Lithium batteries suffer from a number of safety concerns which limits their use in some applications. Nail penetration tests are used by the battery industry to compare the safety of different batteries during an incursion by a metallic object through a battery that physically violates the containment and locally damages the internal structure. For most lithium chemistries, such as mixed metal oxide or lithium iron phosphate cathodes, these tests result in rapid and dangerous failure. Lithium sulfur is an important next generation ultra-high energy density battery chemistry which is also inherently safer. Results are reported for nail penetration tests on 16Ah lithium sulfur batteries showing how they heat up by less than 10°C during a 10min penetration and then cool down rapidly after removal of the nail. Results of a nail penetration test under load are also reported for the first time, showing how the battery was capable of continuing to provide 1.6A (C/10) of current to an external load, with only a 1% drop in voltage. The results should be of interest for applications requiring ultra-high energy densities, improved safety, and continuous provision of power for a short period after damage, particularly military, aviation, portable electronics and automotive industries.