Choosing the right BMS - Amperage rating

msebold

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I'm continuing with my plan to build an 18650 battery pack and I need some more input from smart guys.

I am trying to choose a battery management systems (BMS) from BesTech Power. These are rated, in part, according to amperage. Ida Li, BesTech sales rep, says I should determine the "maximum continuous amperage" of my system.
(Note: I am not addressing battery pack voltage or overall amp hour capacity here.)

I understand that my new direct drive motor (still awaiting delivery) will draw 60 amps.
I plan to build some cushion into my battery pack capacity, which, when completed, could produce as much as 100 amps.

My question is this. Do I choose a BMS based on battery capacity (100 amps), or the estimated draw of the motor (60 amps)?

Educated comments would be appreciated.

Thanks,
Mike S.
 
Your motor will draw whatever you feed it- to a point. If it's rated at 60A continuous, it could conceivably draw 200A wot with locked rotor. Your amps are limited by your controller. You need to base your BMS choice on the controller peak amp rating. So I would choose a BMS that is rated to deliver the controllers peak amps easily or continuously. Your battery in turn needs to be able to deliver controller peak amps continuously with some buffer to give you a good life.
Hope this helps
K
 
The bms needs to protect the battery, so 100 amps would be the maximum you should use. If you really do pull 60 amps, then you'd pop the bms all the time if you use a 60 amps bms.

80 makes sense to me, but my plan would be to adhere to the cut the c rate in half rule. I'd be inclined to run no more than 50 amps controller, and stick with an 80 amps bms.

But,, chances are you won't be seeing 60 amps in the real world for a long time, and your cruise draw will be well under that.
 
As long as you know what you're doing the best bms imo is the one you have between your ears. it won't shut you down in the middle of an intersection you need to get across fast because a 20 ton truck is bearing down on you. If you want it just for charging then you can use any small bms and bypass the output with direct connections to the battery pack. If you plan on normal use, I'd get an 80A minimum bms for a 60A max controller.
 
kdog said:
Your motor will draw whatever you feed it- to a point. If it's rated at 60A continuous, it could conceivably draw 200A wot with locked rotor. Your amps are limited by your controller. You need to base your BMS choice on the controller peak amp rating. So I would choose a BMS that is rated to deliver the controllers peak amps easily or continuously. Your battery in turn needs to be able to deliver controller peak amps continuously with some buffer to give you a good life.
Hope this helps
K

Great advice. Thanks!
 
wesnewell said:
As long as you know what you're doing the best bms imo is the one you have between your ears. it won't shut you down in the middle of an intersection you need to get across fast because a 20 ton truck is bearing down on you. If you want it just for charging then you can use any small bms and bypass the output with direct connections to the battery pack. If you plan on normal use, I'd get an 80A minimum bms for a 60A max controller.

That is what the motor vendor recommends as well. Thanks!
 
dogman dan said:
The bms needs to protect the battery, so 100 amps would be the maximum you should use. If you really do pull 60 amps, then you'd pop the bms all the time if you use a 60 amps bms.

80 makes sense to me, but my plan would be to adhere to the cut the c rate in half rule. I'd be inclined to run no more than 50 amps controller, and stick with an 80 amps bms.

But,, chances are you won't be seeing 60 amps in the real world for a long time, and your cruise draw will be well under that.

Motor is MXUS 3K Turbo direct drive motor. I'm told the motor draws 60 amps. Controller is limited to 60 amps continuous. Probably will go with BMS set for 60amp continuous, 80amp peak. Thanks.
 
So now you need to build a battery capable of feeding the 3k demon.
If you choose a cell with say 10amps continuous discharge rating, you are going to need at least 8p to deliver your 80amp peak. Given you're not going to draw this all the time ( I'm assuming) this should be a reasonable starting point ( and a minimum) to go by. Range, cost, volume, housing, position and weight of the pack are also factors you will need to consider. Then there is the actual build- phew!. Nobuo's repository is a must read if you decide to build yourself. But it's all worth it.
 
You need a bms that is rated at the maximum rating of the battery.

