Li-ion 18650 cell grouping for a battery pack - Page 1
Forum section: electronics / projects

author: santoshgurral
author_rank: Contributor
timestamp: March 25, 2020, 08:02:50 pm
content: I am working on a Li-ion battery pack with used cells from an old laptop battery. I have a doubt on how to correctly group these used cells so that I can get maximum efficiency out of the pack.I am making a 1s5p battery pack with the following capacityCell1- 1909mAhCell2- 1919mAhCell3- 1166mAhCell4- 1483mAhCell5- 2037mAhSo if I group these cells in parallel how does the discharge of these cells happen? I mean since cell3 has the lowest charge capacity with 1166mAh compared to other cells, will other cells also discharge only up to 1166mA instead of their full capacity. Like for example, while discharging the battery pack cell3 would discharge to its bottom faster than any other cell and reach its cut off voltage of 2.65V and the TP4056 module would disconnect the load from battery pack even though other cells in the pack have not fully discharged or reached their bottom?I am using the TP4056 charging module to charge this pack which also has DW01A protection IC on it. Please see the attached image for my setup.

author: sleemanj
author_rank: Super Contributor
timestamp: March 25, 2020, 09:27:29 pm
content: Cells in parallel act as one cell.Sum the capacities, that is the capacity of the pack.By virtue of being parallel they are always balanced.Butthink about what happens in the (unlikely but not impossible) event that one of the cells fails short-circuit.If you put the cells in series, then you would be limited to the capacity of the lowest cell.

author: digsys
author_rank: Unknown
timestamp: March 25, 2020, 09:34:54 pm
content: ... and rate your charge / discharge profiles to the lowest capacity cell .. and as suggested, put say SMT polyfuses from each cell to the bus.

author: santoshgurral
author_rank: Contributor
timestamp: March 25, 2020, 09:42:12 pm
content: @sleemanjI will put fuses from each cell to the bus bar. In case of short-circuit on any cell that cell will be disconnected from the bar.Could you please brief what you mean by virtue of parallel they are always balanced.Is it that each cell will be fully discharged to its bottom irrespective of the cell3? I will be able to utilise the full capacity of all batteries irrespective of the smallest capacity cell?ThanksSantosh

author: tunk
author_rank: Super Contributor
timestamp: March 25, 2020, 09:49:40 pm
content: You should check the self discharge. If one/several of them hashigh self discharge then it will deplete the battery. Chargethe cells, wait maybe an hour and measure the voltage, then waite.g. a week and measure the voltages again.And different internal resistances may lead to current cross-flowwhen you switch the load off.Several charge/discharge cycles may slightly increase the capacity.

author: jbb
author_rank: Super Contributor
timestamp: March 25, 2020, 10:04:11 pm
content: Are cell 3 and cell 4 meant to be the same as the rest? How did you measure the capacity?If so, they are quite aged and should perhaps be thrown out.

author: santoshgurral
author_rank: Contributor
timestamp: March 25, 2020, 10:08:17 pm
content: I have cells from different old laptop battery packs hence the difference in their capacitiesI am testing cell capacities with LiitoKala Engineer Lii500 module

author: sleemanj
author_rank: Super Contributor
timestamp: March 26, 2020, 12:42:09 am
content: Quote from: santoshgurral on March 25, 2020, 09:42:12 pmThe voltage of all cells will always be equal (balanced), cell 1 voltage will always be the same as cell 2 voltage will always be the same as cell 3 voltage...You can think of it like, as one cell "discharges" the other cells "recharge it" so they are always equal.