BMS assumptions - have I got this right ?

[gone_walkabouts]

Really interesting reading. Some of my post outlined I am heading toward LIPO battery pack. Certainly the use ofmy BMS on a LIPO pack would not make sense. I think the point was it would be Ok in terms of Max cut off voltage (3.9v)however the cutover voltage on the discharge would be too low for LIPO. If you did use the BMS, accepting the reduced battery capacity, I guess you would have to cut off from the controller or a CA, but at very conservative levels to account for a individual bad cell.....I can see issues

So in summary in terms of my issue at hand, it seems that the BMS I have is probably not total fit for purpose or possibly something that you could get away with a Balanced PACK. From the above it seems that if your Pack does happen to get out of Balance in a significant way (0.5v or more) then the BMS may not have the capacity to bring the cell/cells in line before the voltage goes up through 3.7v and beyond. This is turn can damage the cells and add to the problem.

What I intend to do since I will be needing this for LIPO Bulk charging, is apply some "manual" balancing using a Battery Medic. This should get my cells in line to the point where the particular BMS I have might be able to get them in line.
In the mean time, while I have no BMS and not yet balance cells, I have turned my charger cutoff on my EMS240 from 56v to around 53v. This will help prevent runaway cells while I wait for a more robust balancing solution.

The challenge I have is that the two dead cells have been replace by other headways I had "lying around" and I not sure if the fact they have had 20% more or less cycles (not sure which) make a big difference here. I know they are from a different prod batch and manufacturing date however they are the same AH and model. I don't have a great idea on how they will affect the ability for those cells to charge and discharge at the same rate are the majority !! Any ideas or experience here ?

 

[friendly1uk]

This has been an illuminating thread. I find it shocking that a lifepo4 bms could think 3.9v acceptable. I don't need to look at the graphs to know it could end badly.

A 12 or 16s you say. For 5-13s I like bms-b's smart bms. You tell them what you want. The cell count, chemistry and your own choice of lvc and hvc. Only $20
I don't really have a second best, but you could look at bestekpower for ideas.

That BMS seems to be fine but is not for LiFePO4 is for LiCo so charge ends at 4.2V for that it will work as advertised I guess I did not try.
But OP needs a BMS that works with LiFePO4.

Before correcting someone knowledgeable it is both polite and wise to first check your facts.

 

[electrodacus]

This has been an illuminating thread. I find it shocking that a lifepo4 bms could think 3.9v acceptable. I don't need to look at the graphs to know it could end badly.

A 12 or 16s you say. For 5-13s I like bms-b's smart bms. You tell them what you want. The cell count, chemistry and your own choice of lvc and hvc. Only $20
I don't really have a second best, but you could look at bestekpower for ideas.

That BMS seems to be fine but is not for LiFePO4 is for LiCo so charge ends at 4.2V for that it will work as advertised I guess I did not try.
But OP needs a BMS that works with LiFePO4.

Before correcting someone knowledgeable it is both polite and wise to first check your facts.

I was on that website and I cheeked the spec the smart one it was referring to dose not seems to have custom lvc and hvc and the limits are set at 4.2V.
I also checked all other BMS on that website and did not found any that will work for LiFePO4.
Can you provide me with a link or model of BMS that will work with LiFePO4 end the charge at 3.6V and not 3.9V
Is definitely not my intention to not be polite.

 

[dnmun]

Really interesting reading. Some of my post outlined I am heading toward LIPO battery pack. Certainly the use ofmy BMS on a LIPO pack would not make sense. I think the point was it would be Ok in terms of Max cut off voltage (3.9v)however the cutover voltage on the discharge would be too low for LIPO. If you did use the BMS, accepting the reduced battery capacity, I guess you would have to cut off from the controller or a CA, but at very conservative levels to account for a individual bad cell.....I can see issues

So in summary in terms of my issue at hand, it seems that the BMS I have is probably not total fit for purpose or possibly something that you could get away with a Balanced PACK. From the above it seems that if your Pack does happen to get out of Balance in a significant way (0.5v or more) then the BMS may not have the capacity to bring the cell/cells in line before the voltage goes up through 3.7v and beyond. This is turn can damage the cells and add to the problem.

What I intend to do since I will be needing this for LIPO Bulk charging, is apply some "manual" balancing using a Battery Medic. This should get my cells in line to the point where the particular BMS I have might be able to get them in line.
In the mean time, while I have no BMS and not yet balance cells, I have turned my charger cutoff on my EMS240 from 56v to around 53v. This will help prevent runaway cells while I wait for a more robust balancing solution.

The challenge I have is that the two dead cells have been replace by other headways I had "lying around" and I not sure if the fact they have had 20% more or less cycles (not sure which) make a big difference here. I know they are from a different prod batch and manufacturing date however they are the same AH and model. I don't have a great idea on how they will affect the ability for those cells to charge and discharge at the same rate are the majority !! Any ideas or experience here ?

i tried to help but you went off on this other tangent because somehow it has now become a problem for you to use the normal BMS for these lifepo4 packs because this guy is saying they are charging to 3.9V without his permission. i cannot stop what people say, i just tried to help you out.

but i told you there is nothing wrong with the BMS and that did not seem to register.

the pack is either severely unbalanced or some of the cells are ruined from over discharging.

i asked if you could measure the cell voltages while it is charging but have not seen any of those numbers either. so without any info there is nothing i can do for you and these people can yak about how much they know about BMSs all they wanna.

