A deeply discharged lithium-ion battery is generally not something you should try to revive yourself. The safest guidance from the community is: don’t do it.

When a Li-ion pack won’t charge after being fully discharged, it may be because its protection circuitry has shut it down for safety. Lithium-ion cells can become hazardous if mishandled, and failure can involve overheating or fire.

A couple of users reported success by temporarily connecting a dead battery to a working one to raise the voltage enough for the charger to recognize it. However, this is risky, not manufacturer-approved, and another user reported it did not work reliably. Because of the fire risk, it’s not a recommended DIY fix.

Best practice: stop using the battery, do not attempt improvised charging methods, and contact Profoto or a qualified battery service/recycling provider for replacement or proper disposal. To reduce the chance of this happening again, store Li-ion batteries partially charged and check/recharge them periodically during long periods of non-use.