The woman, 54, told the court that she picked up the sledgehammer with the intention of handing it over to her 48-year-old boyfriend while the pair were on a sailing trip off the west coast of Sweden.
But as the tool changed hands he accidentally struck himself in the head, she said, banging the back of his head as he fell.
When questioned by police, the woman said that she believed her boyfriend was already dead when she began repeatedly hitting him over the head with the sledgehammer.
“At no point did my client intend to take his life,” the woman’s lawyer, Lennart Borgland, told the court.
But an expert from the Swedish Forensic Science Laboratory (SKL) suggested that fresh evidence obtained from blood samples taken from the scene did not tally with the woman’s version of events.
The suspect gave a calm impression in the courtroom, with her face revealing little emotion. She looked at the pictures presented by the prosecutor but averted her eyes from images depicting the dead man’s injuries.
The pair had been together for 30 years but were in the process of separating when they set out on a sailing trip together in October.
The man had been seeing another woman for the previous four years, a development that had left the 54-year-old in a state of crisis, her lawyer said.
The man had said he was frightened about how the woman would react to the separation.
The woman initially said that her partner had fallen overboard. But significant amounts of blood found on the boat and the injuries on the man’s body, which was found a short while later, led prosecutors to arrest the woman for murder.