How do I to chose discharge current and discharge overcurrent of BMS according to Cell and Motor ?

[Louis68]

Hello everyone,

I have a problem, I need to select a BMS discharge overcurrent and discharge current but I do not know how to chose this BMS parameteraccording to the cell and the moter ? Any one has a tips how to calculate discharge overcurrent and discharge for the BMS.
Which theory expalin this ? any reference , paper, course explain this choice of the value discharge overcurrent and discharge current.


Thank you,

Kind regards,
rand

 

[lnanek]

It's in the data sheet for your cells. Multiply by the number you have in parallel in your battery pack. E.g. a cell with 10A max discharge in a 6p pack would result in a 60A capable battery pack, assuming your BMS max discharge is higher.

If you made the pack yourself, also worth running the pack at full load and pointing a thermal camera at it to check for hot spots. Even if the cells can handle the load, the metal strips between them may be too thin to handle the current without heating up, or may be weakly attached. The forum has ampacity tables if you are building a pack and need to know how thick the metal strips need to be for each material.

 

[Louis68]

It's in the data sheet for your cells. Multiply by the number you have in parallel in your battery pack. E.g. a cell with 10A max discharge in a 6p pack would result in a 60A capable battery pack, assuming your BMS max discharge is higher.

If you made the pack yourself, also worth running the pack at full load and pointing a thermal camera at it to check for hot spots. Even if the cells can handle the load, the metal strips between them may be too thin to handle the current without heating up, or may be weakly attached. The forum has ampacity tables if you are building a pack and need to know how thick the metal strips need to be for each material.

how to calculate the discharge overcurrent ?

 

[eMark]

If the MaximumContinuousDischarge of a 6p battery pack is 60 amps then any greater amp drain is overcurrent discharge. Another example with a Controller cut-off say set at 40amps (for prolonging cycle life). So the BMS discharge overcurrent should be set for amperage drain above 40 amps.

With say a 6p battery (e.g. 60A MCD); while only supplying at most 40 peak amp bursts will result in prolonged cycle life as opposed to a more prolonged MCD drain of 60amps. Supposedly a 60A 6p battery rating means that the battery cells won't overheat pulling 10 amps (MCD) continuously from each cell. However, even with a first-class battery fabrication with A-grade cells, cell separators and pure nickel bus bars (or nickel plated copper bus bars) it's a given that the cycle life is diminished with more and more cycles. Possibly no more than 150 cycles with FULL charging to 4.2v/cell, dicharging to 3.2v per cell and prolonged MCD vs just peak bursts.

For example if a Controller has say a cut-off setting of 45amps you'd use a 60amp BMS with its overcurrent discharge at 46amps. Always buy a 'quality' BMS that is rated for higher amps (safety factor) than its overcurrent discharge setting.

 

[amberwolf]

Overcurrent is a current that is over the current limit.

So you set it to or below the limit your cells can handle, so that they are not stressed.

The regular current limit should be below the limit your cells can handle, so that they are not stressed. As far below as you can live with, preferably.

The harder you push the cells, the shorter their life will be, and the worse they will perform, and the faster they will degrade. Their spec sheets will have some info on this based on lab testing, and depending on the specific cell, there may be end-users that have posted testing results including aging.

Remember also that as cells age, they become less capable, so I recommend building your battery pack to be some percentage better than you need it to be, so that as time passes it still does what you need it to. If you build it so it only just barely does what you need it to do, then it might not be all that long before it can't quite do that anymore, and you'll have to use the vehicle/etc it powers more and more gently and for less and less time/distance as degradation increases.

 

[amberwolf]

Regarding picking the battery relative to your motor/controller, you need to first determine the amount of power you need to do the specific job you have in mind.

If it's a vehicle, then you should note down the worst-case riding conditions, terrain, hills, wind, etc that you will have, how much the whole thing plus rider(s) will weigh, what kind of acceleration and speed you require under the various conditions, and how much range you need out of it. Then you can take that to calculators or simulators like those at ebikes.ca, and guesstimate the required minimum power.

Once you know that, you can then pick a controller and motor that can do everything you need them to.

Then you can pick or design a battery that can do *at least* what those require, and has enough capacity to give the required range, plus at least say, 25% more capacity to account for unexpected detours, headwinds, etc, as well as pack aging over time.

 

[Louis68]

If the MaximumContinuousDischarge of a 6p battery pack is 60 amps then any greater amp drain is overcurrent discharge. Another example with a Controller cut-off say set at 40amps (for prolonging cycle life). So the BMS discharge overcurrent should be set for amperage drain above 40 amps.

With say a 6p battery (e.g. 60A MCD); while only supplying at most 40 peak amp bursts will result in prolonged cycle life as opposed to a more prolonged MCD drain of 60amps. Supposedly a 60A 6p battery rating means that the battery cells won't overheat pulling 10 amps (MCD) continuously from each cell. However, even with a first-class battery fabrication with A-grade cells, cell separators and pure nickel bus bars (or nickel plated copper bus bars) it's a given that the cycle life is diminished with more and more cycles. Possibly no more than 150 cycles with FULL charging to 4.2v/cell, dicharging to 3.2v per cell and prolonged MCD vs just peak bursts.

