Ditto. If the connection is poor, there will be high resistance, whcih causes heating under load.
There's a number of good ways to splice the wires together that won't have this result. Here's one instructable, which itself has other ways in it's comments section
http://www.instructables.com/id/Master- ... everytime/
and google searches will find others.
My favorite splice for thick multistrand cables is to
--cleanly cut the wires to be spliced
--slide a 2" or longer piece of heatshrink over each of the wires, and leave them pushed back as far as possible from the splice point.
--strip the insulation off a short distance (1/2" to 1" depending on what I'm splicing--bigger it is the farther back I strip)
--slightly spread the individual strands apart, like the end of a broom
--insert one set of strands into the other, so they are interleaved.
If i have a crimper that will do the job, I'll crimp the strands together to make a solid connection.
If I don't, I'll twist the strands so they are pressed together as well as possible, then tightly wrap a single strand of copper from some other cable (cat-5 network stuff works well) around the splice point to hold the strands together.
Then either way, I'll solder them.
Then I'll wait for it to cool, slide one of the two heatshrink sections over the splice, and heat that to shrink it. Then wait for it to cool and repeat with the other one over the top of that one.
Additionally, if I have multiple wires in one cable to splice (like 3 phase wires), I'll offset each cut so that none of the splices overlap each other. Then even if the splice insulation fails tehy can't short to each other.