How do I look after cordless tool batteries?

What actually kills a tool battery, the storage charge that matters, and why the cold shed is the wrong place.

Difficulty: beginner
Time: 12 min
Read: 4 min
Safety: warning

Short answer

Heat and extremes of charge are what kill them. Let a pack cool before charging, store at around half charge somewhere at room temperature rather than in a freezing shed, and do not run them completely flat. Batteries usually die from storage, not from use.

Cordless tool batteries cost a substantial fraction of the tool and are treated as disposable. Most fail years earlier than they need to, and almost always because of how they were stored rather than how hard they were worked.

Safety

Damaged lithium batteries are a genuine fire risk. A pack that is swollen, cracked, has been dropped hard or got wet must be taken out of use and disposed of at a recycling point — never in household waste, where they cause fires in bin lorries. Do not charge unattended overnight in a living space, and never charge a pack that is hot or frozen.

Step by step

Let a hot pack cool before charging.A battery straight off a demanding job is hot, and charging it then does more damage than anything else in normal use. Twenty minutes makes a real difference. Good chargers refuse until it cools, which is a feature rather than a fault.
Store at around half charge.Not full, not flat. A pack stored full degrades faster; stored flat it can discharge below the point where the protection circuit will allow charging again, which is permanent.
Store somewhere at room temperature.A freezing shed or a hot loft both age batteries faster. Indoors is better, and it is the change most people could make and do not.
Do not run them completely flat.Swap when the tool starts to labour rather than pushing to a full stop. Deep discharge stresses the cells and shortens life more than moderate use does.
Charge a stored pack occasionally.Every three to six months, top a stored battery back to about half. Self-discharge over a winter can take a pack below the recoverable threshold.
Keep the contacts clean and covered.Dust and debris on the terminals cause poor connections and heat. Most packs have a cover or slot into the tool — do not leave them loose in a toolbox with metal.
Never leave a battery in a cold van or shed overnight.Charging a frozen pack damages it, and a cold pack delivers much less power. Bring them in the night before a winter job.
Use the right charger.The manufacturer's charger, matched to the pack. Third-party fast chargers frequently ignore the temperature signalling that protects the cells.
Do not leave packs on the charger indefinitely.Modern chargers stop when full, but leaving a battery sitting at full charge for months is exactly the storage condition to avoid. Take it off once charged.
Rotate several packs evenly.Using one heavily and leaving another in a drawer ages both — one through cycling, the other through sitting. Alternate them.
Recognise a failing pack.Much shorter run time, running hot, refusing to charge, or the tool cutting out under load. A pack that no longer holds useful charge is a replacement, not a repair.
Recycle them properly.Lithium packs must go to a battery recycling point. In general waste they are crushed and start fires in collection vehicles and at sorting facilities. Most tool retailers and recycling centres take them.

Tips

Buying into one battery platform and staying there is the most useful decision in cordless tools. Batteries cost more than the bare tools over time.
Label packs with the month you bought them. It makes the pattern of failure visible and stops you blaming a tool for a tired battery.
A battery that will not charge at all is sometimes recoverable on a charger that supports a recovery mode — worth trying once before replacing.

Common mistakes

Storing batteries in an unheated shed or garage — Temperature extremes age lithium cells considerably faster than use does. Indoors at room temperature is the single most effective change.
Storing packs fully charged for months — Sitting at full charge accelerates degradation. Around half charge is the storage state, which is why new packs arrive part charged.

If it doesn't work

Run time much shorter than when new

Cause: Normal capacity loss, accelerated by heat or poor storage — Fix: Nothing recovers lost capacity. Store the replacement at half charge indoors and let packs cool before charging.

Charger refuses to charge a pack

Cause: Too hot, too cold, or discharged below the protection threshold — Fix: Let it reach room temperature and try again. A deeply discharged pack sometimes recovers on a charger with a recovery mode.

Battery hot after light use

Cause: Ageing cells with increased internal resistance — Fix: Take it out of use if it is noticeably hot. That is a sign of a failing pack rather than a hard job.

Pack swollen or cracked

Cause: Cell damage — Fix: Stop using it immediately, keep it away from anything flammable, and take it to a battery recycling point.

Questions people ask

How should I store cordless tool batteries?

At around half charge, somewhere at room temperature rather than a cold shed or hot loft, and topped back up every three to six months. Storing full or flat both shorten their life.

Why do my tool batteries die so quickly?

Usually storage rather than use — temperature extremes and long periods at full or empty charge. Charging a hot pack straight after a job is the other common cause.