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[electrictwowheeler]

Even though it's boat cable, which has a higher rating than standard building cable, 80 amps is pushing it, unless it's a fairly short run, like a few feet jump to a starter or only intermittent use.  otherwise it's typically rated at around 60, would be derated to around 50 or so in an engine compartment due to temperatures being hotter. Given it's boat cable, as you claim, it is being rated using marine standards ABYC instead of normal NEC standards for buildings.  nec would rate it only around 30 amps for open 10ga wire.

 if you have it up under cowling or in a box, might want to keep that in mind but I don't think that'll really be an issue for you, you won't be seeing those kinds of amps very often at all.  No need for switches and stuff, just more stuff to add resistance and corrode on you.  Tinned wire is a good thing, fine wire is even better, its easier to work with.  personally i love using welding cable when i can get away with it. 

it's up to you but a simple Anderson connection might be a quick and easy way to connect / disconnect it, and you can even put the fuse inline with it.  that way you are not playing with nuts and bolts, stretching threads and up in that stuff every time you want to use it.  an Anderson would also protect the disconnected ends too from most accidental shorting.

the lg batteries tend to be one of the higher grade batteries on the market, so you are good there, and yes, charging, most manu's prefer 0.5 to 1 C but will allow a bit more safely.  10A - 4C discharge, yah that right in range with a standard grade battery,

another place to look for cheap batteries, sometimes you get good finds, is portable tools, people hunk the pack because one cell dies and it kicks the whole pack but the rest of the cells are usable.  home depot, lowes, or placesl ike that often have free battery disposal, and a quick talk to the manager, many times they are more than happy to give the old ones for free, because they have to otherwise pay to dispose generally.  depending on what brand, size etc, the cells vary but you can build plenty of 'aux power' for essentially free.

aaron

Sorry, I misstated the ampacity of the cable. It's as you say, 60 amps. Remember that the max current draw was 35 amps WOT in sport mode, but in the custom mode I always use, it is only 20 amps max. Of course I'm using an Anderson connector. The SBS75X connector. I use 2 of the small pins to connect the power to the contactor so plugging it in is just one operation. No mistakes. There is plenty of air under the fairing where the wires run for battery cooling. The fairing is open in the front and there is a scoop down low that directs air to the controller and up through the battery so plenty of cooling for the wiring but the compartment that the battery is actually in is not ventilated. I wondered if this would be a heat problem, but I rode 100 miles today and the only thing that was warm at all was the contactor coil. We will see what happens this weekend with 95-degree weather coming. I could always punch a couple of holes in the box and install a 12v computer fan, but I don't really think I'll need it

[Specter]

not necessary,  with that rating of wire,  too hot, is literally too hot to touch.  you'll be fine.
aaron

[ESokoloff]

I rode 100 miles today and the only thing that was warm at all was the contactor coil.

First off, congrats on your achievement of extending your range!!

You could eliminate the extenders contactor & instead just use a switch (just make sure it’s rated for the load & voltage @ DC).

Just a thought… maybe use a high temp wire as a resistive conductor & parallel, the existing 4’ cable? (The small gauge & long length adds resistance).
As suggested before, this would be the #1 on a rotary/cam switch (OFF-#1-#2)   
A small gauge high temp wire sized correctly could offer battery balance.
You could experiment with very light gage standard copper wire until the wire doesn’t vaporize @ the greatest Delta V (incrementally increase wire size until you find a sweet spot/size).
Or as suggested before, use an incandescent light bulb (MR-16?) as the resister.

The main thing is you produced a functional device that you can tweak out the fine details. 

[ESokoloff]

…..
Ampacity is defined as the maximum load a wire can handle continuously and stay within temperature specs.
……..
 I have thought about installing a smaller fuse. That would be easy and now I know what the actual current flow is.
I would be more inclined to use a fast acting fuse designed to protect sensitive electronics.
………

Without looking it up, I think the current rating of a wire size would be its ability to carry it within an acceptable voltage drop.
Voltage drop is a result of consumption which in the case of insufficient conductor circular mills over temps the conductor.

Regarding fuse size, suggest you try small value fuse sizes incrementally until you find a size that holds during a torture test run.
Next try that size fuse @ different Delta V’s & see how that goes.
Up size it a bit for reliability.
If you find a non time-delay fuse that’s works then great.

Here is an explanation of time-delay duel element fuses.

https://youtu.be/f6eVEHBctYM?is=IGhEK0lnIxnoU3ok

[Specter]

@electricwowheeler - whatever you end up doing, please be careful, and always, do a bit of your own research when you can,  try to verify or validate advice that is given to you, that it's good, don't blindly take 'advice' from people as gods rule or being safe, as some often speak of things they understand little about.  Try to understand not only what you are doing, but also preferably why you are doing it that way as well.  This greatly helps you truly learn from your projects but more importantly, keeps you from following bad advice and harming yourself or ending up with a btfu project.

goofing up and letting the magic smoke out of the wires, because you decided to hop on the clown car is one thing, but letting the magic out that keeps your heart beating, because you come across high voltage, is not acceptable, yet almost always, totally preventable.

parts are not cheap either,  why burn up 50 fuses at a buck or two a pop playing whack a mole, when you can get reliable data from one of many verifiable accurate engineering sources and build your project properly, the first time, with maybe only one or two minor adjustments.

technically anything over 50 volts is considered dangerous.  many will debate this, but, just, please, be careful when playing with electricity. it doesn't forget and doesn't cut you a break when you do wrong.

aaron