Hi all, I am having the same type of problem in my assembled ebike with 14s5p 21700 battery pack. After installing a smart BMS, i found out the reason

[ranjithrajaram8]

Hi all,
I am having the same type of problem in my assembled ebike with 14s5p 21700 battery pack. After installing a smart BMS, i found out the reason. It is Low voltage protection shut off by bms. It shows cell 5 is of low voltage. And by the way i do not have a proper 52v liion charger (awaiting my order to arrive) so will charging the whole pack with a proper charger, fix it or do i need to replace all the cells (5P) at no5 position?

 

[amberwolf]

Any cells that are different in voltage from other cells have different properties than those other cells.

That means they will charge and discharge at different rates, and have different capacities, so the pack will never remain balanced, and will always need to be rebalanced on every charge cycle.

If all cells are different from each ohter, then replacing all cells with matched (identical, not just similar) properties, would be the way to fix that.

Without doing that, the pack will only perform as well as it's worst cell, and will need rebalancing every time you charge to get the most out of what it does have to give.

Rebalancing requires leaving the charger on the pack until the charger stops turning on and off, which with a seriously imbalanced pack like this could take hours, days or longer.

The cells shown in your image vary from nearly empty to nearly full, so you have some cells with almost no capacity vs the other cells that have much more capacity. The actual capacity difference can't be determined from the info provided; you'd have to measure that in one of a number of ways or guesstimate it by finding out which specific brand and model of cell you have and referencing the spec sheet for that cell showing the capacity for specific voltages.


Note: I moved your post out of an unrelated troubleshooting thread, since you have a known battery problem vs whatever issue the other person had.

 

[docw009]

That's a horribly unbalanced battery. Group 5 us at 30% and the top three groups are around 85%. The rest are around 50-60%. DId you start with 70 cells that were all the same in voltage and same type?

You might want to share what kind of cells are being used here. their age and condition.

I think a good balance BMS could equalize that battery but it might take days, and it's not going to do so unless you use a 14S 58.8V charger. It's at 53.21V right now, and if you are using a 13S 54.6V charger, not enough voltage to get it into balance mode, unless you have a really flexible BMS where the balance voltage can be temporarily lowered to 3.9V.

 

[amberwolf]

I think a good balance BMS could equalize that battery but it might take days,

Even if it equalizes it, it doesn't fix anything. It just makes the cells the same voltage--they'll still all be wildly different properties, and the pack will always have this problem, which will only get worse with time.

 

[ranjithrajaram8]

All are 21700 but 45 of them are of one generic brand and the rest is of another generic. But all of them are 5000mah. All of them are assembled in to a battery pack (Reention DP7 52v case) in the state of charge they arrived. The pack as a whole has never been charged from day one. (The charger shipping delayed) but i have tried to test the bike with this battery pack that is when the frequent cut off under load occured.

 

[ranjithrajaram8]

That's a horribly unbalanced battery. Group 5 us at 30% and the top three groups are around 85%. The rest are around 50-60%. DId you start with 70 cells that were all the same in voltage and same type?

You might want to share what kind of cells are being used here. their age and condition.

I think a good balance BMS could equalize that battery but it might take days, and it's not going to do so unless you use a 14S 58.8V charger. It's at 53.21V right now, and if you are using a 13S 54.6V charger, not enough voltage to get it into balance mode, unless you have a really flexible BMS where the balance voltage can be temporarily lowered to 3.9V.

Ordered and awaiting the 58.8v charger. In the meantime will try to lower the balance voltage to 3.9

 

[E-HP]

All are 21700 but 45 of them are of one generic brand and the rest is of another generic. But all of them are 5000mah. All of them are assembled in to a battery pack (Reention DP7 52v case) in the state of charge they arrived. The pack as a whole has never been charged from day one. (The charger shipping delayed) but i have tried to test the bike with this battery pack that is when the frequent cut off under load occured.

So you didn’t perform testing/matching of the individual cells before constructing the pack?

 

[BlueSwordM]

All are 21700 but 45 of them are of one generic brand and the rest is of another generic. But all of them are 5000mah. All of them are assembled in to a battery pack (Reention DP7 52v case) in the state of charge they arrived. The pack as a whole has never been charged from day one. (The charger shipping delayed) but i have tried to test the bike with this battery pack that is when the frequent cut off under load occured.

Oh yeah, you should have mixed cells because this can easily cause one set of cells to become unbalanced vs the other ones.

When dealing with used cells, I really recommend getting an IR meter to check how the cells patch as IR is a decent way of gouging cell health.

 

[ranjithrajaram8]

So you didn’t perform testing/matching of the individual cells before constructing the pack?

Kinda know i have to but wanted to drive the ebike so much so just assembled them.

 

[offGridDownUnder]

But all of them are 5000mah

No. All of then are labelled 5000mah by someone, somewhere, sometime.

Maybe they were, maybe they weren't. That doesn't make them ok.