For example if a Controller has say a cut-off setting of 45amps you'd use a 60amp BMS with its overcurrent discharge at 46amps. Always buy a 'quality' BMS that is rated for higher amps (safety factor) than its overcurrent discharge setting.

Thank you, for instance about 6P, If my controller need 50 Amp , Is it correct if my discharge over current is 60 A ? how about the discharge current ? how about the charge current ? how to calculate it ? what is the best practice and how to set parameter

 

[Louis68]

Regarding picking the battery relative to your motor/controller, you need to first determine the amount of power you need to do the specific job you have in mind.

If it's a vehicle, then you should note down the worst-case riding conditions, terrain, hills, wind, etc that you will have, how much the whole thing plus rider(s) will weigh, what kind of acceleration and speed you require under the various conditions, and how much range you need out of it. Then you can take that to calculators or simulators like those at ebikes.ca, and guesstimate the required minimum power.

Once you know that, you can then pick a controller and motor that can do everything you need them to.

Then you can pick or design a battery that can do *at least* what those require, and has enough capacity to give the required range, plus at least say, 25% more capacity to account for unexpected detours, headwinds, etc, as well as pack aging over time.

Thank you, For instance about 6P, If my controller need 50 Amp , Is it correct if my discharge over current is 60 A ? how about the discharge current ? how about the charge current ? how to calculate it ? what is the best practice and how to set parameter

 

[rafalg]

Ok, you can limit the current in BMS to 'avoid stressing the batttery' but this can result in abrupt and unexpected power cutoffs during a ride. Which can be stressing or even dangerous to the rider. So imho a better option is to choose motor controller that will not exceed certain power limit, and use BMS amp limit only for protecting the battery against short circuits and other electric failures.

 

[Louis68]

Ok, you can limit the current in BMS to 'avoid stressing the batttery' but this can result in abrupt and unexpected power cutoffs during a ride. Which can be stressing or even dangerous to the rider. So imho a better option is to choose motor controller that will not exceed certain power limit, and use BMS amp limit only for protecting the battery against short circuits and other electric failures.

Thank you, but how to calculate the discharge overcurrent ? how to calculate discharge current ? what is the difference ? how to calculate charge current ? according to the controller and the cell ? Thank you

 

[rafalg]

Every cell type has such parameter as
- max continuous discharge current (the amount of current you can draw over extended period of time)
- max momentary discharge current (max current allowed during short pulse) - this would be the overcurrent
- max charge current
based on that you can estimate what your battery is capable of.
If each cell has 10A max continuous current and you have 6 cells in parallel, then the battery would be able to provide 60A continuous. And assume 100A overcurrent if each cell is able to supply 15A max in short burst
Charging current is calculated the same way - if one cell can handle up to 3A charging current, then 6 cells in parallel will take up to 18A.
This is how you estimate limits of your battery. The motor/controller should be chosen that you dont exceed the battery limits. Even if motor is 500W, for example, it can still draw more power during start - about twice as much.
So if you run a 42V battery and it's capable of 60A you could run a 2.4kW motor (but i think around 1kW would be more reasonable)

 

[Louis68]

Every cell type has such parameter as
- max continuous discharge current (the amount of current you can draw over extended period of time)
- max momentary discharge current (max current allowed during short pulse) - this would be the overcurrent
- max charge current
based on that you can estimate what your battery is capable of.
If each cell has 10A max continuous current and you have 6 cells in parallel, then the battery would be able to provide 60A continuous. And assume 100A overcurrent if each cell is able to supply 15A max in short burst
Charging current is calculated the same way - if one cell can handle up to 3A charging current, then 6 cells in parallel will take up to 18A.
This is how you estimate limits of your battery. The motor/controller should be chosen that you dont exceed the battery limits. Even if motor is 500W, for example, it can still draw more power during start - about twice as much.
So if you run a 42V battery and it's capable of 60A you could run a 2.4kW motor (but i think around 1kW would be more reasonable)

Thank you for your response, discharge overcurrent ? is not the current for short Circuit ? Thank you

 

[Louis68]

Every cell type has such parameter as
- max continuous discharge current (the amount of current you can draw over extended period of time)
- max momentary discharge current (max current allowed during short pulse) - this would be the overcurrent
- max charge current
based on that you can estimate what your battery is capable of.
If each cell has 10A max continuous current and you have 6 cells in parallel, then the battery would be able to provide 60A continuous. And assume 100A overcurrent if each cell is able to supply 15A max in short burst
Charging current is calculated the same way - if one cell can handle up to 3A charging current, then 6 cells in parallel will take up to 18A.
This is how you estimate limits of your battery. The motor/controller should be chosen that you dont exceed the battery limits. Even if motor is 500W, for example, it can still draw more power during start - about twice as much.
So if you run a 42V battery and it's capable of 60A you could run a 2.4kW motor (but i think around 1kW would be more reasonable)

Here is a sample of A battery : where is max continuous discharge current ? max momentary discharge current ? and where is max charge current ? Thank you

 

[rafalg]

No, overcurrent is for short bursts of extra power needed to get the the motor moving. And short circuit is even more than that...
So you want to allow overcurrent for some limited time, but short circuit should be detected immediately and then battery cut off.
I wouldn't rely on BMS only for this protection, especially for high power setup. I think a simple fuse is more reliable safety measure.

