UVPR issue

[carpetpaul]

Hi, please could I ask your advice, I'm not sure if my device is faulty. for my Li-ion 14S battery pack I set my JK BMS to UVP of 3.0v, it turns off as expected, however turns back on straight away (on App shows 'DISCHARGE OFF' then 'DISCHARGE ON') even though UVPR is set to 3.5v - this behaviour continues in a loop, again and again*

should the BMS not keep the load turned off until the UVPR voltage is reached by the charger ?

* it does this only when my charger is connected.


any help would be most appreciated

thank you

 

[DIYrich]

The BMS is to protect the battery, not manage it.
I would do uvp of 2.75 and uvpr of 3.0

In any case, it doesn't seem to be working right.

What is your charge voltage? Why 14 cells?

 

[carpetpaul]

thanks for your reply. it's a 14S / 52v pack, charge voltage is 58.8v

if the BMS is to protect the battery and not manage it, what is it that keeps the load disconnected until the charger/solar reaches a sensible voltage to reconnect the load ?

 

[DIYrich]

14s is a 42v pack (45.5 operating). 16s is 48v (52.0). 58.0v to charge 16s. 50.75v for 14s.

As long as the battery is in the operating range, you can have it connected to the load. The inverter should have ranges to set. Shutdown at 48.0v (for a 16s battery), resume battery usage at 52.0v.

The BMS is a safety, like a fuse, if the inverter does something wrong. The BMS also watches the individual cells, and keeps each one in range. If the cells go way out of balance, you can be in the operating range of the battery, but an individual cell goes out of range. You can also have multiple batteries, each with its own BMS. You don't want individual banks going offline. That stresses the other batteries. Hence the inverter manages the batteries as a whole to keep them out of a surprise cutoff.

 

[carpetpaul]

are you sure ?


52V
Chemistry: Li-Ion. Configuration: 14s. Max Voltage: 58.8V. Nominal Voltage: 52V.

19 Aug 2022 — Nominal voltage chart for 52V Li-Ion Ebike batteries showing the percentage. 14 Cells x 4.2 Volts/Cell = 58.8 Volts Fully Charged Voltage

58.8V 6A 14S Lithium Battery Charger - for 48V/51.8V Li ...​

58.8V 6A 14S Lithium Battery Charger - for 48V/52V Li-ion

if you use lithium 18650's at 14S, and 4.1V per cell max,14S is 58V at full charge.

 

[timselectric]

Hi, please could I ask your advice, I'm not sure if my device is faulty. for my Li-ion 14S battery pack I set my JK BMS to UVP of 3.0v, it turns off as expected, however turns back on straight away (on App shows 'DISCHARGE OFF' then 'DISCHARGE ON') even though UVPR is set to 3.5v - this behaviour continues in a loop, again and again*

should the BMS not keep the load turned off until the UVPR voltage is reached by the charger ?

* it does this only when my charger is connected.


any help would be most appreciated

thank you

Yes, it should remain disconnected until all cells reach UVPR. So, either the voltage is bouncing back quickly. You have a bad connection. Or the BMS is faulty.

 

[DIYrich]

Oh, you have Li-ion vs LiFePO4.

Voltages are different between the two. In any case, the inverter controls the pack, and the BMS protects the cells.

The BMS does have a problem, unless the cells jump to 3.50v immediately when charging voltage is applied. My LiFePO4 cells can easily swing 0.20v from discharge voltage to charge voltage with less than 0.50v delta. I don't know what is normal for Li-ion, especially when you may be applying a 1.1v difference.

 

[carpetpaul]

thanks for your replies, bad connection is probably unlikely as firstly, I know they're all solid, and the whole thing is as cool as a cucumber under the thermal camera. theres 1400 cells in total, and whilst not perfectly in balance across the pack, at the point immediately after UVP cuts the load, they're certainly not all (and I think "all" is required for re-activation after UVP) above the stupidly high 3.7v UVPR which I set for testing. looking like a faulty BMS. I've emailed JK, but they seen to not respond to contact form or email. I do have another related JK BMS which is a slightly lower output, but wouldn't take too much work to put in place of the one currently connected, I could try that as a good trouble shooting measure, also a factory reset wouldn't go amiss beforehand

 

[carpetpaul]

further tests show the same, pack hits UVP voltage and turns off the pack, BMS turns load back on within 5 seconds whilst massively below UVP-reconnect voltage, load pulls power, BMS shuts off at UVP voltage within 5 seconds and process repeats

I tried to contact JK BMS but no reply via contact form or multiple emails, contacted the JK Store on Aliexpress where I bought the unit, and they're trying to help whilst not admitting if they're the manufacturer or not, but struggling with language and waiting 24 hours between replies, I bought my second (yet unused and slightly smaller) JK BMS from a slightly different seller, so I've also asked them how I might contact some form of technical support for JK.

I could swap out the BMS for my second unit and try that, but it would be a lot of disassembly, and the balance wires don't use the same plug, so it's a fair re-wiring job for a quick test, frustrated !

