Can I revive a dead cordless tool battery?

This question is asked because tool batteries are expensive and the internet is full of confident answers. Almost all of those answers were written for the nickel-cadmium packs of twenty years ago, where a memory effect was real and a shock charge sometimes worked. Lithium chemistry does not behave that way, and the protection circuit refusing to charge is a deliberate safety decision rather than a puzzle to defeat.

Safety

Step by step

1. Check the pack physically first.Swelling, a split case, heat when idle, corrosion or an odd sweetish smell. Any of these and there is nothing to try — the pack is a hazard and goes to recycling.
2. Bring it to room temperature and retry.A pack that is too cold will be rejected by the charger, and a pack left in a shed all winter is very cold indeed. Several hours indoors, then try again. This is the most likely thing to actually work.
3. Clean the terminals and reseat it.Dry cloth or a pencil eraser on the contacts, check the pack clicks fully home, and try a second charger of the same platform if you have one.
4. Try a charger with a recovery mode.Some manufacturers' chargers detect a deeply discharged pack and apply a long, low trickle to bring it back above the threshold. It is slow, it is built into the charger, and it either works or it does not.
5. Understand why deep discharge is usually final.Below a certain cell voltage, internal changes begin that make the cell unsafe to recharge. The protection circuit latches off deliberately. That is not a fault to be worked around.
6. Accept cycle life as the other ending.A pack that charges fully and runs flat in minutes has not failed, it has aged. Capacity falls with cycles and with time, and there is no procedure that restores it.
7. Do not attempt the popular fixes.Jump-starting from another pack or a car battery, freezing, hammering, opening the case to charge cells individually, or running it on a bench supply. Each of these bypasses the protection and each has caused fires.
8. Retire it properly.Tape over the terminals, store it away from anything flammable, and take it to a battery recycling point or a retailer that takes them back. Lithium packs in household waste start fires in bin lorries and at sorting sites.

Tips

• The reliable way to avoid this is storage: half charge, room temperature, and a top-up every few months. Packs almost always die in the cupboard rather than in the tool.
• Before buying a replacement, price a new bare tool with a pack included. On older platforms it is sometimes barely more than a single battery.
• If a pack has failed within its warranty, the manufacturer's registration scheme often extends cover on batteries. It is worth checking before assuming you are buying a new one.

Common mistakes

• Jump-starting a lithium pack from another battery — It forces current past the protection circuit into cells that have been shut off for a safety reason. This is the single most dangerous piece of advice circulating on the subject.
• Putting a battery in the freezer — It is advice inherited from old nickel chemistries and it does nothing for lithium except add condensation to the electronics.
• Opening the pack to replace cells — The cells are spot-welded and the protection board is matched to them. Puncturing a lithium cell during disassembly starts a fire immediately.

Questions people ask