Hi Thanawat,
Can you explain why you test it? Is it because there is a false alarm?
Please note the interface module min working voltage is 15Vdc, the module should not operate below 15Vdc. So it should fault if voltage below 15Vdc.
Question:
1. Where this external power taking from?
2. If come from external charger, does it able to charge up the batteries?
3. Can you try to decrease the voltage started from 15v to 14v wait 1 min, then 14v to 12v wait 1min and etc, see fault coming up or not.