 

[electrodacus]

i tried to help but you went off on this other tangent because somehow it has now become a problem for you to use the normal BMS for these lifepo4 packs because this guy is saying they are charging to 3.9V without his permission. i cannot stop what people say, i just tried to help you out.

but i told you there is nothing wrong with the BMS and that did not seem to register.

the pack is either severely unbalanced or some of the cells are ruined from over discharging.

i asked if you could measure the cell voltages while it is charging but have not seen any of those numbers either. so without any info there is nothing i can do for you and these people can yak about how much they know about BMSs all they wanna.

Most BMS/PCM on the market today sold as LiFePO4 are not design to charge LiFePO4 they are just sold as such is not true for those large one that have individual cell monitors mount on the cell and have programmable thresholds.
I have two of them different models both use the same SOT23-5 chip designed for other more common chemistry and have fixed internal thresholds.
LiCoO2 (most used in laptops and cellphones but not only) have a charge threshold of 4.2V and nominal 3.6 or 3.7V the equivalent for LiFePO4 is 3.6V max charge voltage and 3.2V nominal.
Those so called LiFePO4 BMS use a version of the LiCoO2 SOT23-5 chips designed for military spec chargers at 3.95V for longer life on this Lithium-ion batteries. This guys use this because they are inexpensive a few cents / piece but since they allow 3.9V will damage a LiFePO4 not instantly but will degrade the cell quite fast.
Is this explanation unclear? or you do not believe me ?
I'm an electrical engineer and designed my own fully programmable BMS for my main LiFePO4 battery used on my offgrid house.
I do think I have the necessary knowledge in regards to BMS and battery charging.
You are right that is nothing wrong with the BMS except for the fact that is badly designed and will damage LiFePO4 cells.

 

[friendly1uk]

I was on that website and I cheeked the spec the smart one it was referring to dose not seems to have custom lvc and hvc and the limits are set at 4.2V.
I also checked all other BMS on that website and did not found any that will work for LiFePO4.
Can you provide me with a link or model of BMS that will work with LiFePO4 end the charge at 3.6V and not 3.9V
Is definitely not my intention to not be polite.

I'm looking at this one chap http://www.bmsbattery.com/smart/330-lifepo4lithium-ion-smart-bms-for-513-cells-in-series.html
If you scroll to the bottom, you have to specify series cell count, over voltage, under voltage and chemistry.
It uses the oz890 which will take values between 0-5v per cell.

Here is a great thread on it http://endless-sphere.com/forums/viewtopic.php?f=14&t=48461&hilit=oz890
The datasheet is in the 3rd post down.

 

[electrodacus]

I was on that website and I cheeked the spec the smart one it was referring to dose not seems to have custom lvc and hvc and the limits are set at 4.2V.
I also checked all other BMS on that website and did not found any that will work for LiFePO4.
Can you provide me with a link or model of BMS that will work with LiFePO4 end the charge at 3.6V and not 3.9V
Is definitely not my intention to not be polite.

I'm looking at this one chap http://www.bmsbattery.com/smart/330-lifepo4lithium-ion-smart-bms-for-513-cells-in-series.html
If you scroll to the bottom, you have to specify series cell count, over voltage, under voltage and chemistry.
It uses the oz890 which will take values between 0-5v per cell.

Here is a great thread on it http://endless-sphere.com/forums/viewtopic.php?f=14&t=48461&hilit=oz890
The datasheet is in the 3rd post down.

This BMS is different form what I have found when I searched there. So I will need to apologise.
That one had I2C and was called smart but the I2C was used just for monitoring and had fixed thresholds at 3.9v like the one OP has.
I will take a look at this one but is clear that it has a different construction so SOT23-5 chips for each cell.
I will be back with my opinion but it looks good and the price is excellent.

 

[electrodacus]

It uses the oz890 which will take values between 0-5v per cell.

Here is a great thread on it http://endless-sphere.com/forums/viewtopic.php?f=14&t=48461&hilit=oz890
The datasheet is in the 3rd post down.

Thanks for this. It seems like a nice IC quite similar with what I use but mine is newer from a larger manufacturer and has some additional futures but I can see where they got their inspiration from.
Not sure how I was not able to find this when I did my research last year. I fount probably all other BMS chips except for this one. And this is quite good probably will have been second choice on my list especially that it was available at the time the one I used I needed to wait a bit until I got the first sample directly from the manufacturer.
The OZ890 seems to be very obscure. I made a search to find a price and I only found one place with over 20$/IC sure it must be less expensive how else will they sell the entire BMS at less than that.
Do you have any idea where I can get the OZ890?