Question

why did the indian rulers lose most of the battels in the 18th century ?

Answer 1

here is your answer ,
Because there was lack of unity among rulers. They won't have feelings of nationalism among them. And great rulers who actively supported the British like the Chakma queen from Rangamati (bangladesh) / Dimagiri (Mizoram,India), Kalindi, helped the Brits in capturing a good nos of fugitives during/after the mutiny and among the more famous ones, it was the Scindia family of Gwalior who supported the Brits against the Rani of Jhansi. Indian rulers became lazy and allied with the British. Lack of unity, lack of mutual plans lead to many lost battles. Indian rulers were fooled by them because of these reasons.