Best Place To Connect Usb Charging Wire

I have an '04 Speed Triple and I just ordered a battery powered jump start device for it. It has the option to be charged while the bike is running via a USB port powered by a cigarette lighter charger, and I know the easiest way to hook up the charger is to just run it to the battery terminals. However, I'd prefer to not have the backup battery being charged by the battery while the bike is off, so I'd like to connect it to something that is only on when the bike is on.

That being said, the rectifier on my bike is actually mounted under the back seat of my bike, which makes it really easy to reach, and I'm putting in a new one in the next day or so because I suspect it has gone bad. I was thinking it might be a good idea to splice in the cigarette lighter charger on the battery side of the rectifier, since the previous connector was fried due to a short and I had to solder that side of the wiring anyway. My thought is that the device will only be receiving a charge when the engine is running, so I would avoid draining the battery when the engine is off.

The things I can think of for why it wouldn't work like I hope it will are:

1. Maybe the current would be too high for the device, though on that side of the rectifier my understanding is that the current and voltage should already be regulated to the needs of the system, or;
2. Because the rectifier goes straight to the battery, connecting an accessory to it that draws power might still drain the battery; or
3. Adding a power draw to a wire connected to the rectifier might damage the rectifier. I imagine I would need a charger that has a fuse to prevent that, but it's still something to think about.

I'm hoping someone more knowledgeable than me might be able to help me figure this out. I suppose another option is splicing it into something that is only powered when the key is turned on, but then I have to cut up perfectly good wires to splice the connection in, and I'd like to avoid that if possible.

 

That might not be a bad idea, actually. The wiring is pretty close together and easier to access than at other places. I'll have a look while I'm installing the rectifier.