For what it's worth, it looks like you can't charge the external battery through the frame port if the internal battery isn't mounted, so you'll need to disconnect the external battery and plug it straight into the charger every time you want to charge. Not a huge deal, but annoying all the same.
This is awesome to hear, about to install my 400. Nice to be able to charge one while riding, or to have a lighter bike rocking the 400 on short days. As far as battery drain, guessing the system pulls from whatever battery has more juice left first, then will discharge evenly after?
Nice! Also, I have not found any kind of install guides or instructions online. I would prefer to do it myself if possible. Do you have to drop the motor to access the wiring harness? Any tips would be great!