What is the cheapest way to have an EV battery inspected?

For a single car, a test at an affiliated garage is the most economical: €80 including VAT, including the labor and the certificate. You do not need to purchase any equipment yourself. If you test regularly, for example as a car company, your own equipment is worthwhile: a one-off payment of €99 excluding VAT, then €29.95 excluding VAT per test.