The defendant told the Old Bailey she was “in a trance” when she hit him.
She was found not guilty of murder, but had admitted manslaughter, citing his bullying during their 35-year marriage.
The jury was told the couple had an arranged marriage in 1983 and fled Sri Lanka in the civil war.
Mr Ramanathan was found with serious head injuries and multiple wounds to the body and neck after Packiam Ramanathan told her neighbour she had hit her husband.
Giving evidence, Ramanathan said she lost control after years of abusive behaviour during which her husband had thrown sticks at her and accused her of having an affair with the fishmonger.
Describing the killing, the defendant said: “I don’t know how I did it. For me I still feel like somebody else did it.”
Prosecutor Sally O’Neil said the couple had argued about money and Ramanathan had become very angry at finding out her husband had written to Sri Lankan police accusing her brother of fraud and theft.
However, Stephen Kamlish QC, defending, said if the 73-year-old had wanted to kill her diabetic husband she could have just given him a bigger dose of insulin.
“The fact it was done in the way it was – with a stick – means there was no planning,” he said.
The jury deliberated for half an hour to find Ramanathan not guilty of murder.
She will be sentenced on Friday for manslaughter.