friedpenguin said:
So it comes down to how many 'p' you're comfortable with and what your BMS can do? So a 24v 40A is probably good for 7s4p max? If you fuse each cell and one cell dies what happens to the other cells in that series? I'd like to be able to build a hot swap 7s setup and fuse each 7s1p so if one goes south have a way of notifying and replace just the 7s bank. That's outside the scope of the BMS probably but I could find another solution to monitor the voltage of each 7s that's hopefully affordable.
you're getting your P's and S's mixed up.
1/ if one cell dies, either the fuse blows, or it doesn't (wrong spec of fuse!). IFF the fuse blows, then NOTHING happens to the other cells in series (or parallel). BUT your pack is now swinging out of balance. each group of cells (thats 1s) has a capacity ... and unless the battery was built with too much capacity in that group of cells (unlikely) then things are going to go south ... sooner or later. IFF the fuse doesn't blow, the every other cell in parallel discharge to the failed cell ... then on the next discharge of the battery -- without a BMS -- these cells flip polarity and things get interesting (that'd be the bad type of interesting) real fast. Of course, ACTIVE balancing, or even super smart PASSIVE balancing (including Batrium, if I'm not mistaken) can compensate for this, but then check the logs ... you are losing power to a problem pack ... fix it or ... lose efficiency.
2/ making a battery out of strings of 1p packs is ... very very bad practice. ... like AveRageJoe said to his dad ... 'please don't do that'.
a couple of other things ... cell vs battery -- they are different. words have meanings. suggest you take more notice of what you are saying (genuine/sincere advice here, not picking on you). I know it takes time to learn, but you seem to already have a good grasp of relevant stuff here, so ... best to use the right words, and speak with clarity.
as for individual cell monitoring in a battery ... its a LOT of wires to monitor each cell. Im sorta working on an project to do such a thing, but it would ONLY EVER be used for 'breaking in a battery' (which I do BEFORE battery construction! I balance charge each prospective cell for the battery to a very precise voltage. they can come from different stock/caches or existing cartridge style batteries I have in production use .. I shuffle them, just like I stack 'em ... thus eliminating 99% of 'problem cells' before final battery construction).
To monitor each cell (pair, in my case!) in a
real battery (5+ KWh) composed of little 18650's is ... non-trivial. how big (capacity) are you building? I use 72p for my big builds, that 36 pairs ... in a 13s config ... that'd be 468 channels for an ADC or discrete op-amps to process ... totally non-trivial. and thats not even a 'big' battery! just a string ... the clue I'll give you is this: use the cell-level fuses as shunts ... usingdifferential op-amp's, one could put one input on the busbar (easy!) and the other input on the cell's side of the fuse (lots of wires, and also ... it's generally really hard to actually wire anything to the cell side of the fuse, eh?! unless you use cartridge style, which I do often, and always do for 'breaking in' a battery before construction...).
I admire your enthusiasm friedpenguin. Welcome to the 'board (SLS). Thanks for sharing your Q's. ; ) hope this helped ...
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EDIT: I use string/battery interchangeably, as they are identical, unless. ... (segway, another hint follows!)
you join each interconnect between groups of cells, between each string ... to be clear, what I talking about here is the idea of joining (connecting/shorting) the interconnects of each string, IE then the PACKS are paralleled not just the strings). then things get very exciting. there is a now an easy place to measure performance of each 'pack' in each string of the battery (monitor the current flows between this interconnect of interconnects). so in theory one could isolate a problem 72p (in my case) to investigate, as opposed to 'something in the string' or whatever.
the problem comes IMHO, from the reality that
no-one wants to or can harvest every cell then make their battery. its incremental, kinda organic. it grows. so the question in my mind is how to cope/manage
that. scalability has many many many aspects.