Can a dead battery be reconditioned?
Sometimes—but “dead” describes a symptom, not a diagnosis. The first job is to determine whether the battery is discharged, deteriorated, sulfated, internally failed or physically unsafe.
Do not start with a restoration method
Confirm the battery type and inspect for obvious damage. Then use an appropriate charger/test procedure or professional battery test. A battery that repeatedly loses charge may also be affected by the vehicle or device charging system.
More promising vs less promising situations
Worth diagnosing
A rechargeable battery is intact, its chemistry is known, and the main issue appears to be discharge or reduced capacity.
Stop and replace/recycle
Swelling, leakage, overheating, cracked cases, severe corrosion, internal failure or other unsafe conditions are reasons not to experiment.
Lead-acid batteries and sulfation
Some lead-acid performance loss is associated with sulfation. Early sulfation can differ from permanent hard sulfation, so recoverability depends on the condition rather than the label “dead.”