In Sanskrit, you would need to describe a battery using a combination of words:
* विद्युत कोश (vidyut kosh): This literally translates to "electric container" and could be used to refer to a battery.
* विद्युत संचयक (vidyut sanchayak): This translates to "electric accumulator" and is another possible term for a battery.
Ultimately, the best way to express "akku" in Sanskrit would depend on the context. You could also use a more descriptive phrase to convey the meaning, like "विद्युत-संचालित उपकरणं यस्मिन् विद्युत शक्ति संचित भवति" (vidyut-sanchalit upakaranam yasmin vidyut shakti sanchit bhavati) which means "the device that is powered by electricity where electric power is stored."