Removing/replacing cells during maintenance in a compression fixture.

[Luk88]

I'm looking for some information about how people that use compression go about removing/replacing one cell from a battery that has been under compression for a long time.

If you've done something like this please say how long your battery has been compressed and with which method, what amount of force? Did you charge or discharge the cells before releasing compression? Have you observed any swelling? And so on (any other findings you may have).

Why am I starting this thread? I have a couple of batteries with compression fixtures. I'm planning to reconfigure one (use it's current 8S cells into a new 16S battery) and replace springs with longer ones on the other. It would be much better if I could just remove the compression force for the duration of spring replacement than trying to fabricate some brackets to hold it together.

Currently I'm thinking I'll discharge the cells almost completely beforehand. As batteries "grow slightly" when lithium goes into the graphite matrix during charging. So in theory the cell should be at its thinnest when discharged.


What do you think?

 

[frankrudis65]

Hi Luk,not having had a compressed block for a long time,here is what i have done so far.Compression i have done from the start,because i beleive that the first charge discharge cycles should be done under compression.I have read a paper,where it is stated,that the most swelling occurs at around 50% of charge! and not like everyone assumed at full charge....So i precharged every single cell,when they arrived,which gives me immediately an idea of the capacity of each cell and if there would be something wrong with the charging...When you precharge them in parallel you won't have this information....Then i put the bank under a test for several days with the bms connected.As expected the cell equalization is reduced to a minimum because of the precharging process.In this test pahase,you should be able to find out ,if there is one cell which drifts away a lot....you'll be able to check on this with the bms.If there is an obvious candidate,i would change this cell(when i buy cells,i always get a vew more).This of course means unpressing the whole bank.I have fixed my blocks with 6 bars,3 each side covered with plastic tubes and pressed with springs.This should be done,with some luck only once!Concidering those cells have an expected life time of round about 20 years,this is in my oppinium very little work....Hope this helps...Frank

 

[Steve_S]

Luke, if you bring the pack voltage down to where the cells are at 3.0Vpc that's fine, as it is below 50% and at that point essentially neutral with regards to internal pressures of the cells. Simply undo the pack and extract the cells and reconfigure into the new 16S Casing and reapply pressure. Note that it does NOT ever need to "squish" the cells, firmly snug (hand tight pressure) is all that is needed. (That subject has been beaten to death too many times, no revisit required)