Overkill bms charge options

[John Frum]

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

Opening the fet path with significant current is pretty stressful for the fets.
Also Its my understanding that the upstream voltage can spike.

 

[Gazoo]

If you put the BMS in driving mode and use the lock shown on the main screen, that works as well.

 

[DerpsyDoodler]

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.


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

Tbh, I think you’re spinning your wheels. I had jbd 4s 120a BMS and when I tried to disable one or the other it did not work (I didn’t expect it to, being a common port BMS and all).

 

[Gazoo]

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?



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.


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.

Please read my review of the UPS. I explained everything there.

 

[Gazoo]

Tbh, I think you’re spinning your wheels. I had jbd 4s 120a BMS and when I tried to disable one or the other it did not work (I didn’t expect it to, being a common port BMS and all).

Can you be more specific? If you turned off charging did the BMS still discharge?

 

[DerpsyDoodler]

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.

And does it allow for charging when the discharge path is enabled too? I don’t know anything about the jst connector you’re talking about. Mine only had 2 connectors. 1 for balance leads, one for uart (bluetooth in my case).

 

[John Frum]

Please read my review of the UPS. I explained everything there.

Is that in this thread or elsewhere?
It its elsewhere how bout a link please?

 

[John Frum]

And does it allow for charging when the discharge path is enabled too?

Yes it will accept charge current when the discharge path is disabled.

 

[Ampster]

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..... Thanks for any responses!

Maybe this was discussed or I missed something. If you have a separate charger just put a switch on it. I would not try to force a BMS to control a charger for that purpose. Tell us more about your charger?

 

[Gazoo]

Is that in this thread or elsewhere?
It its elsewhere how bout a link please?

Sorry, I meant to link to it.

Gazoo's Review of the Snadi 24 Volt, 2000 Watt UPS

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[DerpsyDoodler]

Yes it will accept charge current when the discharge path is disabled.

I didn’t say disabled, but not trying to nitpic over that. Just trying to see if you verified that one is most definitely disabled while the other is most definitely enabled (so simultaneously).

 

[John Frum]

Maybe this was discussed or I missed something. If you have a separate charger just put a switch on it. I would not try to force a BMS to control a charger for that purpose.

I've been thinking about this for a while.
If it was really important I could run 2 bms.
One for charging and one for discharge.
The charge one would not have fets in the current path it would just use one of these to disable the charge source.

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[John Frum]

I didn’t say disabled, but not trying to nitpic over that. Just trying to see if you verified that one is most definitely disabled while the other is most definitely enabled (so simultaneously).

I only have administrative control of the discharge path but it is verified orthogonal to the charge path.

 

[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.

Right now the pack voltage is 27.05 volts and it's clear the cells voltages have dropped a bit. I will wait a little longer, and then unplug the UPS from the grid and see if it will discharge. Of course discharge is turned on in the app.

 

[DerpsyDoodler]

I only have administrative control of the discharge path but it is verified orthogonal to the charge path.

Mind posting a pic of your BMS as well as the jst connector you jumpered?

 

[Gazoo]

I think that holding cells at >3.4 volts per is abuse.

I agree. The highest cell voltage when the UPS is floating is 3.390 and the lowest is 3.375. The delta was worse at 25mv's so I put a bit of charge in the lowest cell and now have a delta of .015mv's. I am sure I could tighten it up a bit further.

 

[John Frum]

I agree. The highest cell voltage when the UPS is floating is 3.390 and the lowest is 3.375. The delta was worse at 25mv's so I put a bit of charge in the lowest cell and now have a delta of .015mv's. I am sure I could tighten it up a bit further.

I saw something similar.
On discharge my cells pull a wobbly at ~26.2 and are back to normal at ~26.1.
On the charge cycle its at ~26.3 to 26.4.
The cells rapidly go from 3mv delta to 20mv delta and back.

 

[Gazoo]

I saw something similar.
On discharge my cells pull a wobbly at ~26.2 and are back to normal at ~26.1.
On the charge cycle its at ~26.3 to 26.4.
The cells rapidly go from 3mv delta to 20mv delta and back.

For me it only happens while charging or discharging. But not while floating. The BMS shows the voltage and the delta wobble, and current wobble too. My DROK meter shows the wobble too. But my DMM doesn't see any voltage wobble at the cells. I wish I had a scope.

 

[Ampster]

I've been thinking about this for a while.

I was actually trying to respond to the OP's original question. He hasn't replied since his initial post. I think the issue @Gazoo is having is unique to using a UPS.

 

[John Frum]

I was actually trying to respond to the OP's original question. He hasn't replied since his initial post. I think the issue @Gazoo is having is unique to using a UPS.

I assumed he pulled an "Elvis" and we were just talking to amongst ourselves .