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Old 11-17-2020, 03:29 PM   #1
dipduo
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Default Looking for the best BMS for custom 32650 cell build

I've been lurking through this forum for several months now- attempting to consume information as best I can. Finally decided to make an account & post about my current situation.

I'm building a 36v LIFEPO4 conversion kit. The plan is to use battery hookup's new 36650 cells that have the following specs:
NOMINAL VOLTAGE: 3.2V
CAPACITY: 5000MAH
MAX CONTINUOUS DISCHARGE: 15A (3C)
MAX PULSE DISCHARGE (10 SECONDS): 50A (10C)

Going to build a 12S 40P pack & am looking for guidance on the best approach for a BMS.

Should I have a BMS on each of the 12 3.2v (40P) packs?
Should I use a 36v BMS on the combined packs?

My biggest concern is the 350A+ current draw on the BMS- was considering putting multiple BMSes in parallel to remediate this.

I recently lost a brand new (to me) RV in a fire due to a custom wiring setup made by the previous owner...

So doing this the RIGHT way really means something to me.
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Old 11-17-2020, 05:24 PM   #2
Volt_Ampere
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Default Re: Looking for the best BMS for custom 32650 cell build

I would not put the BMS in series with the pack discharge. Only use it to control overcharge / overdischarge. Yes - you will want a 12S BMS. Compared to most builds here, 40P is overkill. That will give you 200AH capacity and 600AMP capability which is about double what most folks need. 480 cells (12 * 40) is a lot to deal with. Do you have experience doing a large lithium pack?
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Old 11-17-2020, 05:42 PM   #3
dipduo
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Default Re: Looking for the best BMS for custom 32650 cell build

I totally get that 40P is overkill. the goal of the whole project is to make a long range cart & maximize battery life by not putting a huge load on the cells.

I am working on a 4S 50P 18650 pack as a bit of a "training/test" build. Doing everything the "right" way with fuse wire, spot welds, pre-battery balancing etc.

I'll probably buy half of the 26650 cells & check out the size of combining 40 together before making any final decisions.

what if I split up each of the (~)40P cells into groups of 4 (12V) then put BMSes on the three of those (combined 36v)?

Would any special wiring need to be used to achieve this?

My real problem is finding a BMS that is rated for ~350A discharge..
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Old 11-17-2020, 06:02 PM   #4
Volt_Ampere
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Default Re: Looking for the best BMS for custom 32650 cell build

I don't see why you need a BMS rated for 350 Amps. I built my own BMS that prevents overcharge by monitoring cell voltages and warns of undercharge with a very loud beeper. You can use a contactor if you really want to shut the cart down due to undercharge. The BMS itself does not need to see the full battery current.
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Old 11-17-2020, 11:30 PM   #5
dipduo
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Default Re: Looking for the best BMS for custom 32650 cell build

Quote:
Originally Posted by Volt_Ampere View Post
I don't see why you need a BMS rated for 350 Amps. I built my own BMS that prevents overcharge by monitoring cell voltages and warns of undercharge with a very loud beeper. You can use a contactor if you really want to shut the cart down due to undercharge. The BMS itself does not need to see the full battery current.
so to be clear, you only use the BMS for charging- discharge is wired around it?
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Old 11-18-2020, 09:09 AM   #6
Volt_Ampere
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Default Re: Looking for the best BMS for custom 32650 cell build

That is how mine is wired. Some systems do have the BMS controlling overcurrent but I don't favor that for a golf cart or electric car application. You should include a fuse in the battery circuit to protect against shorts. My Alltrax controller (500Amp) came with a 300 amp fuse. I have my max current set at 400 amps in the Alltrax programming - and the fuse does not blow because I never pull that much current for more than a few seconds.
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