Need brand and model type, too. Many really small types used another set of dry cell alkaline batteries in the "charger" to charge the onboard batts. So there's absolutely no telling what the requirements are without more information.
But one thing is certain -- the wrong charger, wrong charge rate, or even wrong charge time with a substitute charger and yes the batteries can catch on fire or explode.
Almost all helicopters have onboard lipos, necessary for the charge density to lift without wings.