Mixing 18650 and 21700 cells in same parallel pack in battery pack ok?

[YoshiMoshi]

TL;DR Summary

Mix 21700 and 18650 cells in parallel?

I'm doing a 3S3P pack. My box can hold 9 18650s. It can also hold 3 18650s and 6 21700s. Is it ok to mix 18650s and 21700s in the same p pack? I would use the same 21700s and the same 18650s in each p pack. I would arrange them to maximize capacity.

So say 18650 cells are Model A cells (making up model number her for discussion purposes). All 3 18650 cells in my battery be model A.
The six 21700 cells are Model B cells.

One P pack would contain 2 model B 21700 cells and 1 model A 18650 cell.
I make three of these identical P packs described above. I would then put these P packs in series to create my battery pack.

Good idea? I don't see any issues. Each P pack would have roughly the same capacity and max current draw as each other (each cell slightly different even from same model) and would be identical.

 

[berkeman]

I have no idea what you are asking, but hopefully somebody else who is familiar with those battery designations will be able to answer. I just know that hooking batteries in parallel is problematic unless they are identical with identical past histories (age, charging histories, etc.).

 

[Baluncore]

The difference is form factor and capacity.
18650 = 18 mm diameter x 65 mm long.
21700 = 21 mm diameter x 70 mm long.

When using different capacity cells, or cells from different manufacturers in parallel, you will have reduced capacity before damage occurs.
When using cells in series, a battery management controller should be used.

Rather than two parallel batteries of different cells, use three series strings in parallel. That will require three series battery management controllers.

 

[Flyboy]

Yeah, definitely don’t mix different capacity cells in the same string. If anything, try to match your capacities as much as possible in each string.

 

[Baluncore]

Battery management.
https://en.wikipedia.org/wiki/Battery_management_system

 

[YoshiMoshi]

Thanks for the help!

Yea, they have different dimensions, and that 21700s have a larger energy density than 18650s. Some 21700 also have higher maximum continuous discharge current than 18650s

Just making things up here:
Model A 18650 cell has capacity of 3,000 mAh, maximum continuous discharge current of 25 A
Model B 21700 cell has capacity of 3,500 mAh, maximum continuous discharge current of 30 A

Say my container has physical dimensions that allows for the following configurations of 9 cells:
1) 9x18650s
2) 6x21700s + 3x18650s
3) You can expand 2 to get rid of any number of 21700s and replace them with 18650s, such that no more than 9 cells exist in the container

So physically, due to higher energy capacity, I figured option 2 would be the most optimum solution, and then I asked myself if combining cells of different sizes into a parallel pack would be a good idea or not. I would do a 3S3P configuration. Meaning I would have something like this:

1) If I had in parallel A-A-A my overall capacity would be 9 Ah, maximum continuous discharge current of 75 A
2) If I had in parallel A-B-B (shown in the picture above) my overall capacity would be 10 Ah, maximum continuous discharge current of 85 A
3) My container will not fit this If I had in parallel B-B-B my overall capacity would be 10.5 Ah, maximum discharge current of 90 A

I do suffer .5 Ah capacity and 5 A maximum discharge current, by not using all the same 21700 cell from the same manufacturer (Option 3), nine of them. But if that's all I suffer, is capacity and maximum continuous discharge current, and I'm willing to take those losses, than is that ok? Because in the end I still have a battery pack going with Option 2 over Option 1 (and my container won't physically fit Option 3)?

I would need a BMS in there (not shown) to have charge balancing. Each parallel pack would be the same configuration A-B-B. So each parallel pack/string would be the same.

I know that I shouldn't have in parallel batteries with drastically different voltages for long periods of time. That's a big "no no". Although people do try this to revive dead batteries, probably not to safe, creates a lot of heat I believe. I see that the nominal voltage of 18650s and 21700s is the same, so as long as I fully charge them than I think I should be ok on that front.

I know the capacities and maximum discharge current would be different, but I think I would be ok if I arranged them in the configuration shown in the picture above, each parallel pack of same configuration? Each instance of say the B cell would be slightly different. One could be 3502 mAh while another could be 3498 mAh. I would obviously do capacity matching, such that the overall capacity of each parallel pack was as high as possible, getting the optimal capacity.

Thanks for help in trying to understand this!

 

[Vanadium 50]

I think this is a terrible idea.

When you start out, and everything is putting our 4.2 V, no problem. Then as the batteries discharge, the two different kinds of battery have two different voltages. And you have them in parallel.