18650 capacity question

[Ezrider_028]

Hi there newbie question,

I was given an about 70 18650s - I tested capacity and about 80% of them average about 2,400ah, however about 20% are below 2k - avg 1,800ah. .

Question : If I build a 48v battery pack would it be good to include the 20% lower amp cells ? This to add more capacity or will it somehow degrade the battery pack ? My plan would be to mix high and low capacity cells in each series and use a calculator to ensure each pack levels with each other.

So just debating to leave out the lower cap cells or take advantage of them.

Thanks.

 

[docw009]

Are these new or used cells? How does the measured capacity compare with the original value? Do you know the internal resistance? All the same cell brand and type?

In general, I believe mixing cells of different capacities, brands, and internal resistance doesn't make for a very good ebike battery. I know people do successfully this for powerwalls, but when you draw higher currents, the groups will have different discharge characteristics and get unbalanced.

Depends on what you have. A 3000 mah cell that tests out at 2400mah is going to act a lot different than a 2600 mah testing 2400 mah. The first one will probably be dead weight in your battery, Anything under 2000 mah probably will have little to contribute too.

 

[harrisonpatm]

Hi there newbie question,

I was given an about 70 18650s - I tested capacity and about 80% of them average about 2,400ah, however about 20% are below 2k - avg 1,800ah. .

Question : If I build a 48v battery pack would it be good to include the 20% lower amp cells ? This to add more capacity or will it somehow degrade the battery pack ? My plan would be to mix high and low capacity cells in each series and use a calculator to ensure each pack levels with each other.

So just debating to leave out the lower cap cells or take advantage of them.

Thanks.

Short answer is leave them out. If they're lower capacity, that means they've degraded, and will continue to degrade, faster than the other ones in the pack, bringing down the pack as a whole.

But that's still not the whole story. What cells are they? What's their capacity supposed to be? You said you tested a bunch at 2400mah. That's irrelevant if you don't know their original capacity. If they were originally rated for 2500mah, then sure, they're at 96% of their original spec. If they were originally rated for 3200mah, then you've got degraded cells already at 75% of their capacity.

Furthermore, you don't know what their rated discharge is. This is important when dealing with an ebike, but for any application, if you push the cells past what they're rated for, you could have a bad day.

Can you share the exact model of the cell you're using, as well as the intended application?

 

[Ezrider_028]

These anre used cells, I asked the person who gave me these he said they are Samsung, but couldn’t tell me the original capacity. So honestly, I thought of this being a quick hobby project, as I have a 14s BMS lying around so thought of building a spare battery. I do have a relatively new ‘off the shelf battery (52v)’ for my Ebike - but was being a bit “greedy” thinking, if I can squeeze a bit more “capacity juice” then it would be better. But I get these lower capacity cells - despite holding their charge - as I re-measured voltage after a week - these might go bad sooner - hence unless I’m willing to re-assemble the pack frequently to replace old cells - might as well leave them out. Thanks all. I’m kinda new to this 18650 cell world.

 

[Diggs]

You should be able to find your cells in the database to see what their original capacity was -

Cell Database

secondlifestorage.com

 

[E-HP]

You should be able to find your cells in the database to see what their original capacity was -

Cell Database

secondlifestorage.com

Great resource. I added the link to my newbie FAQ battery section, along with the entry in the change log. Thanks!

 

[Ezrider_028]

Yes, thank you for sending the cell database - very helpful.