The battery doubles the meager range of the C21. Unfortunately it fixes to the frame on the only bottle mounts, so no longer space for a drink!
Works well once you have figured it out. There is an installation video, but no instructions on how to charge the battery whilst on the bike or how to use it alongside the main battery.
It turns out that each battery needs to be charged separately and then used one at a time, switching over only when one battery is empty. You loose the trip mileage in the process.
Could be better!

È un acquisto indispensabile.
Quando torno a casa ho sempre una batteria carica per riutilizzare subito la bici oppure quando faccio giri lunghi per sicurezza mi porto quella di scorta nello zaino

I'm delighted with my bike: it looks great, light, very easy to ride even without support due to excellent transmission. Quickly charging battery lasts for over 45 m with 5 different levels of support. With external battery I can ride almost 100 miles! Incredible value for money! I'm 71 and riding became a joy again! Don't hesitate - buy it! You'll love it, too! George

All works very well. Like the usb port for my phone and the torch could be handy.

Excellent service - prompt and efficient! Thank you!