Battery Build Questions - 2170 cells - 20s10p

[skyeg3]

Hey all. Looking to build a badass battery to replace my current setup. I have the orientation of cells figured out but have questions about how to wire the pack. I'm not sure whether I'll hire somebody to build the pack or do it myself. If I can get enough information here I may just do it myself. Specific things I have questions about:

Wiring. This will be a 20s 10p pack. So do you wire 20 10p packs in series? Or 10 20s packs in parallel? It seems the first option would be the easiest because you can just take a chunk of 10 cells and bridge the connections on all of both sides. Then it's just a matter of wiring those in series. But in terms of electrical flow seems the second option might be better because with the first option you have more current flowing through some batteries with less reaching the end of the line. It seems with the second option all the cells would charge and discharge more evenly. I hope that makes sense. I'm not sure if this matters or not.

My other question is about sourcing a BMS. Would definitely like to find an American brand as opposed to finding something off Ali Baba. If I use tesla model 3 cells I'll need something that can do 150 amps. (72v)

I can answer those two questions I may just build the pack myself. If not I'll have to find somebody. Can anyone recommend somebody? I am near San Francisco.

 

[fechter]

You want to wire 20 10p packs in series, but you want the series connections to be distributed as much as possible to avoid having all the current in one conductor. Having each series group connected in parallel ensures balance between cells in that group and you will only need one BMS.

I haven't seen any US made BMS units that are reasonably priced. A 20s, 150A BMS is going to be a bit hard to find too. One approach for serious power is to use a low current 20s BMS and have that only run the coil on a big contactor that is on the main battery lead. If the BMS trips, the contactor opens. The other approach that I have used is to get a lower amp rated BMS and beef up the FETs, shunts and traces so it can handle more.

Below is a pack I built that is 16s, 12p using nickel sheet. I pull about 100A on peaks. In your setup, you may want to do double rows and nickel strips but the strips may have a hard time handling 150A. Some people use double layers of nickel to increase the rating.

 

[skyeg3]

Ok thank you. That answers my question about cell orientation. Where do you buy the metal strips that you use to make these connections?

As for the BMS. Does it still manage the charging of the cells? And then it will shut the whole battery down if it senses one of the packs is lower then acceptable voltage.. Is that right?

How are your cells that together? I was planning on using hot melt glue.

 

[fechter]

The BMS is only supposed to be a safety backup in case something else goes wrong. Normally the charger takes care of charging control but if any cell goes too high, it cuts the charge current. Similar thing when it gets too low. Normally your motor controller should turn off when the pack gets too low but if any cell gets too low, the BMS will also cut off the discharge.

The nickel sheet I bought surplus. You'd have to shop around to find some (and beware of fake stuff). The sheet was 0.15mm. Most people use the narrow strips.

I used silicone glue to hold the cells together. Hot melt will work too, as long as the batteries don't get hot. :wink:

It's hard to see in the picture, but my cells are also bound with high temp fiberglass tape, which will prevent the cell cans from touching if a wrapper decides to melt off. Kapton tape or Nomex paper can also be used for this. You don't need to worry about insulating between cells in the same parallel group.