You are allowed to carry max 2 power banks in your hand luggage and non in your check in luggage. That is the security law and not a matter for custom to handle. Any other advice from those above are based on luck. As sometimes the Malaysian security is not too strict. However China security is very strict , so dont take any chances by checking in power banks.
Anyway please be aware that most power banks bought from street vendors are fake. so be prepared to regret later.