What you did, depending on battery chemistry, was very bad: the consequences could have been an explosion or fire.
Testing a battery of any type would be in this fashion:
apply a known rated load for a given time (rated to some fraction of the battery capacity and AH rating), vehicle lead acid batteries (trucks etc) can provide 1000A or more so make sure the loads are chosen suitably.
measure the voltage, current and time.
Calculate the result as needed. Repeat at various intervals of time and / or voltage - be aware of voltage and current limits for the type of battery tested.
This procedure must be tailored to the battery chemistry under test, what is valid for one battery may not be safe for another. Fuses etc should be used liberally as well as ppe (personal protective equipment).