Battery bank and BMS questions, LiFePO4

My plan is to run only a 6S system but as using lifepo4 batteries, will make it 7S.

These are the cells, at this stage will use 7S x 4P = 28 of these :

The question no 1: is there any disadvantage to handle and build these as 4 separate 7S strings/packs and run them in parallel? In this alternative each small pack would have an own BMS . Alternative would be to make one big pack 7s 4p and one BMS?

The question no 2: it worth while to invest in a powerful BMS and use that also for discharge, or use a less expensive one and only use it for charging, discharge directly from battery poles?

Thanks in advance for any input on this.

Better discharge protection than ESC since BMS watches every individual cell unlike ESC which checks pack discharge in total. if u can, i would say go for it.

For me not the best choice , low voltage = high amp for the same power, more high more heat less time , you will need a big bms to handle the discharge and that can be done with a good esc , is it a choice money wise?

This cells are not bad but , let say you need 2500w to have fun , the voltage drop fast to 2,75v , you will have around 20v continiously under load for 5A / cells total discharge capacity , so about 15-20 min of ride ( at the best )

I want with nese modules and 30Q , just finish to charge the first pack for the first test next week … Will see but it s very easy to built and safe as well

Thanks for the link to the cells! Good enough for me, I don’t consider anything else than LifPo4 in marine environment for my DIY projects…Price for cells was about 40 usd for 12, all included. Not bad?

You are right, in some respect not best choice. For me starting point was to use as simple and easily available parts( dont have a 3d printer, yet…) and use as much as possible existing solutions. Using a standard prop having 6s lipo or 7s lifepo and a basic 120kv 63 size motor, will without gear obtain rpm that matches the propeller. As doing now only a e surfboard, will have short wires.

Later on discovered that the 6384 will not as 1/1 gear create enough torgue so my solution is to use 2 x 6384 and two esc. So when using 2500w and 7s, this load is shared by 2 esc and two 6384 motors. So basically 55 amp for each set. Better solution would probably have been to use 14s and one large diameter 60kv motor but thats another story.

Still would like to know any issues related to use 4 separate 7S strings/packs and run them in parallel compared to one big pack 7s4p?

The issue would be to get 55a through to next parallel set , solder nickel strip , Springs … Not easy

Yes. In fact total consumption would be roughly 110A from a power bank consisting of 4 single cell pack’s (7S1P) connected in parallel. That means each pack would need to deliver about 28a, and that would go through a single nickel strip(later parallel connected with 8Agw) That is just within specification and the cells are delivered with factory spot welded nickel strip. Wonder, can the nickel strip manage the 28a…?

I guess you may will need to double : 2x 0.15 nickel strip … I recall 10a max before heating for a 7mm 0.15 strip , the Spot weld may Heat and unsolder …
I want with full Cooper 0.5mm i think