 

[Louis68]

No, overcurrent is for short bursts of extra power needed to get the the motor moving. And short circuit is even more than that...
So you want to allow overcurrent for some limited time, but short circuit should be detected immediately and then battery cut off.
I wouldn't rely on BMS only for this protection, especially for high power setup. I think a simple fuse is more reliable safety measure.

Thank you, you say " So if you run a 42V battery and it's capable of 60A you could run a 2.4kW motor (but i think around 1kW would be more reasonable) " , Why 1kW is reasoble for the Motor even if I have 2.4 kW on my battery ?

 

[eMark]

Checkout these informative ES threads... overcurrent discharge, undercurrent discharge endless sphere - Google Search

I wouldn't rely on BMS only for this protection, especially for high power setup. I think a simple fuse is more reliable safety measure.

If your ebiking enjoyment is more for power/speed performance as opposed to casual urban tripping you might want to consider a [so-called] SMART balancing BMS. Allowing you to monitor each parallel group of cells after each discharge cycle and after charging to 90 or 95% some of the time instead of a FULL charge everytime. With a quality fabricated battery (new A-grade name brand cells) that's used more for casual/moderate urban tripping ... then a non-balancing BMS with just over and under voltage protection should suffice.

 

[batteryGOLD]

Hello everyone,

I have a problem, I need to select a BMS discharge overcurrent and discharge current but I do not know how to chose this BMS parameteraccording to the cell and the motor?

take this example
citycoco scooter 1000W 60V system
at acellaration max power to motor is limited by controller 30A. so 30x60V means 1800W delivery
soo battery BMS needs to be a 30 or 40A rated (a 30A bms has a teorical peak 60A handle capability anda maybe shutdown protection at 100A)
some citycoco chinease batteries use 30A bms with aceptable performance plus a car fuse holder with a 30A fuse (some times this part melts or the worst case one ignited fire inside tha battery! carefull with fuse holders!!)

other example
trotinete xiaomi 365 (motor rated 250W) draws peak 15A at battery 36V means 36x15 = 500W peak draw to motor (the more peak current allowed, the more acellaration) there are some firmwares to tune some parameters at xiaomi trotinete/patinete. some able to setup up current to get 750W from motor.

there is a rule for max watts draw to a motor. any motor is able to handle twice rated power. ex. a 1000W is capable of handling 2000W peaks during acellaration and short time periods. maybe also possible to handle more than 2kW for very short time periods.


make sure to test any BMS at any battery to see if low/high limits cells voltage cut (many bms's random worlwide do not have this low and high level cut protecction.. soo carefull)

 

[E-HP]

View attachment 339573

Here is a sample of A battery : where is max continuous discharge current ? max momentary discharge current ? and where is max charge current ? Thank you

Take a look a the spec sheets for other batteries/manufacturers. They all use slightly different nomenclature, when describing Max. continuous Discharge Current. I've never seen a value for overcurrent on a spec sheet, and that's somewhat of a arbitrary value based on how conservatively you want to treat your battery, since any pack is capable of discharging several times the continuous discharge rating, but doing so to an extreme, or regularly, will hurt the cells.
The continuous rating is like a slow acting fuse/breaker, where discharging continuously over the value will eventually lead the the BMS tripping. I've never had that happen, even though I've drawn much more current than the rating, going up a long hill; eventually it should trip though. The overcurrent value is like an instantaneous fuse/breaker, where exceeding that value (which includes short circuits) even momentarily, will trip the BMS. I've had that occur a few times with my old pack when removing all of the limits on the controller and hitting full throttle from a stop.
Personally, I'd want the max continuous set to the calculated value of the cells and parallel groups, and the overcurrent to something much higher, mainly because I don't really want anything tripping (unless there's a short). when making a left with oncoming traffic.
If you look at the specs for various battery pack, you may see them rated for continuous discharge and max discharge, which I believe, in those cases, are the values for continuous and overcurrent. EM3EV does so with the max discharge set conservatively to 10A to 15A above continuous. All EM3EV ratings are conservative in order to protect the cells and their longevity. It's your choice whether to follow suit. EM3ev 52V (14S6P) Hard-Case Triangle Ebike Battery Pack With Bag | EM3ev