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What Size BMS do I need 12S7P 48V 5000W Inverter

DeanR181

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Sep 23, 2021
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Hello,

I am making a 12s7p system with 50Ah 4V (3.7V) lithium battery cells.

I believe I need a BMS for each string (7 BMS's) but I am confused to what amp BMS I need for each string of cells.

I have read where some people say that each BMS should be able to handle the full current load of the entire system ( assuming 350Ah based on a 5000W inverter 48V and 7 x 50Ah strings)) which means I would need 7 BMS's each rated a minimum of 350Amp in case one or more of the 7 BSM's fails, but others say just to use the amp for the String (5000W / 48V = 104Amp.) Could you please confirm which setup is correct, and also, how are all 7 BMS’s connected? Do I connect them in parallel?

Also, the busbars to connect each cell. Again, I am confused with this and not sure if they only need to handle the current for the string of cells (50aH battery, 5000W inverter at 48V). I am getting very confused to which ratings I need to work off. i.e. the full system current or the single string current or the battery current.

Finally, if I have 7 separate BMS's with Bluetooth connection to my phone app, would this mean that I need to swap in and out of each BMS on the phone app to check each string individually or is there a BMS that will link all the BMS's together so that I can see all 88 cells in the one app page?

Maybe there is a BMS available where I only need one BMS with 88 or more cell connections? I have looked but not sure if I can get away with one BMS for the whole system or whether I do need a BMS for each string.

Any help with this would be greatly appreciated.
Thank you very much, regards Dean
 
Look at (and email) Batrium. This is an Australian company that make a very nice (but expensive) BMS.

I am not exactly sure which way they will recommend the setup - but it can handle (and report) on that many cells.
 
Look at (and email) Batrium. This is an Australian company that make a very nice (but expensive) BMS.

I am not exactly sure which way they will recommend the setup - but it can handle (and report) on that many cells.
Great, thanks
 
Hi,

Have you thought of using 14S6P? You could split it up into 3 or 2 banks of 14S2P or 14S3P, so you'd only need 2 or 3 BMSs. Gives you some redundancy. Depends if your inverter will handle the higher voltage. I use an 18S with my LFP cells, my MPP Solar inverter will handle 66V, 64V max charging.

I am assuming you have 84, not 88 cells.

dRdoS7
 
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Hi,

Have you thought of using 14S6P? You could split it up into 3 or 2 banks of 14S2P or 14S3P, so you'd only need 2 or 3 BMSs. Gives you some redundancy. Depends if your inverter will handle the higher voltage. I use an 18S with my LFP cells, my MPP Solar inverter will handle 66V, 64V max charging.

I am assuming you have 84, not 88 cells.

dRdoS7
Yes, my mistake, 84 cells (I have 4 spare). We want to keep it 12s7p but just confused to the rating of the BMS for each string. As mentioned, some say to divide the total current by the amount of strings, but others say each BMS should be rated for the whole system and not divided by the amount of strings. I.e. 5000W / 48V = 104A. So assume a BMS rated 120A. Some say that each BMS should therefore be rated 120A but others say to divide this 104A by the number of strings. I.e. 104A / 7 = 14.9A So assume a 20A BMS on each battery. I am leaning toward 120A for each. This is the part I am confused with due to conflicting information
 
Hi,

Yes, my mistake, 84 cells (I have 4 spare). We want to keep it 12s7p but just confused to the rating of the BMS for each string. As mentioned, some say to divide the total current by the amount of strings, but others say each BMS should be rated for the whole system and not divided by the amount of strings. I.e. 5000W / 48V = 104A. So assume a BMS rated 120A. Some say that each BMS should therefore be rated 120A but others say to divide this 104A by the number of strings. I.e. 104A / 7 = 14.9A So assume a 20A BMS on each battery. I am leaning toward 120A for each. This is the part I am confused with due to conflicting information

7 x BMSs sounds very expensive!

What's the voltage range of your cells?

Split your 84 cells into 2 or 3 strings. You'll be able to run off only 1 if required. You didn't say what voltages your inverter will handle. My original 3kW, could run at 60V (62V trip).

I can recommend the JK/Heltec, no experience with any other brand.


I have 18 x 280Ah cells, and a 5000W inverter. I figured a 200A discharge BMS would handle the ~100A no problems.

dRdoS7
 
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