Creating 50ah bricks of 21700 cells to be used in larger install - BMS what?

[Cliff Rodriguez]

Hello, I've been studying battery building for a while and am finally doing my first project. I ordered my first lot of 100 21700 3.7v 4000ah cells. I want to build a brick that is 12v at 50ah. If it works as expected and I'm comfortable, I will build 3 more until I have a 24v 100ah battery in a case. That makes 4 of these bricks per enclosure.

My plan is to make these enclosures portable. Will be 2 or 3 eventually. They will lock into a spot in my RV and also be portable to take out and use in various power emergencies. The enclosure will have 110v and 12v outlets (inverter somehow included inside or outside).

The question I have is around the BMS and monitoring. Is it one BMS per brick or one per eventual 24v 100ah enclosure? Any recommendations/questions appreciated!
Thanks in advance for the great info.

...Cliff

 

[snoobler]

NMC cells are a poor choice for 12V. Their operating range of 12-16.8V does not lend itself well to typical 12V applications.

You would be MUCH better served by going straight to 24V via a 7S arrangement with a working range of 21-29.4V.

IMHO, based on my own testing, I recommend you should limit cells to 3.3-4.05V or 23.1V to 28.35V for optimal life using 60-80% of the cell's total capacity.

100/7 = 14P7S arrangement with two cells left over.

You should also test 100% of cells for internal resistance and capacity and make all of your 14P groups have the same matched parameters, i.e., when you add up all the cell capacities, each group of 7 will be the same, and calculate total resistance for each 14 cell group.

All NMC cells should be individually fused either with a fusible wire or via purpose built nickel plates with fusible links for each cell.

NMC chemistry (3.7V Lithium) is toxic, flammable and may explode if operated outside its specifications. Do lots of research and educate yourself before you take the plunge and implement a battery. Fear of a negative outcome is your best ally in this endeavor.

 

[Cliff Rodriguez]

Thank you for the info! One more question, how should I fuse the cells? Nickel sheets or solder wire? Since I'm using 21700 cells, its hard to find nickel fuse sheets. I found one at aliexpress. What do you think? https://www.aliexpress.com/item/33027203565.html?spm=2114.12010612.8148356.1.c2656a390NJ01C

 

[Cliff Rodriguez]

I'm looking at Daly Smart BMS and noticed that all the ones I saw are listed for 3.2v and I need 3.7v. Do you know of a BMS for the 21700 cells you would recommend that have a blue tooth or other "smart" connection to an app? Based on the recommendations from this thread I am looking at a battery pack that is 22P7S (24v 100Ah).

 

[snoobler]

Missed your prior inquiry. Yes. Fuse them. Doesn't matter how, but that link you provided is NOT for fusible sheets.

Daly makes them for 3.6/3.7 nominal:

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Just make sure you pick a suitable amperage and consider any differences in amps on discharge and charge.

 

[Ampster]

I'm looking at Daly Smart BMS and noticed that all the ones I saw are listed for 3.2v and I need 3.7v. Do you know of a BMS for the 21700 cells you would recommend that have a blue tooth or other "smart" connection to an app?

I like the Overkill BMS I have for a 4S pack. You would have to ask them what the work around might be to get their 8S BMS to be programmed for 7S. I know that my Orion only comes in increments of 12 cells so for a 16S pack I had to crimp the leads for 17 to 24 to one ring rerminal and put it on #16. I also have a user setting for number of cells and I think my Overkill has a cell # setting as well.

 

[snoobler]

I was under the impression the Overkill was only 8S/16S and only 3.2V. If that ever changes, I'll recommend it too.

 

[Ampster]

I was under the impression the Overkill was only 8S/16S and only 3.2V. If that ever changes, I'll recommend it too.

I bought one and configured it for NMC for a while until I found out the inverter could not handle the 16 Volts. I think Overkill is only 4S and 8S. It is all on the web site.