Do the math on your cell specs and calculate what your pack can deliver safely. That way you can always upgrade the rest of your system without needing to replace your bms.

The entire point of the bms is to protect your pack, not the rest of the system. Look at the limit of your battery, not your specific usage at this point.
 
limited to 60 amps continuous

This doesn't quite ring true. The limit isn't generally the continuous. The limit is the max.

If your controller is limited to 60A then you don't need a bms any bigger than that. Though it's trip current should be higher.

I have a 30A bms. It will do more. With enough metal wicking away the heat it will hold 60A. It takes 100A before it considers there is a fault and disconnects. I can't speak for the bestech as one look over and I didn't fit it. However getting one designed for that power level is as much as can be expected of you. Remember it is a fuse of sorts and protects the cabling to the controller, so it shouldn't be too big for their current carrying capacity
 
friendly1uk said:
This doesn't quite ring true. The limit isn't generally the continuous. The limit is the max.

If your controller is limited to 60A then you don't need a bms any bigger than that. Though it's trip current should be higher.

Thanks Friendly One. From what I've read and found out I think you are absolutely right. I've already ordered a BMS rated for 60 amps discharge, 80 amps peak (simply to account for any malfunction that would cause a spike).
 
I have a question related. I am using 14 leaf batteries in series for 104 volts total. I will need 14 or 28 balance connections (i think) what do I use for bms and charger ? I am new to this so I am a bit in the dark
 
Zener-Ecite said:
I have a question related. I am using 14 leaf batteries in series for 104 volts total. I will need 14 or 28 balance connections (i think) what do I use for bms and charger ? I am new to this so I am a bit in the dark
https://www.youtube.com/watch?v=eawsLxQ6HXY ... pretty neat -- perhaps a little pricey? Is there another option?
https://www.youtube.com/watch?v=w3fQOjoGq0U ... there must be a less involved ... KIS ... easier method ... :thumb:
LeafMon_365c2734-7378-46ce-90ad-fd0d19105df2_1080x.JPG

https://www.batrium.com/collections/cell-monitor/products/leafmon-7x ... pretty neat and also expensive. Is there another option?

Devise your own KIS Battery Maintenance System that gets the job done (with fusing) -- far less expensive than above.
Manually check module volts at bottom/top ... discharge/charge for imbalance and balance modules when necessary.

https://www.evdrive.store/product-page/48v-14s-bms-assembly-kit-7-nissan-leaf-lithium-ion-battery-g1-g2-60a-breaker ... FWIW
https://www.techdirectclub.com/4s-bms-30-amps-assembly-kit-for-2-nissan-leaf-lithium-ion-battery-g1-g2/ ... FWIW
https://www.diyelectriccar.com/threads/leaf-battery-bms.202363/ ... FWIW
 
The Chargery Charger and BMS is another option ... https://www.chargerystore.com/

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https://www.chargerystore.com/index.php?route=product/category&path=33

Checkout this ES Battery Thread ... https://endless-sphere.com/forums/viewtopic.php?t=105323 ... here's a couple excerpts ....
John in CR said:
Automotive grade batteries stay incredibly well balanced and the chemistry Nissan uses in their packs is a quite safe one, so your primary risk of fire is from a short circuit or charger failure (ie if over-charging can even light a Leaf module on fire.). At first you'll check the pack for balance frequently, and then as you become familiar with it far less frequency is required. On my packs I only check for balance a few times a year plus if I notice anything different in voltage at rest and/or under load while riding. Checking balance of a 20s pack with easy access to the terminals takes less than a minute.
999zip999 said:
The number one thing you have going for you you're using a high grade automotive cell proven and they keep well balanced and you know what you got.
 
I have a question , i am not trying to still this thread to its original owner but my question is pretty similar so please my apology, i have a trike all wheel drive 48v 3 motor 3 controller , 2 controller had a peak draw of 18 amps and the other 1 is 23 peak amps this will total close to 60amps, should i use 80amps bms and at least 8p of 10 amps peak battery?
 
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