BMS capacity in parallel connection.

[Pawel1976]

Hi
I have a 2000W hub motor
2x48V 17.5aH batteries in parrael
Controller 45Amps max
Each battery pack BMS is 30Amps.

1. How many amps should I set the Controller to to be safe?

2. Can you explain to me, do we add BMS together if two batteries are connected in parallel with separate BMS, so the BMS capacity is 60Amps, or its 30Amps?
Should I set my controller under 60Amps or under 30Amps?

Thank you in advance.
Pav.

 

[chuyskywalker]

If you parallel the two batteries, you will get double the amp draw and double the capacity. If each pack pack can support 30A, then in parallel you can pull 60A.

If you ran them in series, you would double the voltage and keep the same amps/ah

 

[Pawel1976]

If you parallel the two batteries, you will get double the amp draw and double the capacity. If each pack pack can support 30A, then in parallel you can pull 60A.

If you ran them in series, you would double the voltage and keep the same amps/ah

Ok. So, O can set my controller for 50Amps for example and it will be safe for sure?
Thank you for your reply

 

[glennb]

The BMS’ provide most of your safety. If they’re good, and the temperature sensors are suitably located, then you’re good, regardless of what goes on downstream from them …

… but without knowing the cell models and configuration and battery design and construction quality it’s still not possible to verify that it’s “safe”. Furthermore, your wires could still melt, connections arc, etc. There’s many potential points of failure that could lead to dangerous situations.

Numbers are useful, but there’s more to it.

 

[Pawel1976]

The BMS’ provide most of your safety. If they’re good, and the temperature sensors are suitably located, then you’re good, regardless of what goes on downstream from them …

… but without knowing the cell models and configuration and battery design and construction quality it’s still not possible to verify that it’s “safe”. Furthermore, your wires could still melt, connections arc, etc. There’s many potential points of failure that could lead to dangerous situations.

Numbers are useful, but there’s more to it.

I really don't understand people. I was expecting simple answer
Yes, you can set yr controller below 60Amps because in parallel connection your BMS will be 60Amps.
No, you can't, you have to set it below 30Amps

I really don't understand why did you write all of this, what for? It make no sense to me, its not relevant. You wrote so many useless sentences, glennb.

 

[lnanek]

He was trying to help you. That information is far from useless. You asked if it was safe for sure. The answer is no, it is not safe for sure.

Double the amps is double the heat in your wires. If your wires are too thin, the connector to your motor will melt when you double the amps.

 

[Pawel1976]

Ok, so the answer should be NO!!!
NOooo, you can not, its not safe, set it under 30Amps. Period.
Simple as that. Thanks

 

[docw009]

If the motor is rated 2000W on a 48V system, then 40A thru the controller is about 2000W input, It's the controller that determines the amperage. Yes. your 45A controller is a little high and you could adjust it if that's possible, Or you could just ride slower.

By the way, you could connect ten of those batteries to your 45A controller, and it will never take more than 45A. The controller sets the maximum amps it can draw. As long as the battery supplies it, no problem.

Now if you had a 60A controller, 60A wiring, etc, when you put those two 30A batteries in parallel, each battery. if matched, would supply 30A and stay within the BMS ratings. That part would be safe in my opinion. If you only had one of those batteries by itself, it could not power a 60A or 45A controller. I believe you would overheat the cells or the BMS.

If all of this is just in the planning stage, it would probably be wiser to have a single battery big enough to power your motor, If you are going to parallel two smaller batteries, it's also probably safer to use a battery blender. This will prevent you from being careless or making a mistake connecting them.

 

[Chalo]

If you are going to parallel two smaller batteries, it's also probably safer to use a battery blender.

I think that would be a mistake, because it would create situations in which the entire load is borne by a single battery that isn't rated for it. You can't assure full parallel power without actually putting the batteries in parallel (with or without diodes, which can create other problems). Using a battery food processor can take one or the other battery offline without warning.

 

[Pawel1976]

If the motor is rated 2000W on a 48V system, then 40A thru the controller is about 2000W input, It's the controller that determines the amperage. Yes. your 45A controller is a little high and you could adjust it if that's possible, Or you could just ride slower.

By the way, you could connect ten of those batteries to your 45A controller, and it will never take more than 45A. The controller sets the maximum amps it can draw. As long as the battery supplies it, no problem.

Now if you had a 60A controller, 60A wiring, etc, when you put those two 30A batteries in parallel, each battery. if matched, would supply 30A and stay within the BMS ratings. That part would be safe in my opinion. If you only had one of those batteries by itself, it could not power a 60A or 45A controller. I believe you would overheat the cells or the BMS.

