Lithium Battery Safety notes
P. McGill
21Oct13

* Dorado AUV batteries are comprised of modules made by Inspired Energy, part number PH2059MB. These were customized for MBARI according to Hans Thomas' wishes.

* The Inspired Energy modules have an 8S1P arrangement of Panasonic NCR18650 cells, each 2900 mAh. There are 55 modules in one Benthos sphere, and up to two spheres on the AUV.

* LRAUV battery is also made of Inspired Energy modules, but of lower-capacity 2200 mAh Panasonic 18650 cells.

* Paper "Simulation of abuse tolerance of lithium-ion battery packs" [Spotnitz 2007] shows that whether or not a pack goes into thermal runaway when one of its cells overheats depends how much heat is released by the failed cell and the thermal conductivity of the battery pack housing.

* Paper "An alternative cooling system to enhance the safety of Li-ion battery packs" [Kizilel 2009] tested a paraffin/graphite composite phase-change material to absorb the heat of a runaway cell. The PCM matrix is a graphite sponge saturated with wax. They claim it's much more effective than air cooling. They found that better thermal conductivity between cells keeps a hot cell from reaching critical runaway temperatures. A 1- or 2-mm nylon spacer between air-cooled cells also prevented thermal runaway propagation, but only if sufficient heat removal was provided by the air cooling.

* The original goals of project 901215, Lithium Battery Safety Guidelines, are:
"We plan to determine how much gas is released when a typical cell fails, and how much heat energy is directed at adjacent cells. We will also attempt to determine how much heat the cells can sustain before they fail, and if there are ways to minimize the probability of cell failure in cascade. Finally, we will investigate how a typical pressure vessel is affected by a battery failure, and how it could be successfully vented to release internal pressure."

The deliverables are:
"The product of this research will be a set of recommendations to MBARI Management regarding design principles and safety measures that will increase the safety of large lithium battery packs used at MBARI. This will take the form of a report to the MBARI Safety Officer outlining the tests that were performed, and making recommendations regarding:
• Best industry practice for battery control electronics
• Pack design to reduce chance of failure propagation
• Pressure housing and vent design to reduce chance of explosion
• Storage and handling practices."

* My calculations show that 1 to 2 liters of gas are produced when an 18650 fails. A LiCoO2 cell reaches 262 psig in a 116 cc calorimeter, so we have (262 / 14.7) x 116 = 2.07 liters. Based on the maximum dP/dt of 15122 psi/min, or 252 psi/sec, the gas generation takes more than a second and likely several seconds.

* Will the particulates in the evolved gas prevent the pressure relief valve from sealing again? Is a particulate filter needed?

* NCR18650 testing:
- At 1C continuous discharge, I see about 3.3 Ah of capacity.
- Discharging at 1C for 6 min then resting (no current) for 15, I see 3142 mAh. Final Voc is 3.32 V.

* Driving cells to thermal runaway:
- 2 ft of 24 AWG nichrome wire has a resistance of 3.5 ohms at 315 degC. It will output 30 W of heat when driven by 10.2 V at 2.9 A. McMaster-Carr 400 ft spool is p/n 8880K82.
- The nichrome must be wrapped in heat-resistant insulation to prevent it from shorting against the 18650 cell's case. We can use fiberglass sleeving from Omega Engineering, p/n FBGS-N-24-25. The fiberglass sleeving is sized so "24 gage" sleeving fits a 24 AWG wire.
- We must measure up to 800 degC. An Omega K-type 0.38 mm thermocouple is good to 871 degC. Part number CHAL-015-BW has 12"-long 0.015"-dia bare wire leads and is $22 per pack of 5. The response time in still air is 10 sec, but should be much shorter for metal-to-metal conduction.
- Dataq sells thermocouple modules for their data recorders. DI-8B47K-04 is for K-type thermocouples, over the range 0 to 1000 degC, with +/- 2.4 degC accuracy. The module costs $113.