Overkill bms charge options

[7caper7]

Need help. Which methods allow the Overkill bms to turn off charging? In my application (marine) I need to be able to easily turn off charging at times. Charging current is present but I need to prevent it. (long term storage needing 50% SOC for long life, and other batteries (AGM) in the circuit needing float charge.) Thanks for any responses!

 

[John Frum]

Some of the versions of the android app should allow the bms to open the charge circuit.
Doing that with significant charge current is probably going to severely shorten the life span of the bms though.
Is this an existing system or one that you are designing?

 

[7caper7]

Some of the versions of the android app should allow the bms to open the charge circuit.
Doing that with significant charge current is probably going to severely shorten the life span of the bms though.
Is this an existing system or one that you are designing?

Thanks for the response! I have the bms on backorder and will examine the android app when received. Your answer is helpful. I will be able to temporarly stop charging voltage, disable charging in the app, then reestablish charging voltage. I think that would solve any possible issues and have a workaround for my problem. Thanks.

 

[Gazoo]

I tried turning off charging and it didn't work. Maybe someone else will chime in here who has tried it. However using the app to shut off the BMS does work. But then you can't charge or discharge.

 

[John Frum]

I tried turning off charging and it didn't work. Maybe someone else will chime in here who has tried it. However using the app to shut off the BMS does work. But then you can't charge or discharge.

Does your android app have the option?
Mine doesn't have the option but I've seen screen shots of versions that do.

 

[Gazoo]

Does your android app have the option?
Mine doesn't have the option but I've seen screen shots of versions that do.

Yes the newer version does have the option and I have it. I got the app from Overkills website. I like the newer version better but some don't.

 

[time2roll]

I would think you can drop the BMS maximum battery voltage during periods of storage.

 

[John Frum]

Yes the newer version does have the option and I have it I got the app from Overkills website. I like the newer version better but some don't.

So just to be super pedantic, you have the option but the option doesn't work, confirm?

 

[Gazoo]

So just to be super pedantic, you have the option but the option doesn't work, confirm?

Right but I only tried it once and while the inverter was floating the cells. I turned charging off and the inverter continued to float the cells. I just tried it again and the same. Maybe the BMS needs to see more charge current to shut off charging?

 

[John Frum]

Right but I only tried it once and while the inverter was floating the cells. I turned charging off and the inverter continued to float the cells. I just tried it again and the same. Maybe the BMS needs to see more charge current to shut off charging?

It should not need to see any charge current at all to stop charging.
The charge fets and discharge fets are disparate paths.
The physical p- is like a physical door.
The charge and discharge options are like virtual doors that can be opened or closed whether or not someone is trying to go in or out.

 

[Gazoo]

Yeah that's what I was thinking too. Let me come back in about an hour to confirm. It does appear my cells voltages are starting to drop a bit...like 1mv so far. These cells self discharge is so slow with no load on them I might have not waited long enough. I think I might have been wrong and turning charge off is working. Starting with a total battery voltage of 27.07 volts. Let's see if it drops.

 

[John Frum]

Yeah that's what I was thinking too. Let me come back in about an hour to confirm. It does appear my cells voltages are starting to drop a bit...like 1mv so far. These cells self discharge is so slow with no load on them I might have not waited long enough. I think I might have been wrong and turning charge off is working.

I would think you would validate the function primarily with current flow.
Maybe I missed something from earlier in the thread.

 

[Gazoo]

I would think you would validate the function primarily with current flow.
Maybe I missed something from earlier in the thread.

There is practically no current flow while floating. I don't have anything in the circuit that can measure that low a current.

 

[John Frum]

There is practically no current flow while floating. I don't have anything in the circuit that can measure that low a current.

Increase the load and eventually the battery will share the load.
You can also decrease the charge current to enhance the effect.

 

[DerpsyDoodler]

Right but I only tried it once and while the inverter was floating the cells. I turned charging off and the inverter continued to float the cells. I just tried it again and the same. Maybe the BMS needs to see more charge current to shut off charging?

Or maybe that is just a broad option in the app to cover the BMS which do have separate port charge/discharge. I believe jbd (jiabaida) provides separate port charge/discharge option.

 

[Gazoo]

I will be very happy if this works. The lowest I can set my charge voltage to is 26.8 volts. But still the inverter always stops charging at 27.07 volts and then floats. That's a little less than 3.4 volts per cell. I have no way to change any other settings.

I am not convinced holding cells at 3.4 volts over an extended period of time will reduce the capacity of a cell and plan to test this. Remember I have a UPS I ordered from China and other than some quirks I really like it. It's used in the event of a power outage which rarely happens.

Increase the load and eventually the battery will share the load.
You can also decrease the charge current to enhance the effect.

I know but I am lazy...lol. We will know for sure soon enough.

 

[Gazoo]

Or maybe that is just a broad option in the app to cover the BMS which do have separate port charge/discharge. I believe jbd (jiabaida) provides separate port charge/discharge option.

We will know soon enough. I am going to unplug the UPS from the grid with charging shut off and make sure the BMS will still discharge. This new app might be even better than I thought.

 

[mikefitz]

There are lots of parameters on the BMS you could change to stop charging , for example the low temperature charge disable, set this to a high value, the pack maximum voltage to a low value and so on.

I suggest a more practical method would to leave the BMS alone and use a power relay or isolating switch in the charge path to the battery.

Mike

 

[John Frum]

I will be very happy if this works. The lowest I can set my charge voltage to is 26.8 volts. But still the inverter always stops charging at 27.07 volts and then floats. That's a little less than 3.4 volts per cell. I have no way to change any other settings.

What is your bulk/absorb voltage?
What is your charge termination criteria?
Do you have a float stage?
If yes what is the float voltage?

I am not convinced holding cells at 3.4 volts over an extended period of time will reduce the capacity of a cell and plan to test this.

If you are correct then you should set bulk/absorb and float as high as your top balance will allow.
Personally I think that holding cells at >3.4 volts per is abuse.

Remember I have a UPS I ordered from China and other than some quirks I really like it.

I can't guarantee that I will.

I've recently made my own online 7kwh UPS.
I use a Powermax converter/charger as my charge source.
Its charges at 29.0 volts for ~30 minutes(which is really meaningless as the battery will never get to constant voltage in that time frame)
The it charges at 27.2 volts for ~12 hours and usually doesn't make it to saturation.
Then it switches to 25.9 volts at which point the battery takes the full load.
At 11:30pm the outlet timer disconnects the charger for 30 minutes and the cycle repeats itself.

 

[John Frum]

Or maybe that is just a broad option in the app to cover the BMS which do have separate port charge/discharge. I believe jbd (jiabaida) provides separate port charge/discharge option.

I don't know about that but my overkill has a 2 pin jst connector that when shorted enables the discharge path.
It works as expected.