Despite popular misconceptions, the PRIMARY PURPOSE OF A SOLDERING FLUX is to provide a barrier between the hot metal and the oxygen in the air so it doesn't oxidize, a process that happens in less than a second at elevated temperatures. The most common fluxes today are resin. The resin melts long before the surfaces reach the melting point of the solder and coats the surfaces without itself oxidizing or burning to create contaminants. After the junction cools, the resin solidifies back to a neutral solid on the outer surface since its surface tension is far less than the surface tension of the molten solder.

All electrical soldering should be done between surfaces that are already "tinned" or coated with solder. If the item to be soldered is not tinned, then this should be done before attempting to make a solder joint. If the tinned surface has been exposed for an extended time it may have oxidized however the tin/lead metals in solder are much more stable than copper so the oxidation is much less severe. When you make the solder joint, this oxidation is on the surface of the solder which is going to melt and so it is mechanically released. The flux then floats this contaminant away from the surface allowing intimate contact between the solder and the base metals. This does not happen on an oxidized copper surface because the copper surface does not melt to dislodge the oxidized components.

There are "active" fluxes that are designed to reduce or etch existing oxidation to provide a clean surface for soldering. They are intended for use on surfaces that have not been tinned and where mechanical cleaning is inadequate. These fluxes should never be used for electrical or electronic work. If your electrical cable is old and oxidized, throw it away - you will NEVER be able to clean it enough to solder, EVEN USING AN ACTIVE FLUX, which you shouldn't.

When assembling a battery cable termination I use the following proceedure. You should not rely on the flux in a resin cored soler to provide the protection you need for a joint this size because much of the metal will reach oxidizing temperatures before the solder melts to release the flux. For electrical work a SN60PB40 solder or thereabouts is an excellent choice since it has a lower melting point which helps with large amounts of copper to heat up.

Start with new, tinned, marine grade cable and clean tinned terminals. If the terminals are not tinned then you need to clean them inside thoroughly, (steel wool is fine) apply a thin coat of NON ACID, NON "ACTIVE" electronic style flux and apply a coat of solder on the inside. Kester makes a number of suitable flux products. If they are clean then the solder should wet the whole surface rapidly when you get it to the right temperature, preferably using a large electrical iron (eg 180 watts or more for big terminals - 60 watts for smaller). You don't need any build-up, just enough to turn the surface silver.

Coat the inside of the terminal barrel with flux and dip the end of the cable in the flux. It doesn't take much, just enough to coat all surfaces.

Assemble them and crimp, prefably with a tool designed for the terminal in use.

I then support the terminal vertically in a vice or vice-grips with the cable vertical up. Use a couple of pieces of wood to thermally insulate the terminal from the vice so you can heat it. Apply heat, again either with a large iron, or sometimes you will need a small blow torch. Apply and allow the solder to melt and flow in until it either fills the cup, or it flows out the vent hole if there is one. Allow to cool and solifify before moving. If you allow it to move before it solidifies, you will have to reheat it until it all melts and try again. A touch of paste flux on the junction will help reduce oxidation if you have to reheat it.

Wipe off any loose flux, clean with steel or bronze wool if you like, and then put a light coat of electrical grease on it. I forget the name of it but most electrical supply houses carry it for housing installations. Then put a piece of heat shrink tubing over the barrel and at least an inch of the insulation and shrink it into place. A little silicone caulking under the HS tubing will keep it watertight.