A couple of things here.
1) As Thomas mentioned, a 50' cord is 100 feet of wire, 50 foot there and 50 feet back.
2) "The current" is NOT being delivered. The current is being wasted as heat (power loss).
To make the math easy to understand assume that the battery is a 1 Ohm load and the solar panel
is delivering 12 V. Put 12 volts across a 1 ohm load = 12 amps. I = E/R. Now add .1 ohm resistance in the wire as loss (just an example). The wire is placed in series with the original load and so the resulting load is 1.1 ohms. 12V / 1.1 Ohm = 10.909 amps. You have just "lost" .9 amps. Where did it go? To heat loss.
3) And finally solder is an excellent conductor, assuming you are not using plumbing solder.
Solder is a tin (or silver) / lead mix.
The flux in electrical
solders help the wire surfaces to "wet" with the solder and form an immaculate electrical
connection. Solder also absolutely 100% prevents poor connections in the crimp since there is no crimp. Poor connections lose power in... high resistance... which causes... heat.
I had an electrician come in to look at my power panel in my home. The lights
would dim when stuff turned on around the house. It turns out that every screw in the panel (to each individual breaker) had loosened over the years. The main power cable from the power company is a massive thing which came into a massive screw. It too was loose. The black plastic block around the screw was MELTED from the heat generated from the poor connection.
Obviously this is an extreme example, but it amply demonstrates the issue behind the scenes. Crimps will never be as good as a soldered connection. They will ALWAYS corrode over time. Heat causes them to flex. Flexing causes the connection to slowly deteriorate. Poor connection causes resistance which causes heat (which is power loss), which causes more flexing causing worse connection and so a vicious cycle ensues. A poor connection also allows oxidation of the wire surface (corrosion). Corrosion causes resistance, which causes heat, which causes (is) power loss which feeds back into that vicious cycle.