RobbMelanon said:
[...]
Is the only way to balance before building the 100p pack? And they you don't ever balance again?
Great question -- and great thread, thanks OP.
I am currently practicing my 'tuning' skills, in building packs that are capacity matched -- precisely matched. My theory is, that if I can top balance, then discharge to bottom, and its still balanced then heh ... that would be awesome, eh?
my aim is to run 10-20kWh without any real BMS. NB I aim to monitor these packs, and top them off at ~ 3.92V (normally).
so YES, to answer your question here, I believe that if you solder/weld up a pack, with mismatched (even 1%) capacities then you will be stuck with a battery that goes out of balance....
Cells in parallel is like having two (or more) tanks of water that have their bottoms all interconnected. No matter how much water you take out of one tank, the others will flow and balance out. No matter how much you dump into one cell/tank, it will flow out into the other cells/tanks in parallel. They cannot be in any other way.
um ... no, not quite.
in my experience this very rationale expectation is only true when charge/discharge rate is relatively low. think < 0.1C ...
once the current gets up to 0.5C and certainly any more,
then the cell's relatively different internal resistances start messing with everything. IE effective capacity changes, differently for each pack, and therefore battery goes out of balance.
so my practice/idea/tuning thoughts (above) are ONLY valid with loads and charging at relatively low rates.
if the cells were all homogeneous (IE new, from same-ish batch etc) then their internal resistances would generally all be matched, so then one can go higher in the C rate before things get wonky. but I'm using harvested (used) cells exclusively, so ... tuning practice for me.
I no longer use the cell repacker on this site, as it 'only' (great job though) works with matching capacity. since I'm only making smallish batteries (1-3kWh) at the moment, which don't have enough cell in each pack (low P count) then the standard deviation on internal resistances in each one is far too high. hence me developing another way to 'tune' the construction to _try_ to match this as well as pack capacities.
*DCkiwi puts on fireproof jacket, so flame away* ; P
ps as I noticed on one of Jehu's vids, a smart guy at EV West said the trick is to have it bottom balnced, then charge it, and the battery still balanced at the top. this is the trick, Im quite convinced. its not about one, or the other, really. albeit they both can be useful in their own way..