Comrade said:
There are plenty of pretty manual switches rated for hundreds of amps. They are called battery disconnects.
Very true. They do tend to be larger and bulkier though, leading you to not be able to mount it on the handlebar. Leading to your next point...
Comrade said:
What's the benefit of turning it on from the handlebar though? Versus flipping a physical switch somewhere else.
Also very true. And that's where the DIY comes in. You don't have to turn it on from the handlebar if you're building it yourself, you can do whatever you want. Battery disconnects are easy enough to mount on the side of an e-moto, just reach down when you're ready to turn it on. I've seen several like that when I was researching, I think the youtube channel RatherBWelding did that on his recent build.
The benefit of a contactor coil is that your on switch for the coil can be small, and in most cases, people will choose to turn on the coil using the ignition key. Turning the key closes the contactor. In fact, I was lucky enough to have the original ignition and keys from my 1984 honda motorcycle when I got it. Lucky because it was one key that not only turned the ignition switch, but also locked both the seat and front forks, and I love a good analog backup if I'm nervous about where I'm parking my bike. Ignition switches are convenient, common, good for security because they're keyed. And certainly not rated for hundreds of amps. For a motorcycle or ebike, sure, you can mount a heavy duty battery disconnect anywhere and easily reach it, but contactors really come in handy once you start building golf carts, forklifts, and electric cars, thinks that you really want to keep a keyed ignition switch for.
There's a little bit of a safety issue coming into play as well: if your hand is the thing that's physically touching the switch to connect and disconnect the power from battery to controller, you want to make sure it's totally safe. When that switch is throwing 24 or 48 volts at 30-40amps on a smaller ebike, your hand is not in much danger if the switch is underrated for it's application. On the other end of the spectrum, an electric car battery could be 200-400V. You don't want that level switch to be anywhere near the steering column, in case of failure. A contactor coil is simply a relay, using 12v or whatever to switch something much bigger. In between these two examples is the spectrum of what battery voltage you feel comfortable switching with your hand. Personally, for my setup, my 72v contactor actually uses a 72v coil as well. This means my ignition, rated for 12v, is switching the 72v coil on and off. I've chosen to take this risk, because these older ignition switches use quite a bit more copper than the newer ones, and I've also put a 1A fuse on that line, so if there is a failure, the fuse will blow before the switch welds itself shut or sends current through the key into my hand.
But hey, you're 100% correct. A battery disconnect switch could easily be used for on-off between battery and controller, if that's what you want and you are able to mount it on a spot that you like. And if you still want to have something keyed for security purposes, most motor controllers require an on-off switch to be wired, so there's no reason that you can't buy a cheap $10 SPST keyed ignition switch for that purpose.