Bluetooth BMS and high voltage cutoff

respire

1 mW
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Apr 23, 2018
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I'm having a battery built with a bluetooth BMS (this one) and would like to know if using it to limit high voltage to 4V per cell is a substitute for using a charger with a 80% or 90% cutoff.

My (uneducated) assumption is that the charging voltage output on a 58.8V charger set to charge at 90% would be around 56V, and that limiting to 4V per cell through the BMS, the voltage output would still be 58.8 but that somehow the current wouldn't be allowed to pass when each cell has reached 4V (or 56V for the whole pack).

Is that assumption valid and what is best for maxing out on the battery life time?
 
it will start to hiccup massivly and probably refuse to work if you keep charging it and keeps triggering the OVP.

just buy something like a ELG or HLG meanwell in the proper range and use that.
 
flippy said:
just buy something like a ELG or HLG meanwell in the proper range and use that.
Thanks for the recommendations. They have an external potentiometer for voltage adjustment right? I've seen the product sheet and these charger seem to support a 90-300+ voltage range. I'm in Taiwan with 110/60Hz, do you think there will be noticable differences in terms of output current and charging time?

flippy said:
it will start to hiccup massivly and probably refuse to work if you keep charging it and keeps triggering the OVP.
Is it the BMS that would refuse to work or just the OVP function on the BMS. Would that be permanent damage?

My battery builder included a 2A charger in the quote so I hoped not to have to buy a different charger.
 
meanwells A versions have a internal pot for current and voltage you can set.
they dont care about voltage, anything above 80ish volts will turn them on.
the bms will cut off the output of the battery until the voltage has dropped. if you keep the charger on it you get massive hiccupping and keep pumping in low amounts of current and slowly get the resting voltage above the set OVP. then it will never release the OVP as the cell resting voltage is slighty higher then the OVP voltage and you basically have a fully charged pack that is dead until you bypassed the bms and discharged it a bit.

ask the builder to lower the voltage on the charger. if that cant be done you need a different charger. be weary for battery builders that cant deliver such a simple request.

side note: does the bms start activly balancing as soon as you charge it or can it only do a top balance? if you have the latter and the builder cant give you a smart bms that starts balacing immediatly: run away.
 
respire said:
They have an external potentiometer for voltage adjustment right?
As long as you get the *LG-***H-***A version (A at the end) then it has adjsutable voltage and current limits using two pots under rubber caps.


I'm in Taiwan with 110/60Hz, do you think there will be noticable differences in terms of output current and charging time?
They are rated for wattage, so how fast it can charge depends on your pack voltage and the current limit you set, and the version of PSU you get. For example, the HLG-600H-54A I use is 600W, so at 58V I get about 12A max out of it (so it peaks around 650-680W, IIRC). Im also on 110(115VAC)/60Hz.

Is it the BMS that would refuse to work or just the OVP function on the BMS.
When a cell reaches full, the BMS will first begin bleeding votlage off of it, then when it reaches HVC, the BMS will cut off input current from teh charger. Then the BMS will finish bleeding down the high cells and then reallow input from the charger. Once all the cellsa re the same voltage, then assuming theyre all the same internal resistance, then every time the BMS reallows current, theyll all begin to hit HVC pretty mcuh instantly.

At that point, the charger will probably be cycled on and off by teh BMS fairly rapidly. I dont know what effect that will have on it.

It shouldnt have much effect on the BMS except that depending on its design, it may sit there bleeding off voltage from cells much of the time after that point, and so it will be warmer than normal (and if its not ventilated, it could get hot over enough time)


My battery builder included a 2A charger in the quote so I hoped not to have to buy a different charger.
Then have them set the charger to the max voltage you want the pack to charge to, and then you dont have to worry about any of the above. ;)
 
amberwolf said:
When a cell reaches full, the BMS will first begin bleeding votlage off of it, then when it reaches HVC, the BMS will cut off input current from teh charger. Then the BMS will finish bleeding down the high cells and then reallow input from the charger. Once all the cellsa re the same voltage, then assuming theyre all the same internal resistance, then every time the BMS reallows current, theyll all begin to hit HVC pretty mcuh instantly.

