| Electronics > Projects, Designs, and Technical Stuff |
| Deliberate Lithium cell overcharging |
| << < (11/11) |
| grizewald:
To avoid the problem of internal protection circuits stopping your fun, look for lithium-ion cells described as "IMR". This type is for very high discharge current applications and does not have a protection circuit included. |
| johnkenyon:
As I see it, there are two parts to this - the electronic side - getting a cell to fail (and fail quickly with lots of heat generation), and then the physical side of getting a failed cell to trigger failure in other cells. A look at the accident reports for the Boeing Dreamliner battery failure will yield information how they mitigated the risk from individual cell failure - do the opposite. I reckon that placing cells in close thermal contact with each other, restricting ventilation (to retain heat and emitted gases) and possibly generating a spark to ignite any hydrogen given off will probably give the extreme/worst case failure mode for a battery pack that would be useful for firefighting training. |
| Rerouter:
You don't have to get that crazy on the math to work out some numbers. Air has a specific heat of about 1kJ/kg e.g. how much energy it takes to heat it by 1C, Then you have a Convective Heat Transfer Coefficient of 10 to 100 W/m2K for most passive cooled devices. https://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html So your losing 5-50w of heat by convection by the time the exterior of the battery has reached 150C, tiny in the scheme of a short. http://www.inforlab-chimie.fr/doc/document_fichier_279.pdf Now a 18650 cell has a specific heat of about 80 kJ/kg, so to heat a battery up from 20C to the ignition point of 150, saying we have a 50g battery, takes at-least 520 kJ to reach that point. or just shy of about 4 minutes heated by a 2.2KW heating element. Now in the feild, the issue is generally the battery pack, essentially wrapping the product in a plastic wrap along side other cells reduces the available surface area, equally they tend to be fitted in other containers to protect the pack from impact like Fibreglass insulators, and being heated to significant temperatures over a longer time period, it leaves a lot less head room to peak to that temperature. when a fault happens, https://www.nasa.gov/sites/default/files/atoms/files/therm_runaway_test_18650_li-ion_clobato.pdf Previous testing was also done with a heater, but they did far lower power over a longer time period. |
| Navigation |
| Message Index |
| Previous page |