Here's the info from the Alien Motion website:
https://www.alienmotion.com/faq.html
"Just as you can not use any charger with any lead acid battery, you also need to be aware of what you are putting into your Alien Motion battery as well. To achieve a full charge, our AM4 battery will need to be charged at 14.4V DC at a maximum of 10 amps, the AM8 and AM12 will be happy with the same voltage, but can utilize as much as 20 amps for a quicker charge time. The advantage to you is extremely fast charge times compared to lead acid. Our products will fully recharge in as little as 15 minutes as opposed to the hours that lead acid requires. When you combine this with extremely low self discharge rates, charging should not be a frequent proposition for Alien Motion batteries."
I don't claim to be an electrical engineer, but with the balance charger maybe it can charge that fast because it is charging the cells individually. That would make sense to me. If you are simultaneously charging multiple smaller cells at a fast rate as opposed to charging one large cell.