If all of this is just in the planning stage, it would probably be wiser to have a single battery big enough to power your motor, If you are going to parallel two smaller batteries, it's also probably safer to use a battery blender. This will prevent you from being careless or making a mistake connecting them.

Thank you for your reply. Its very importanto me. But, if I set my controller for 35Amps for example the batteries 30Amps BMS will stop it at 30Amps, then the battery BMS will switch the battery off. Yes or not?
Yes, I connected the batteries through 40Amps blender. Im going to change for bigger battery in the future, but now ive got what I've got.

My question was, do we add each battery BMS capacity if connected in parallel or not (30Amps +30Amps =60amps) or not? The bms will be 30Amps.
Thank you

 

[Pawel1976]

If the motor is rated 2000W on a 48V system, then 40A thru the controller is about 2000W input, It's the controller that determines the amperage. Yes. your 45A controller is a little high and you could adjust it if that's possible, Or you could just ride slower.

By the way, you could connect ten of those batteries to your 45A controller, and it will never take more than 45A. The controller sets the maximum amps it can draw. As long as the battery supplies it, no problem.

Now if you had a 60A controller, 60A wiring, etc, when you put those two 30A batteries in parallel, each battery. if matched, would supply 30A and stay within the BMS ratings. That part would be safe in my opinion. If you only had one of those batteries by itself, it could not power a 60A or 45A controller. I believe you would overheat the cells or the BMS.

If all of this is just in the planning stage, it would probably be wiser to have a single battery big enough to power your motor, If you are going to parallel two smaller batteries, it's also probably safer to use a battery blender. This will prevent you from being careless or making a mistake connecting them.

Thank you for your reply. Its very importanto me. But, if I set my controller for 35Amps for example the batteries 30Amps BMS will stop it at 30Amps, then the battery BMS will switch the battery off. Yes or not?
Yes, I connected the batteries through 40Amps blender. Im going to change for bigger battery in the future, but now ive got what I've got.

My question was, do we add each battery BMS capacity if connected in parallel or not (30Amps +30Amps =60amps) or not? The bms will be 30Amps.
Thank you

 

[Pawel1976]

If the motor is rated 2000W on a 48V system, then 40A thru the controller is about 2000W input, It's the controller that determines the amperage. Yes. your 45A controller is a little high and you could adjust it if that's possible, Or you could just ride slower.

By the way, you could connect ten of those batteries to your 45A controller, and it will never take more than 45A. The controller sets the maximum amps it can draw. As long as the battery supplies it, no problem.

Now if you had a 60A controller, 60A wiring, etc, when you put those two 30A batteries in parallel, each battery. if matched, would supply 30A and stay within the BMS ratings. That part would be safe in my opinion. If you only had one of those batteries by itself, it could not power a 60A or 45A controller. I believe you would overheat the cells or the BMS.

If all of this is just in the planning stage, it would probably be wiser to have a single battery big enough to power your motor, If you are going to parallel two smaller batteries, it's also probably safer to use a battery blender. This will prevent you from being careless or making a mistake connecting them.

 

[Pawel1976]

Battery 1. BMS 30AMPS
Battery 2. BMS 30AMPS
Batteries 1 and 2 connected in parallel.

What will be total BMS capacity in the system?
A. 60Amps
B. 30Amps

 

[docw009]

Your two 30A batteries in parallel are rated to push a total of 60A, 30A from each pack. The BMS has nothing to do with it, You could get more tha 60A if the controller were capable. Also the wiring, connectors, etc.

A BMS doesn't have ability to shut off current, unless its a short circuit. You can run 40A thru a 30A BMS for short time if the cells are capable. The 30A refers to the continuous current the BMS can handle without overheating. It will probably get too hot at 40A and cook itself if you run over current for too long, but you could probably run 33A all day unless you lived in Phoneix, Arizona which is celebrationg 100 days of faily highs ovef 100F,.

I've got one battery with a 15A BMS, but the cells were rated to push 30A . It will allow 26A on one bike, which is the max on the controller, if I blip the throttle. I don't hold throttle for more than a few seconds. so nothing overheats,

 

[chuyskywalker]

A BMS doesn't have ability to shut off current, unless its a short circuit.

This is incorrect. Anything which is not just a charging BMS' absolutely has current cut off. They shut off current for plenty reasons (low voltage, over voltage, temperature, etc). The whole point of the BMS' mosfets interrupting the negative power path is to control current flow based on pack conditions.