 

[DIYrich]

I suggest trying to top balance. Charge with the voltage just below the point where it will UVP. What is the delta between high and low?
Let the cells balance for hours with that voltage applied. What is the delta now?
Can you increase the charge voltage without UVP? If so, rinse and repeat.

 

[carpetpaul]

thanks for your reply, top balance is close, 0.007A I think, granted, it does drift off as the cells discharge, but I'm not convinced this is the cause of the issue, when the BMS turns off due to OVP voltage, all of the other cells are way below the UVP-recovery/restart voltage, yet only as soon as the BMS (and inverter) are off, "discharge on" is seen on the JK app, and the inverter starts up again, runs for a couple of seconds, then cuts out due to the UVP.

my points/problems are :

1, once UVP triggers the BMS to turn discharge off (ie, no output to load) this should remain off until all cells reach UVPR, such that it can allow my solar to charge the battery to a point that I'd like to allow the load to connect again

THIS IS MORE THAN LIKELY THE CAUSE OF THE PROBLEM :
2, if my (tiny 4 amp) charger is connected, and simulating a low level of charging from the solar panels, this somehow causes the BMS to connect the load



in the attached photo (check if you think this is the correct settup) :

"1" Pos/Neg come in from the pack, through the breaker and
pos goes through to the terminal, and neg goes through the BMS, BMS P- then to Terminal

"4" Obviously the main terminals, here is where the charger is connected, and solar (later) will connect

"2" Inverter connects to terminals, I'm currently trying to rebuild slightly as everything is too tight here.

"3" is the charger socket, and as you can see, it's connected straight to the terminals, could this somehow be in the wrong place ?


In the screen shots of the App below, bear in mind that for testing, I'd set the OCP to 4.0v, and the UVP to 3.1V (UVPR was set to 3.4v, and on a later test 3.6v, but the re-starting loop happened both tests)

 

[timselectric]

I believe it's time for a refund or replacement.

 

[carpetpaul]

Probably one of my issues is that my inverter is rather dumb, in the case of 'wanting to continue charging whilst inverting' the inverter just doesn't know, and can't be programmed to stay off until the voltage in the pack is restored by charging.

JK have responded numerous time saying their devices don't support 'linked inverters, only straight to battery', but even then, once inverter or BMS shuts down due to UVP, neither the BMS can stay shut off (till a particular UVPR level, nor the inverter can be programmed to do the same

although the BMS doesn't seem to operate as intended, I'm thinking that I bought too much of a simple inverter for my application, and perhaps looking at an inverter/charge controller which will have some programmability might be the best bet, after all, forum members keep telling me that it's your charge controller that should look after these aspects, that the BMS is the last line of defence

 

[DIYrich]

screen shots of:
1) Right before cutoff
2) Right after cutoff
3) Settings page.

 

[DIYrich]

thanks for your reply. it's a 14S / 52v pack, charge voltage is 58.8v

BMS settings should be just outside the inverter settings. The inverter should cutoff before the BMS. The BMS is for backup.

If you are charging at 58.8v, that is 4.2v per cell. Isn't that the limit for Li-Ion? Maybe set your charger to 57.4v, or 4.1v per cell.

UVP of 4.20v, and UVPR of 4.10v.

 

[carpetpaul]

Thanks Rich. You’ve got UVP and OVP mixed up. It’s the bottom end where the problem lies.

You’re right though, but the issue seems regardless of the settings

Look at it this way :

1, if inverter has a under voltage cut off, the dumb inverter will continually start back up each time the sun comes out and the battery voltage raises above that threshold

2, if the BMS cut off at it’s under voltage cut off, my thoughts were that it should stay off until the set under voltage release/recovery voltage is reached during charging, this is the problem, and what doesn’t seem to be working

I may have to buy a dedicated proper inverter/charger

 

[DIYrich]

Thanks Rich. You’ve got UVP and OVP mixed up. It’s the bottom end where the problem lies.

On the low end, how about 3.1v and 3.3v, with 46.0 as the inverter cutoff.

I wonder if bottom balancing would help?

 

[carpetpaul]

Some times you feel like your in over your head !

I’ve just been looking at the user manual for my Epever Tracer AN charge controller (yet to be installed) and whilst it has versions setting for UVP and OVP, it would seem that these setting are for its load terminals which are either intended for a relay and not directly driving a large load

I wonder if this could be set such that at UVP, the relay turns off my inverter switch ? Then the Charge controller puts the inverter switch back on via the same relay at the UVP-release/recovery voltage ?

 

[carpetpaul]

Re your reply above, the inverter itself has no setting, but I think we could take control of its on switch vis the relay / load output terminals of the charge controller ?

 

[timselectric]

If your seat belt doesn't work. The solution is not better brakes. While better brakes are a good idea. Fixing the seat belt is the first thing that you should do.
You need to replace the faulty BMS.