8s JK BMS configuration?

[snaker]

@shirley_hkg: I just bought a powerbank 24V/314Ah 8s lifepo4 + 50A charger. It has a 8s JK-BMS 250A/2A fitted. Below images are the current configuration of the JK-BMS. Can you have a look to check if there is any parameter I should adjust. I want it cut off sooner e.g at 3.000V (instead of 2.700V) and not charge too full e.g at 3.600 or 3.550V (instead of 3.650V).

[shirley_hkg]

I use the BMS for CHARGING only.

Here is my settings

[shirley_hkg]

[shirley_hkg]

Can you have a look to check if there is any parameter I should adjust.

Those settings are killing the cells.

Now I believe BM is right, that many people really don't have a clue.

[snaker]

Thanks shirley_hkg. So, I have to ask carefully before using it

[snaker]

Does the "Power off Vol" parameter mean a single cell volt or a total pack volt? It seems a single cell volt but chatgpt says it's pack volt

[shirley_hkg]

Cell voltage.

One more thing :
the wire resistance adjustment is individually pack related. I had the fuse between cells 4 & 5 .
Do not copy.

[shirley_hkg]

@ I want it cut off sooner e.g at 3.000V (instead of 2.700V)

(Cell under volt protect) 3.00v is 100% discharged. No need at all in routine operation.

I prefer 3.12V, and this is nearly 90% discharged already.

[snaker]

How can you adjust the "Power off vol" parameter to 3.000V? My BMS only allows to adjust it to max 2.499V, it does not accept any value higher than 2.499.

[shirley_hkg]

Are you in LiFePO4 mode ?

This parameter is insignificant. Cell under volt will cut your output already.

[snaker]

Yes, it is in lifepo4 mode. The status tab shows "Cell Type: LFP".

[Williamclark77]

Those settings Shirley shared are almost exactly what I use and have used for years on a few packs. Great settings! I start balance at 3.45, OVP at 3.55, and OVPR at 3.5. Evan when charged at higher watts the cells never go over about 3.58v.

I just checked the pack I'm sitting on. It will not allow setting the "Power Off" value over 2.5v either. It says "Sending Failure" when I try. Maybe Shirley has a different version? This one I just tested is Hardware Version V11.XW. Software Version V11.261H. Version V5.4.0

You should have no worry about reaching the "Power Off" point if your pack is large. I personally would not want it set high in case of a poor wire connection or similar issue. I would rather run the cell low than get stuck somewhere. My personal feeling only.

[snaker]

I found the way to adjust the "Power off Vol" parameter. All parameters in JK BMS must be set in the order from top to bottom. The following parameters depend on the previous parameters. To adjust "Power off Vol" to 3.000V, all 3 previous parameters UVP, UVPR, SOC 0% must be set higher than 3.000V first.

[shirley_hkg]

Right. It compares with related parameters to see if it is logical or not.

BTW, it saves whatever you set anyway. Ignore the warning message just.

You could avoid cooking the high cells, by setting the OVPR low, but not lower than (Start balance). It will pull the high ones down and keeps on balancing. Charging resumes when they are low enough.

BM suggests NOT cooking the high cells for prolong period.

[shirley_hkg]

I start balance at 3.45, OVP at 3.55, and OVPR at 3.5. .

[snaker]

On the powerbank there is a power button to turn the BMS on/off. When the BMS is off, it cuts off the output. In this off state, does the balancing keep working or the BMS is disconnected from the pack and stops balancing?

[snaker]

Here is my settings.

[snaker]

I charged the powerbank @50A (in fact 46A) and I see the BMS works surprisingly good. When the highest cell hits 3.550V, the BMS cuts off the charge current. During about a hour, the charger only restarts 1 time and keeps charging @5A in a couple minutes, then stops. After 1 hour, all cells are balanced at 3.400V. The difference between the highest and lowest cells is only 0.002V. Then I turn off the BMS, the balancing stops too. That means nothing can harm the cells. This cell balancing sounds as good as PL8.