You are right, however, that most BMS' have a continuous rating that you can surpass. This Daly, for example, claims that the 30A version can handle 100A for an unspecified amount of time. (Though I don't believe that at all; sadly Daly has a history of over-inflating their BMS' capabilities.)

The point, here, @Pawel1976 is that these electrical systems (bms, battery pack/cells, motor, controller, wiring, and more) all interact with each other in complex ways; it's rarely as simple as a simple yes/no answer when you start to DIY you e-things.

 

[gromike]

You have 2 batteries that are rated a 30a each, so your total amp draw can be 60a.
You then multiply that 60a by .707 and get 42a. That is what you should set your controller's max amps at.
Why .707? Root mean square - Wikipedia .

 

[Dui ni shuo de dui]

Battery 1. BMS 30AMPS
Battery 2. BMS 30AMPS
Batteries 1 and 2 connected in parallel.

What will be total BMS capacity in the system?
A. 60Amps
B. 30Amps

None of the above. Capacity is expressed in Ah. You said that each of your battery is 17.5Ah, so in this case your total capacity would be 17.5+17.5 = 35Ah.

Now what you are talking about here is the battery maximum discharge current, which is expressed in Amps.
You have two batteries, each one can deliver 30Amps. So both combined will give you a maximum discharge current of 30+30=60Amps.

So, to answer your original question, you should set your controller to limit the current at any number of your choice as long as it is below 60Amps.

 

[Dui ni shuo de dui]

You have 2 batteries that are rated a 30a each, so your total amp draw can be 60a.
You then multiply that 60a by .707 and get 42a. That is what you should set your controller's max amps at.
Why .707? Root mean square - Wikipedia .

No.
This is a continuous current circuit, it has nothing to do with sinewave.
The maximum available current is simply the addition of each individual battery maximum current.

 

[gromike]

I just like the number 0.707. It's as close to perfection as I ever get.

 

[Dui ni shuo de dui]

Your two 30A batteries in parallel are rated to push a total of 60A, 30A from each pack. The BMS has nothing to do with it, You could get more tha 60A if the controller were capable. Also the wiring, connectors, etc.

No. The BMS is here to monitor the current and it will shut off the power if it goes beyond its 30A rating.
That's the whole purpose of this thing.

A BMS doesn't have ability to shut off current, unless its a short circuit. You can run 40A thru a 30A BMS for short time if the cells are capable. The 30A refers to the continuous current the BMS can handle without overheating.

A BMS absolutely has the ability to shut off current, it is the very reason why it exists in the first place.
Depending on the BMS it can cut the current under the following circumstances:
-Over current discharge
-Over current charge
-Over voltage/UnderVoltage event
-Cell out of balance
-Over temperature and sometimes under temperature
-short circuit detection
-by a button press from the user, when the users wants to switch the BMS off
-possibly some other condition I forgot

BMS have current sensors integrated in their DC bus and they use it to sense the current that is being drawn from the battery. You are correct saying that the 30A usually likely refers to the continuous current, indeed. That is the case for most BMS, they will allow for let's say 30Amps continuous and 45Amps peak for maybe 10 seconds.
Problem is: you can't know that for sure unless you can check the BMS configuration and see how it has been set up by the manufacturer. Some BMSs will allow you to do that, but most won't.

What is correct in your post is the fact that the BMS rated power is likely continuous, so there might be a little wiggle room to go over 60Amps peak if the user wants to try to get to the very limit of what can be achievable with his setup. There shouldn't be too much risks in doing that, but it could result in damage if the user is very unlucky (because when the first BMS cuts power then all the power needs will be instantly transfered to the second one. So the second one might see 60+ amps for a very short moment before it triggers the overcurrent shut off. Meanwhile the excess current could overheat and blow its mosfets. Unlikely to happen in my opinion, but still possible).

Anyway, the OP is searching for a simple and straight answer, let's not overcomplicate things for no reason. The simple answer is 60Amps or less.

 

[docw009]

This is incorrect. Anything which is not just a charging BMS' absolutely has current cut off. They shut off current for plenty reasons (low voltage, over voltage, temperature, etc). The whole point of the BMS' mosfets interrupting the negative power path is to control current flow based on pack conditions.

You are right, however, that most BMS' have a continuous rating that you can surpass. This Daly, for example, claims that the 30A version can handle 100A for an unspecified amount of time. (Though I don't believe that at all; sadly Daly has a history of over-inflating their BMS' capabilities.)

The point, here, @Pawel1976 is that these electrical systems (bms, battery pack/cells, motor, controller, wiring, and more) all interact with each other in complex ways; it's rarely as simple as a simple yes/no answer when you start to DIY you e-things.

