How do i size my bms

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Troche

New member
Joined
Mar 25, 2021
Messages
3
Hi guys, I'm new here and kind of new to diy battery builds. I'll give you a little background to my project so to paint a proper picture.
I bought a sailboat in the middle of a refit. The previous owner scraped the deisel engine to go electric. He got a far as purchasing the motor and controller as well as a few other things related to the boat before needing to sell. Here I come into the picture oblivious but eager to learn and plop down some cash and realize there is a lot to learn.
The drive motor is 72v 32kw. Oubre worked out how to charge the batteries. I should say I sources all of the components and still fighting out whether they all will play nicely together.
I purchased lithium battery cells from battery hookup. The plan is to build a 72v 200ah battery as directed by the supplier of the motor. I have evening worked out on how I want to configure the packs but I'm stumped on how to size the bms. I know it's going to be a 20s but the amperage. That's the part I don't have a clue. Do you size it based on the charging parameters of the battery or the draw the motor will have.
I'm sure someone out there will have an answer for me that I can understand.
The batteries are being delivered today so I'll be busy testing and sorting for the next little while. Pretty excited.
 

daromer

Moderator
Joined
Oct 8, 2016
Messages
5,659
You design it based on your max load so the bms must be able to cope with your surge current going to the motor. is that 32kw or is that nominal?

Your motordriver you should be able to set a max current in terms of amperage. That need to be lower than the bms can handle. preferable 20% less.
When we talk about sizes above its high current. Many people then go to mechanical switches instead of mosfet based systems that turn the battery on/off.

because 32000/72 = 450A. You can find BMS s of that size suited for it. Daly among others are commonly used for this purpose.
 

Troche

New member
Joined
Mar 25, 2021
Messages
3
Thank you for your time daromer. Yes 32kw is peak. The guys at thunder struck said to use a 400 amp breaker between the battery and the load. They also said I probably would never see those kind of loads in my application. I don't have tons of money to throw at this project so I don't want to overkill everything but I do want to be sure that I am properly protected from overcurrent and obviously FIRE. Does that 450amp rating effect the rate of charge. The batteries I have are good for .5c charge rate.
 

Roland W

Member
Joined
Oct 9, 2017
Messages
137
Thank you for your time daromer. Yes 32kw is peak. The guys at thunder struck said to use a 400 amp breaker between the battery and the load. They also said I probably would never see those kind of loads in my application. I don't have tons of money to throw at this project so I don't want to overkill everything but I do want to be sure that I am properly protected from overcurrent and obviously FIRE. Does that 450amp rating effect the rate of charge. The batteries I have are good for .5c charge rate.

Charging is not the problem! You will only see maybe 100A there. In your case, the limiting factor is the motor current, especially when it is started up and surging.
 
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