APC Smart UPS pop outbreak and burning smell after electrical surge

[light767_apc]

Hi 

I have an old APC smart UPS perfectly working DLA-1500I, so basically it had alwasy worked fine, this evening happen an electrical surge in my house, APC UPS immediadly turn on as usualy but, this time, a pop outbreak( like a small explosion) happens and then a burning smell comes out. Actually after the electrical surge, everything come back to normal and even the UPS keep working fine;  anyway I have turned off everything because I was concern from the event. 

I have disassmbled the UPS but I cannot see any burned component on the PCB....what can have been done? 

The battery itself? 

[light767_apc]

I have provided a new batteyr pack for the UPS because I disassmbled it and by a visual test I cannot spot any burned component on the PCB Board. The only thing that smell of burned plastic was the battery pack so I assume the pop outbreak that I heard during the electrical surge was coming from the battery pack even if I'm not familiar with battery that pop without acutally burning or make something worst. 

Anyway I'm gonna install the new battery pack but I was wondering how can I simply check if the output of the UPS is correct or not? Is using an oscilloscope in a lab the only good way? 

[light767_apc]

Any help?