Of course. Trying to keep my answer shorter/concise for the OP, I omitted the other ways it shuts off current. Thanks for the correction,

 

[glennb]

I really don't understand people. I was expecting simple answer
Yes, you can set yr controller below 60Amps because in parallel connection your BMS will be 60Amps.
No, you can't, you have to set it below 30Amps

I really don't understand why did you write all of this, what for? It make no sense to me, its not relevant. You wrote so many useless sentences, glennb.

Posts concerning safety get my attention.

Your focus on batteries led me to assume that it’s battery safety that concerns you. I said you can do whatever you like downstream from BMS, because they (should) protect you. That was me keeping things simple.

The caveat seemed necessary, for a number of reasons. But that’s by the by. The idea’s to protect the cells, and the BMS can only do so much, and not a lot for external from sources of ignition, so obviously no balsa wood battery boxes, and keep all the 60A wiring and components well away from them.

 

[Pawel1976]

Your two 30A batteries in parallel are rated to push a total of 60A, 30A from each pack. The BMS has nothing to do with it, You could get more tha 60A if the controller were capable. Also the wiring, connectors, etc.

A BMS doesn't have ability to shut off current, unless its a short circuit. You can run 40

None of the above. Capacity is expressed in Ah. You said that each of your battery is 17.5Ah, so in this case your total capacity would be 17.5+17.5 = 35Ah.

Now what you are talking about here is the battery maximum discharge current, which is expressed in Amps.
You have two batteries, each one can deliver 30Amps. So both combined will give you a maximum discharge current of 30+30=60Amps.

So, to answer your original question, you should set your controller to limit the current at any number of your choice as long as it is below 60Amps.

Ok, Understand, under 60Amps. Thank you. But, what happen if I set my controller for 40Amps for example and I forget to turn one battery ON?.

 

[Pawel1976]

No. The BMS is here to monitor the current and it will shut off the power if it goes beyond its 30A rating.
That's the whole purpose of this thing.


A BMS absolutely has the ability to shut off current, it is the very reason why it exists in the first place.
Depending on the BMS it can cut the current under the following circumstances:
-Over current discharge
-Over current charge
-Over voltage/UnderVoltage event
-Cell out of balance
-Over temperature and sometimes under temperature
-short circuit detection
-by a button press from the user, when the users wants to switch the BMS off
-possibly some other condition I forgot

BMS have current sensors integrated in their DC bus and they use it to sense the current that is being drawn from the battery. You are correct saying that the 30A usually likely refers to the continuous current, indeed. That is the case for most BMS, they will allow for let's say 30Amps continuous and 45Amps peak for maybe 10 seconds.
Problem is: you can't know that for sure unless you can check the BMS configuration and see how it has been set up by the manufacturer. Some BMSs will allow you to do that, but most won't.

What is correct in your post is the fact that the BMS rated power is likely continuous, so there might be a little wiggle room to go over 60Amps peak if

None of the above. Capacity is expressed in Ah. You said that each of your battery is 17.5Ah, so in this case your total capacity would be 17.5+17.5 = 35Ah.

Now what you are talking about here is the battery maximum discharge current, which is expressed in Amps.
You have two batteries, each one can deliver 30Amps. So both combined will give you a maximum discharge current of 30+30=60Amps.

So, to answer your original question, you should set your controller to limit the current at any number of your choice as long as it is below 60Amps.

You said " So, to answer your original question, you should set your controller to limit the current at any number of your choice as long as it is below 60Amps."
What happen if I set my controller at 40Amps for example and forget to turn one battery ON?
Knowing, that my battery pack BMS is 30Amps. Does it mean the controller or BMS will shut the system? Because of overcurrent?

 

[docw009]

"What happen if I set my controller at 40Amps for example and forget to turn one battery ON?
Knowing, that my battery pack BMS is 30Amps. Does it mean the controller or BMS will shut the system? Because of overcurrent?"

It's already been established that there is no overcurrent protection from the BMS, unless you have a short circuit. It will not stop you from running at 40A, if the cells in the single battery are capable of that. It could overheat and melt. That will stop you,

It should also be emphasized that you're not allowed to forget things when you run batteries in parallel. Forget to turn on one battery, Ride 20 miles, Better not turn the other battery back on. because the voltages re no longer equal. That's why I like battery blenders. CHalo points out their limits, but humans have worse limits,

What kind of bike are you riding that needs to pull 40A anyway. I have to assume it's a pre-built commercial scooter?