Are battery ring terminals and the like really necessary? It seems like twisted copper and a couple of flat washers should do the job just fine.

Please crucify me now. 

Crucified..... Cable termination is a weak point  in terms of current flow. If  you have a 35mm2  cable for instance that now has to conduct through a flat surface which has less than ½ the cross-sectional area of the cable.  When aluminium cabling was common aluminium lugs were a weak point due to the formation of Al2O3. A couple of washers is a bad idea.

Heya.

I watch a fair amount of solar youtube videos. Especially in the USA. It seams that the lugs are filled with solder to fill the gaps?

Jay

10 hours ago, Energy said:

. It seams that the lugs are filled with solder to fill the gaps?

Not solder; solder seeps into the copper and makes an abrupt stiff to flexible interface that raises stress. Plus, soldering 35 mm^2 cable is nearly impossible. 

Use a compound to exclude the air and water from the crimp joint. The idea is that it squeezes out of the junction and only fills the gaps. It is mildly conductive, but of course nothing like the conductivity of copper. So don't use excessive amounts.

Proper crimped lugs are essential for anything over about five amps. 

6 hours ago, Coulomb said:

Proper crimped lugs are essential for anything over about five amps. 

Yup. When working with thinner wires and low-current things I prefer soldering. It's mechanically stronger than just twisting things together, and much neater when you also seal it up with some heat-shrink insulation. But the moment it gets a bit thicker, about house-wiring level, then you got to use terminals, crimps, or other methods, even if those of us who (for reasons I still don't understand) received training in doing cleat wiring were taught* to solder the joints, though I cannot for the life of me remember when you're allowed to make joints in the first place :-)

* High school, which for me was a hybrid Technical/Academic affair, instead of useless subjects like... you know... BIOLOGY... we did other technical things like cars, electrical stuff, metal work and technical drawings :-) I was in grade 8 when we did that course, it was basic electrical work, consisted mostly of some math, wiring a three-prong plug, and solder work.

11 hours ago, Coulomb said:

Use a compound to exclude the air and water from the crimp joint. The idea is that it squeezes out of the junction and only fills the gaps.

Would this be appropriate?

Carplan/Carlube Mullti-Purpose Grease Copper 20g

https://www.lazada.com.ph/products/carplancarlube-mullti-purpose-grease-copper-20g-i120057417-s124749710.html?spm=a2o4l.searchlist.list.7.25027cccXRCKYf&search=1

 

11 hours ago, stuvo said:

Would this be appropriate?

I would not. It's not designed for electrical joints.

Something like this (seems expensive, you can likely do much better):

https://www.wantitall.co.za/industrial/burndy-p8a-oxide-inhibiting-joint-compounds-penetrox-a-8-oz-container-size-squeeze-bottle-container-__b008klx2ry

7 hours ago, Coulomb said:

seems expensive

wantitall is an import site for foreign stuff. You only buy from them if you really really really want it. They are not cheap.