These are bootlace ferrules (front row non-insulated ferrules, middle row twin insulated ferrules and the back row are insulated ferrules):
Ferrules are used to crimp stranded wire ends and improve electrical connections. They also protect the stranded wire ends from damage, if it is screwed down (like when installing them into a Victron charger).
They come in insulated and non-insulated forms. Both are useful. The Doc has a range of sizes as they must be matched to the wire ends you are crimping.
Twin bootlace ferules
Twin bootlace ferrules allow you to insert and crimp two separate wires into the ferrule. The Doc has only seen them come in insulated ferrules. The twin ferrules have the broader bases in the middle row of the above picture.
Buy non-insulated ferrules with a bell housing
If buying non-insulated ferrules, a bell housing at one end makes life simpler – it is easier to insert the thin copper strands. The insulation also acts as a bell housing on insulated ferrules.
The bell housing is the bell shaped end you see above on the non-insulated ferules (the bottom end of the ferrule).
Copper or brass ferrules?
The best bootlace ferules are copper with tin coating, not brass with tin coating. Copper is a better electrical conductor, but copper ferrules do cost more.
The tin coating reduces corrosion, especially in wet environments.
Buy a 4-sided or 6-sided self-adjusting crimping dye?
Self-adjusting bootlace ferrule crimpers come in 4 or 6 sides. Depending on the intended use, The Doc may use a 4 or 6-sided crimper.
NOTE: The Doc has never seen bootlace ferrule crimps as smooth and uniform as shown in those 2 pictures!
Broadly speaking, The Doc has found the 6 sided or hex crimper gives a better crimp, as the 4 sided dye crushes the ferrule on the 4 corners. If limited to one crimper, The Doc would buy a 6-sided crimper over the 4-sided crimper.
Tip: however, 6-sided crimpers are good up to 6mm diameter ferrules, any bigger and you will probably need a 4-sided crimper. Eventually The Doc had to buy both styles.
If you have quick-change style crimper like the S&G Tool Aid 18960, see if a dye is suitable for bootlace ferules. These dyes will crimp the bootlace ferrules differently to the 4 or 6-sided self-adjusting crimpers. These crimpers have a different profile for bootlace ferrules, which is neither 4-sdied or 6-sided, they usually look like this:
What ferrule crimper to buy?
Work out what you need to crimp, look at the wire gauge you need to crimp, then work out what ferrules and self-adjusting crimper you need to buy.
WORD OF WARNING: sometimes you need to finesse the ferrule dye shape and size to get them to fit the device, like a Victron charger. It is not always straight forward. Bootlace ferrules are not as simple as terminal crimps or Anderson plugs.
Buying bootlace ferrules or crimpers
Wiha and Knippex make professional grade bootlace ferrule crimpers, with a very expensive price tag to match. The Doc prefers Taiwanese crimpers, to Chinese made ones, on eBay or Amazon. In Australia owners can buy quality crimpers and ferrules from Rhino Tools here and crimps here.
Why are they called bootlace ferrules?
They were originally used to crimp ferrules onto boot laces. Another use for your crimper! The Doc makes his own boot laces.