The frame around the stock battery looks like it would allow a battery pack with one more layer of cells. It would be great to get a little more range out of it with something like a 17.5Ah battery for example. Anyone know or tried this yet?
I found a couple, listed below. My stock Rad battery is 4 inches deep (front to back), and these are 4.5 inches. It appears you have about half an inch to work with, so these MAY work. It will be a tight fit, and I'm not certain there will be enough workable space to take the battery off and on the bike.
The first battery is 48v and 16Ah with LG cells, and is made by Unit Pack Power which has a very good reputation. I have 3 of their batteries. The second battery is 48v and 17Ah with unknown cells, and is made by Alalito, which I'm not familiar with. Neither of these batteries fit the Rad battery mount, so you will need to install the mounting plate that comes with the battery.
https://amzn.to/3xIbcIn
https://amzn.to/3fdUKt4