One thing I still do not understand. I set OVPR at 3.475V, I see all cell voltages are 3.400V but the charge current is still 0A. Should the charger have restarted and kept charging as all cell voltages are under 3.475V? Anyway, I prefer this actual behavior (BMS cuts off charge current), I just do not understand why.

[shirley_hkg]

I have no idea too.

May increase this and see, Recommended Float Voltage.

[Burgerman]

After 1 hour, all cells are balanced at 3.400V.

And thats the problem.
You cannot balance cells at 3.4V. Or say they are balanced at all, as they will all be / read the same voltage even if unbalanced...

You must have balance start above 3.45V minimum. And hold voltage above this until balance ends. Thats the part BMS cant do. And unless the battery is held at 3.450 to 3.600V (3.550 best) then the cells will never be properly balanced. How do you balance a difference in v that keeps dissapearing?

A BMS stops charging when it sees the "charged" voltage, but then tries to keep on balancing as the voltage falls away again. But the difference in cell volts dissapears so it cant.

When charging with a real charger, you will see all the voltages stay close together even if miles out o balance. Until it hits 3.450V at which point they BEGIN to diverge, be different. Above this voltage they rapidly become full at different times. So make sure that the BMS cannot or does not balance below 3.45V minimum. If it tries thaen it can actually UNBALANCE a pack. In a perfect world you would only charge to 3.550V and you would start balance at that same 3.550V and hold the CV voltage there until the current fell to a low value and all cells are the same voltage. THEN stop the charger. Then its properly balanced.

[shirley_hkg]

No. He meant all the cells felt to and stayed the same at 3.400V after an hour.

The BMS will not be doing any balance below the (Start balance), which is 3.45V in this case.

[Burgerman]

I know. But even if they were out of balance they would do the same.

[Williamclark77]

Set your OVPR to 3.5.

After 1 hour, all cells are balanced at 3.400V.

You must have balance start above 3.45V minimum. And hold voltage above this until balance ends. Thats the part BMS cant do. And unless the battery is held at 3.450 to 3.600V (3.550 best) then the cells will never be properly balanced. How do you balance a difference in v that keeps dissapearing?

A BMS stops charging when it sees the "charged" voltage, but then tries to keep on balancing as the voltage falls away again. But the difference in cell volts dissapears so it cant.

It will turn current back on. At least it should. Mine does. Mine hold all cells at 3.5v then turns off. Once one cell drops below 3.45v (the start balance voltage), it turns current back on and equalizes all cells at 3.5v again with the OVP at 3.55v. None ever go over 3.57 or 3.58v.

I just updated my app and now things are moved around a bit and labelled differently. I would prefer to be able to set it to balance at 3.55v for XX amount of time then just shut off completely instead of turning current back on once one cell drops below 3.45 (or whatever your start balance voltage is set to).

They balance very well and very fast. Better than any "charger" I've used. The way mine is setup, and I think Shirley does similar, it's just an onboard charger and monitor. It does nothing unless a power supply above XX voltage is connected. Basically if you permanently mounted a PL8 to the pack.

[Williamclark77]

I just updated my app and now things are moved around a bit and labelled differently. I would prefer to be able to set it to balance at 3.55v for XX amount of time then just shut off completely instead of turning current back on once one cell drops below 3.45 (or whatever your start balance voltage is set to).

Your apps appear to have this ability. Mine does not. Maybe my region has not received this yet? My app version is 5.5.0. That's the latest available for my device.

[Burgerman]

Basically if you permanently mounted a PL8 to the pack.

That would be correct. If it had the ability to control the charger output in proportional fashion and HOLD the correct accurate CV voltage inn a steady state while it balanced but it cant. So it keeps shutting off, and reconnecting. While the volts go up/down. Thats way it has to do that dsconnect and reconnect bounce bounce thing...

I would prefer to be able to set it to balance at 3.55v for XX amount of time then just shut off completely instead of turning current back on once one cell drops below 3.45 (or whatever your start balance voltage is set to).

Logically you want this:
Charge and hold steady 3.550V per cell until both of the following are true:
Balance falls to a couple of mA max to hold at 3.550V.
CV until x CHOOSEN mA termination current has been achieved, (or 15 mins CV post balance complete).

