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New battery pack buid -- cell questions
#1
I am planning to build a 3P6S battery pack. Right now I use LG MJ1 cells. I always charge these cells to full.

With the new pack I will be charging only up to 3.92V to increase the cycles I can get out the pack. How should I size the new pack?

I chose 3.92V because I read an article from battery university :
Quote:In terms of longevity, the optimal charge voltage is 3.92V/cell. Battery experts believe that this threshold eliminates all voltage-related stresses; going lower may not gain further benefits but induce other symptoms.

How much capacity am I losing if I charge the cells only upto 3.92V opposed to the usual 4.2V. The cells have nominal capacity of 3500mah.

How many cycles can I expect from the pack?

Also, I read that it is optimal to operate the battery in 85-25% SOC, is that really necessary if I don't charge them above 3.92v?

I will be pulling 15A at max from my 3P pack. Should I buy cells like the Sony-VTC6 rated for high discharge? The cells rated for high discharge should have less internal resistance and should give more cycles when operated well below their discharge capabilities. I will charging the cells at max 0.5C.

Please suggest me some good cells to build my pack.


Thanks
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#2
If you've read the Battery University articles, it shows you in graphs how much capacity you could potentially loose. It also shows the cycle counts.

The only sure fire way to know is to capacity test the cell to 4.2, then capacity test to 3.92V. Everything outside of that is conjecture, especially if the cells are used.

There is no difference in wiring up a pack for 4.2V/cell or 3.92V/cell. However, if you are planning on a "24V" based system, you want to go with at least 7s, not 6s.

Operating up to 85% SOC "IS" only going up to ~3.92V (3.92 / 4.2 = 93.3%). Actual 85% is 3.57V, but you loose a LOT of capacity between 3.57V and 3.92V, maybe as much as 15-20%.

15A / 3p = 5A/cell, which is 2.5C. You need 2 times as many cells to reach .5C. 1C = 3.5A (3500mAh / 1000).

We need more details to give better and more accurate suggestions. Your request is too vague.

Ohh, also, I'm going to assume you mean 6s3p. Please check the FAQ located on the main page to get a better understanding of the terminologies we use here on the forum.
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#3
(08-05-2019, 11:59 AM)Korishan Wrote: 15A / 3p = 5A/cell, which is 2.5C. You need 2 times as many cells to reach .5C. 1C = 3.5A (3500mAh / 1000).
Its the max current of my motor and not nominal current.

(08-05-2019, 11:59 AM)Korishan Wrote: We need more details to give better and more accurate suggestions. Your request is too vague.
Sorry! What details have I missed sharing?
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#4
(08-08-2019, 05:31 AM)evbkz Wrote:
(08-05-2019, 11:59 AM)Korishan Wrote: 15A / 3p = 5A/cell, which is 2.5C. You need 2 times as many cells to reach .5C. 1C = 3.5A (3500mAh / 1000).
Its the max current of my motor and not nominal current.

(08-05-2019, 11:59 AM)Korishan Wrote: We need more details to give better and more accurate suggestions. Your request is too vague.
Sorry! What details have I missed sharing?

The motor used is one detail that we'd need. If 15A is surge current, what is run current? Again, if the motor is pulling even 8A under nominal load/current, 3p is not enough. That would still be 3.3A /cell
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
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#5
(08-08-2019, 11:50 AM)Korishan Wrote:
(08-08-2019, 05:31 AM)evbkz Wrote:
(08-05-2019, 11:59 AM)Korishan Wrote: 15A / 3p = 5A/cell, which is 2.5C. You need 2 times as many cells to reach .5C.  1C = 3.5A (3500mAh / 1000).
Its the max current of my motor and not nominal current.

(08-05-2019, 11:59 AM)Korishan Wrote: We need more details to give better and more accurate suggestions. Your request is too vague.
Sorry! What details have I missed sharing?

The motor used is one detail that we'd need. If 15A is surge current, what is run current? Again, if the motor is pulling even 8A under nominal load/current, 3p is not enough. That would still be 3.3A /cell
Hi, 
Sorry for late reply. I have changed my motor choice. This is the motor I will be using. The motor has max current of 45A and max volts is 12S. I am planning to use it only with a 6S pack. Will the max current still be same?
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