Generally what happens is that one or more of the cells inside the battery ... the 18650 will be damaged if over-discharged, which is again a Bad Thing. It's worth noting that there are "protected ...

[Andreas] wanted to use protected 18650 cells, but didn’t want to ... [Andreas] found that battery holders with a coiled spring would accommodate the extra length, but those with metal leaf ...

Accommodating three 18650, 3.7V li-ion rechargeable batteries, the BK-18650-PC6 battery holder targets consumer, industrial, and communication products. Accommodating three 18650, 3.7V li-ion ...

The company’s latest SMT and THM battery holders support li-ion 18650 cells. The holders feature a low profile, heat-resistant Nylon housings, and gold-plated phosphor bronze contacts.