And thats what it cant do as the charger output needs to be controlled to below whatever the balance current cannot take away. Requires proportional charge power control.

[snaker]

BMS cannot deeply balance cells as it has no control to proportionally reduce the charge current to a very low value (e.g 100mAh). But this balancing is already good enough. I think the difference between cells (if any) is less than 1Ah. That's very small (comparing to 314Ah) and no need to bother.

Comparing to a balance charger e.g iCharger DX8, this JK BMS is much simpler (in electronic parts) therefore less error prone. 2 of my 3 PL8s are broken, if my last PL8 fails I consider to use JK BMS (just to charge) instead of iCharger.

[Burgerman]

You can.
Just not so good.

The biggest issue is that people try to fit them on the chair. Where theres no space. And where if a mosfet fails shorted you have a fire... And where they do the safety nazi thing and chop of power instead of controling the parameters proportionally properly.

But they can be use to turn a decent power supply like the ZXD + BMS externally to a reasonable charger.

[Williamclark77]

Did you figure out why it was going down to 3.4v?

BMS cannot deeply balance cells as it has no control to proportionally reduce the charge current to a very low value (e.g 100mAh). But this balancing is already good enough. I think the difference between cells (if any) is less than 1Ah. That's very small (comparing to 314Ah) and no need to bother

You are correct and I don't think it matters, at least not enough to even know without precise long term testing - over ten years. I would bet that the manufacturing tolerances are greater than 1ah between different batches, not to mention losses due to wiring/connections. All of the cells are held above 3.5v and below 3.6v for XX amount of time and become more equalized than necessary for a long life span. The whole point of balancing is to never go above or below a certain voltage on any cell. This is accomplished perfectly.

The battery is going to outlast the mechanical part of your chair, which is the goal. I am in the process of repurposing a pack I built in 2018 to give someone in my community to use as an add on pack. Cells still capacity tested good enough that I would have thought they were new if I didn't know.

You can always turn your power supply down to low current once the bulk is returned and the balance has started if you want. I have done that several times. Couldn't tell any difference. All cells sitting on 3.5v +/- .002v. Tilt a few times or move 30 meters and they're all at 3.3X volts.

The unit Snaker is using is not big. About the size of your hand when flat and add maybe 3/4" to the length of the pack if put on the front or back. For my application, it's actually smaller and more tidy. Once built, it is a solid contained unit with only two wires coming out (connecting bolts actually, like a car battery with screw terminals). Better than 15 tiny equal length balance wires and two medium sized charging wires going to an external connection and two heavy power wires. All the same wiring in each configuration, just different packaging.

You're already sitting on a bunch of mosfets and capacitors in your controller. I try not to think of just how many components we are dependent on! Properly configured and applied they will never cut power.

[snaker]

No, I have not figured out. That requires charging and monitoring several times. I only use this powerbank to power an inverter in the cases of power outage. After the first charge with shirley_hkg's recommended settings, I see the BMS works as I want. I think it's Ok and my dad moved it to the upstairs room. Now I cannot access it via phone to monitor.

The things I do not like generic BMS are:
- It keeps charging too full (3.650V) and discharging too deep (2.700V).
- It turns the charge current on/off too frequently during charging.
- It tortures the cells by keep balancing all the time.

But this JK BMS can avoid all of those things with proper settings.

I only see it turned the charge current on/off only 1 time during charging. It's fairly strange but that's good. Maybe, that's due to the dump charger I am using is a little special. The seller gave it to me without documentation, so I do not know how it works. Actually, I only know it automatically switches charging in 2 modes (50A and 5A).

[shirley_hkg]

Did you figure out why it was going down to 3.4v?


Maybe CV of charger is too high after he reset OVP down (3.620→3.545). That renders the BMS cutoff it directly, leaving no time for absorption , when it was charging at 50A.

I set CV @28.45V, so current will fall gradually when approaching this voltage.

I only see it turned the charge current on/off only 1 time during charging. It's fairly strange but that's good. Maybe, that's due to the dump charger I am using is a little special. .


Some charger may not restart when it saw high volt (battery is full)

Most E-bike users here prefer charging lithium with PSU.