BMS

pongyboy1

1 mW
Joined
Jan 20, 2021
Messages
15
I have a 4000w motor and will be putting a 72v battery on it. What is the minimum rated BMS I will need?
 
How many amps can your battery output?
 
Jonno said:
How many amps can your battery output?
Is it as simple as 4000 watts divided by 72 volts equals 55 amps assuming the battery output is capable of 55amps without breaking a sweat and the controller is rated at no more than 55 amps output.

Just because the motor is rated at 4000 watts doesn't mean it should be taxed at 4000 watts OR does it if there's a built-in safety factor above 4000 watts ?

If controller is no more than 55 amp rating then a BT programmable BMS rated at least 75 amps should provide sufficient buffer.
 
Well you are assuming several things there.
1. battery amperage capability
2. controller max amperage

If 1 and 2 are for instance 150a then you would need a 150a to 200a BMS Unless you choose the BMS bypass option

The motor rating @ 4000w is a guide, it will run at 4000w happily without burning out.
All the motors I own QS205s, QS273s, QS138s all run at between 10 to 30kw in bursts without a problem.

I hope this helps

Jonno
 
eMark said:
Jonno said:
How many amps can your battery output?
Is it as simple as 4000 watts divided by 72 volts equals 55 amps assuming the battery output is capable of 55amps without breaking a sweat and the controller is rated at no more than 55 amps output.

Just because the motor is rated at 4000 watts doesn't mean it should be taxed at 4000 watts OR does it if there's a built-in safety factor above 4000 watts ?

If controller is no more than 55 amp rating then a BT programmable BMS rated at least 75 amps should provide sufficient buffer.
the bms has nothing to do with the controller. the bms only has 1 job: protect the battery.

you rate the bms according to the battery, not the consumer.

so as no information about the battery is given you simply cant answer the question.
 
So what you are saying is a 200 amp discharge battery and a 200 - 250a bms is ok for a 300a controller?

Been there done that and it doesn't work. The battery shuts down.
(got one here for BMS bypass at the moment)
 
Jonno said:
So what you are saying is a 200 amp discharge battery and a 200 - 250a bms is ok for a 300a controller?
Don't see how you arrive at that from his post. Yes it's a good idea to oversize the BMS (250a BMS) with a 200a discharge battery, but why in the world would you use a 300a controller instead of a 200a controller when the battery is only rated at 200 amp discharge.

Isn't it possible guessimate (figure close enuf) the proper/safe BMS in this 20S5P 72v pack having Panasonic NCR18650BD 3180mAh 10A cells ... http://kinstarbattery.com/Product/35/76 ... and therefore also the best controller rating to use with this pack ?

In a like manner shouldn't pongyboy1 be able to determine the size of the BMS he needs knowing his cells rating and the proper controller amperage rating for his 72v battery. What if he isn't sure of his battery cells' rating ?

For the sake of discussion let's assume his cells are 10A maximum continuous rating. Does it make any difference whether his pack is 5P or 7P or 2800mAh cells or 3180mAh cells when it comes to determining what size BMS he needs.
___________________
when i said "Is it as simple as 4000 watts divided by 72 volts equals 55 amps" i was referring to a good level cruising speed amperage; whereas for "raw performance" climbing hills he may need at least a 100 amp controller for shorter periods with the cells maximum continuous discharge.
 
Jonno said:
How many amps can your battery output?
I am not sure. I have been looking online at different 72v options with different BMS ratings. 50A, 60A, 70A, 80A, and 100A. I want to choose the right one that would be a perfect fit for my 4000w motor.
 
Jonno said:
So what you are saying is a 200 amp discharge battery and a 200 - 250a bms is ok for a 300a controller?

Been there done that and it doesn't work. The battery shuts down.
(got one here for BMS bypass at the moment)

that means you need to reduce the load, aka: reprogram your controller to draw less current as you are overloading the battery and the bms is doing EXACTLY what its supposed to: protect the battery.

pongyboy1 said:
Jonno said:
How many amps can your battery output?
I am not sure. I have been looking online at different 72v options with different BMS ratings. 50A, 60A, 70A, 80A, and 100A. I want to choose the right one that would be a perfect fit for my 4000w motor.

read the posts i stated again. the controller does not matter, your bms needs to be rated for the BATTERY specs, not the consumer/controller.

if your controller is capacble of consuming more power then the battery is capable of (As seen in the above example) you need to reprogram it to draw less current or upgrade your battery and get a bms to match.

the bms rating is ONLY rated to what your battery can do. not what you WANT your battery to do. using a 100A battery and hook up a 300A bms so yo can run a 200A controller is NOT the correct way to do this. that is litteraly just like warpping tin foil over your fuses in your house because you blow them constantly with a 40A machine that is running off a 20A circuit.

stop mentioning your wattage of the motor, it does not matter in the slightest in what you are asking. we are asking what fuel needs to go into your car, not what type of bulb you have in your headlights.

say your exact battery specs (cells model, xxSxxP and so on) and nothing else.
 
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