takyka said:
Lot's of false information here!
Fuse wire is usable in low P high(ish) current per cell application too!
I have a 6S4P pack where the cell connections are 0.3mm dia. copper wires. I regularly discharge the pack with 40A (10A/cell) without any issue. The fuse wire resistance is ~5% of the cell's internal resistance only, so the extra voltage drop is negligible. During testing, a dead short blown four parallel fuses in fraction of a second.
I don't see any reason, this could not be scaled up to 20A/cell, as high energy cells having less IR (than my Pana 16850PFs).
The higher short circuit current could blown a thicker fuse wire without any issue.
10A/cell is not high current for 18650, so your user case is not applicable.
- A 10mm length of Ø0.3mm copper wire has resistance of ~2.38mOhm.
- At 10A each wire converts 0.24W into heat (@141A/mm^2).
- At 20A each wire converts 0.95W into heat (@283A/mm^2).
- Anything over 40A/mm^2 would be considered "hot" for copper.
Regardless, you've missed the point of cell fusing entirely anyway. Cells don't just instantly fail in short-circuit. It can happen from extreme electrical or physical abuse, but then you probably have bigger problems to sort out. Your short circuit "test" isn't really a real world situation for 99.9% of user cases. Almost all pack fires that I have heard about are caused during charging.
Bad cells usually just degrade over time (i.e. self discharge, whether due to dentritic crystal growth or other reasons), and the sagging voltage puts stress on the surrounding cells, which silently kills the pack. In large P groups, the cell fuses can blow at relatively low delta V (nowhere near a dead short). This situation is ideal for large sealed packs with hundreds of P cells (think large EVs), because it automatically isolates bad cells before too much damage is done, and allows the pack to remain servicable without intervention.
For relatively low P count packs in ebikes, fuse wires make far less sense.
If you're worried about a cell spontaneously going dead short, you've got much bigger issues, and a cell fuse isn't going to stop the fireball.
And did you solder the copper to the cell ?