Why My Uninterrupted Power Supply Need to be slapped EveryTime To charge ?

I've installed a UPS for my house (220V mains), connected with 2 car batterys. In my area the daily power outage is 4 hours, So When the power is out, my rooms and computer still function from the UPS. but suddenly the Red Light Of the Charger that is Called ( MAIN ) is not On even when there's electricity . so i climb to my UPS and Slapped or give it a physical Impact ,In order for The Red light to Function, but after some time the light go off again and need to be reforced . Any Suggestion on what is causing this ?? I send it a technician 2 times and he was not able to solve my problem

A corroded or loose connection would cause this. Us technicians look for this by poking connections with a plastic stick.

Can You plz make a Detailed explanation on how to solve this and what is causing this , cause i'm just not a technician

Detailed explanation: Open the box, connect power, and start poking connections with a plastic stick. When the machine goes on and off as a result of poking something, that is the bad connection.

THank you sir , i will try it, and the strange thing is , that when the Main Light go off , a very weak touch or punch can switch it on again .if there's anything else plz Let me know

Interesting choice of username.

Can you translate the username? Holy war against samarji?