Diy 18650 pack issues

[charlen10]

Hi all,

I have assembled my firts 18650 pack, however I am facing some issues with it. Before assembling it, I measured all the cells voltage and they were around 4.12V, however, after having connected all the batteries in a 4s3p configuration, when I measure the pack voltage it is at 14.2V and the cells are at 3.56V and the BMS is therefore cutting the battery. If I measure the voltage of the parallel packs(from possitive to negative on each cell), the voltage measured is again 4.12V.

Any idea of what is happening here? Is there any problem in the connections or I just have to charge the complete pack? Since the individual cells were fully charged, I am afraid of over charging it.

Regards

Carlos

 

[Diysolar123]

lifepo4 cells quickly settle in voltage from their max of 3.65v once charging is complete because the 99% charge voltage is around 3.4V/cell!
This is why when it comes to lifepo4 cells voltage only tells you when it is all full, and mostly empty.

you state that you measured "all the cells voltage" as 4.12V...if you mean individual cells, that would be very bad (unless you do not have lifepo4)!
in a 4s3p, just to make sure, you have four batteries wired in series...and you have THREE of those that are connected to a positive and negative busbar/connector???right??

so its something like this (4S3P):


OR...
are you putting 3 cells in parallel (3P) and then stringing 4 of those together 3P4S?


FYI, the voltage of EACH cell should never exceed 3.65V which is what your BMS is doing while it watches cell voltages.
the voltage of cells in parallel will be the same. the voltage between different parallel cells may be different.

for example the 3P4S configuration means that the voltage of cells IN a parallel group will be the same, however, the voltage of EACH parallel group may be different!!

in each case this is why having a BMS is critical because the parallel groups can charge at different rates and their voltages will drift based on each parallel group.
consider a charge voltage of 3.65v * 4 = 14.6V... so you would think that charging at 14.6V would fully charge all your cells which sadly is not the case depending on the state of each group of parallel cells.

think of all the way the cell voltages can add up to 14.6v:



if you have a BMS protecting your cells no need to worry, if you do not you need to be very careful about overcharging any cell (or group of cells).