DD Contact: To use in between two batteries on an 18650 tube and DoubleDouble. also for use with an 18650 DDiesel top (contact should be placed on top of the battery). **Copper contact included with each 18650 DD purchase.
XL Donut Contact: To use with 20700/21700 size tubes ONLY when you're using an 18650/20650 battery (instead of a 21700/20700 battery). Also needed for 21700 stacked devices ONLY if you're using smaller batteries.
Magnum Donut Contact: Also referred to as 'Thick' or '21700'. To use in between two batteries on a 21700 tube and DoubleDouble. also for use with a 21700 DDiesel top (contact should be placed on TOP of the battery). **Copper contact included with each 21700 DD purchase.