Yeah it’s hard to draw the line really, for example let’s say both of the redundant BMS temperature sensors fail and it doesn’t stop the battery temperature at 60C. Well then in order to continuously drain a high current from the cells in order to get them to heat you would need to:
- Have the throttle/pedal assist system (PAS) stuck on full or a high level
- Be going up a hill or somewhere that puts a high load on the motor continuously
- Have both the bike computer and motor controller temperature sensors also fail to detect the resulting high temperatures in the motor and motor wiring respectively and not cut the power.
And this is on top of the other safety features built into the battery such as individually fused cells to prevent a dead cell from shorting the others in its parallel group, or equally a dead short on the output would blow all the fuses on the cells if for some reason the BMS doesn’t cut the power first as it should, as well as enough cell-to-cell spacing to prevent a thermal runaway in one cell from propagating to its neighbors and then across the rest of the whole pack, etc.
So yes, I am relying on the BMS to do most of the heavy lifting with regards to safety (over/under current, voltage and temperature, as well as cell balancing), but there are other systems in the chain that have their own safety features, as well as built in hardware safety features, and so I think you could demonstrate that all of those combined means that you’ve done everything reasonable to ensure that this remote scenario of the cells getting dangerously hot does not happen.
There are of course a few things that would make the design even safer such as:
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Milling the cell holders out of an aluminium alloy (Al 6061T6) for heatsinking purposes, as well as preventing the holder from melting during a thermal runaway (I first discovered it in this NASA presentation https://web.archive.org/web/20220126005918/https://ec.europa.eu/jrc/sites/default/files/eric-darcy-nasa-lessons-learned-passive-thermal-runaway-propagation-resistant-designs-spacecraft-batteries.pdf)
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Incorporating a metal heatsink into the wall of the enclosure to transfer heat from inside to outside whilst still maintaining watertightness, as well as active cooling on the outside (as is seen on some electric car batteries).
However, these would significantly increase the cost and complexity of the design and would be overkill considering that no ebike batteries have those features, and a lot of them don’t even have some of the basics such as individually fused cells or watertightness. So for now I’m going to stick with plastic internals and externals since that is the norm for ebike batteries anyway.
Also, I agree it is a bit frustrating that you can’t take items with batteries over say 100Wh on a lot of transportation, so for example taking a regular bicycle on public transport such as busses, trains or planes, or having it shipped somewhere is fine, but ebikes are either banned on public transport or incur extra costs if shipped.
It’s understandable on the one hand because the transport/shipping company have no idea what the quality of the pack is and whether it was put together with some low grade nickel plated steel strips with sharp edges in China with the cells just glued together, or whether it was built with attention to detail and the intent to be safe and repairable.
It’s ironic, because having just talked about the features I’ve implemented in the design, the final product in this case would probably be put in the same category as a dodgy chinese battery for general purposes simply because it’s a one-off DIY pack that I probably can’t afford to have UL certified or similar.