Today, i find the original MINI PROFAQ DOC

https://www.ninebot.com/Aftersales-faq.html

 

(translate from chinese to english with microsoft and google, sorry)

They said: 

44. Q : The battery is detachable for users? User-removable batteries will lose warranty?

A : Battery is user removable, do not affect the warranty. After you remove the battery pack, it is close to the body on the side of the battery cover has a squareLED lamp, if the light is intermittent flashing green light, indicating that batteries are working properly. If the light does not shine or some other color, indicating battery failure, you need to repair / replace. Note: battery removed will expose circuits within the body, users cannot disassemble inner circuit, remove body circuit will lose warranty eligibility. Internal circuit with high voltage, and still remains a high voltage with the battery removed. Touch the internal circuit is likely to be a shock, or risk of injury.

 

Maybe use this document when you are in touch with your ninebot reseller for know if this is available in your country

Well, if the battery light is flashing in green: all is ok

but if your battery light don' flash or other: it's a bad day for you