Well the first setback occurred, and boy was it spectacular... I wish I had a video of it.

So what happened? 

My very expensive Sabertooth 2x60 shorted out and caught fire. And not just like a bunch of sparks, but literally caught fire to where I started to run and grab my fire extinguisher. But luckily at the last second the fuse tripped and stopped it from getting out of control.

Where did I go wrong?

Well I'm still not 100% sure, but maybe some folks here have an idea. First of all, I don't think it was an issue with the Sabertooth controller, because it happened after I made some changes to wiring, so it's more than likely 100% my fault.

What were the changes I made to the wiring?

One of the things I've been wanting to do is to be able to monitor my voltage remotely from my radio transmitter so I can start to calculate how much battery life I'm going to get out of Lawnny in real working conditions. My Spektrum receiver has a voltage sensor port which it uses to send telemetry to the transmitter, so this seemed like a perfect solution.

Spektrum sells a battery sensor cable for $12, but after looking online I found that it's just a standard JST ZH connector and I could buy 12 of them for $8. Seemed like a no-brainer.

From everything I could read online, this voltage sensor port is capable of handling up to 24v, so it seemed like I should just be able to hook it up in parallel to the battery and be done with it. The closest and easiest place I could hook it up was at the battery positive and negative terminals on the motor controller.

Now I wasn't fully sure if the JST ZH connectors I ordered were properly colored for the positive and negative inputs on the receiver, but I figured in a worst case scenario that I would just get a negative voltage reading instead of a positive one and would have to swap the wires around. So rather than clamping the wires into the terminals I figured I would just hold them against the terminals by hand like I would my multimeter. This may have been the fatal flaw...

When I switched on the power I immediately saw huge sparks and a fireball surrounding the motor controller, followed by a scream of "Noooooooooooo!!!" (I think that was me). I immediately removed the wires, but it kept sparking and shooting flames. After what seemed like a long time (even though it was probably only 5-10 seconds), I started to run for the fire extinguisher, and a moment later it stopped. I suspect that's when the 60 amp fuse finally popped.

So many questions...

Did I hook up the voltage sensor backwards? This is my working theory, but I'm still not sure why that would have caused the motor controller to go up in a blaze— if I hook up my multimeter backwards, it doesn't cause things to blow up. If anything would have gone up in smoke my guess is that it should have been the receiver. 

The only thing I can think of is that maybe the BEC circuitry in the motor controller (which powers the receiver) shares a common ground with the high power motor ground. By hooking it up backwards I suppose I created a short across the motor controller, and the reason this is different than hooking up my handheld multimeter backwards is that my handheld doesn't share a common ground with the battery and controller. What do you all think?

The second question: what took the fuse so damn long to blow? The fuse for the motor controller is this: https://www.amazon.com/dp/B08LYG7ZYF.

Did this short draw less than 60 amps? I doubt it— the fireball I witnessed seems to beg to differ.

Maybe the fuse is more of a slow-blow type? Maybe it's just a cheaply made Chinese part and doesn't work well? Who knows, but I don't think I can trust it and will need to find a better replacement.

Anyway, it was a setback, but not worth crying over too much. Within 10 minutes of the accident I painfully shelled over another $200 for a new motor controller, and figured it'd be a good time to take Lawnny completely apart for a coat of paint on the bare steel to prevent rust. I'm still not sure if the receiver also got fried, but it looks okay— we'll find out when I fire him up next (maybe fire isn't the best word to use).