At that point, the charger will probably be cycled on and off by teh BMS fairly rapidly. I dont know what effect that will have on it.

It shouldnt have much effect on the BMS except that depending on its design, it may sit there bleeding off voltage from cells much of the time after that point, and so it will be warmer than normal (and if its not ventilated, it could get hot over enough time)

most cheap bms will not start balancing until they get above 4.21V. when you charge until 4V it wont balance at all.
 
flippy said:
most cheap bms will not start balancing until they get above 4.21V. when you charge until 4V it wont balance at all.

Im assuming ;) that his BMS will balance at 4v because thats what he said hes going to have them set it to charge each cell to, and thats the only way the BMS could do that, is to have its HVC and balancing reduced to that per cell.
 
amberwolf said:
flippy said:
most cheap bms will not start balancing until they get above 4.21V. when you charge until 4V it wont balance at all.

Im assuming ;) that his BMS will balance at 4v because thats what he said hes going to have them set it to charge each cell to, and thats the only way the BMS could do that, is to have its HVC and balancing reduced to that per cell.
If its one thing i have learned when dealing with the mess other companies have left a customer with: dont assume, verify.
 
The battery builder is Paco from Cyclone Taiwan... I met him and I'm sure he knows his stuff, but I think the language barrier (his english//my chinese) is preventing us to communicate very effectively. Here's his last message after I asked him to adjust the voltage of the charger he'll provide, and redirected him to this ES thread:

The charger can't adjust the voltage!
You just need to set the battery charge voltage 4.1 and release voltage 3.85v [in the BMS] it will not keep changing.

Now I don't find any setting called « battery charge voltage » in the BMS specs but maybe he's referring to the « Overvoltage » setting. There is also a « Overvoltage release » which sounds similar to what he mentions and a « balanced turned-on voltage » that would address the concerns expressed by flippy. See screen capture below (or product specs). Perhaps a careful combination of these three settings will get the battery charged in a nice way to the voltage I need no matter what cheap 58.8V charger I use?

[img=https://ae01.alicdn.com/kf/HTB1jGhLQVXXXXXuXFXXq6xXFXXXz/222529769/HTB1jGhLQVXXXXXuXFXXq6xXFXXXz.jpg?size=79732&height=600&width=338&hash=ab1f1c09f1dfaa69f44589c203783124]
 
amberwolf said:
When a cell reaches full, the BMS will first begin bleeding votlage off of it, then when it reaches HVC, the BMS will cut off input current from teh charger. Then the BMS will finish bleeding down the high cells and then reallow input from the charger. Once all the cellsa re the same voltage, then assuming theyre all the same internal resistance, then every time the BMS reallows current, theyll all begin to hit HVC pretty mcuh instantly.
the problem with letting the battery charge high enough to reach HVC is, that each BMS has a HVC-release voltage. and this release voltage is quite lower then maximum cell voltage (though it can be changed in this BT BMS' software).
eg.:
- max cell voltage: 4.20V
- HVC: 4.30V
- HVC release voltage 4.15V.

so the charger will cut off if voltage of 4.30V is reached, and then uses it's super tiny bleeding resistors to pull the according cell down to 4.15V. during that time (which can be hours depending on pack size), NOTHING else happens. the low cells remain low, and only this one super high cell get's bleeded down. and then all cells are pulled down to 4.15V and the whole process starts again.
that means there will be no flipping on/off of the charger. this is a very long process, and HVC cutoff should really be the last gate of protection.
 
respire said:
[img=https://ae01.alicdn.com/kf/HTB1jGhLQVXXXXXuXFXXq6xXFXXXz/222529769/HTB1jGhLQVXXXXXuXFXXq6xXFXXXz.jpg?size=79732&height=600&width=338&hash=ab1f1c09f1dfaa69f44589c203783124]
with these settings the following will happen:

.) at 3.6V balancing will start. no matter if a charger is connected or not
.) it will balance (bleed individual cells down) until they are all in a range of 20mV between lowest and highest cell
.) whenever any cell reaches 4.15V during charge it will turn off charging FETs immediately
.) then it will bleed down this cell to 4.05V and start charging again
 
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