Does hot weather damage cordless tool batteries?

Cold hides charge and gives it back. Heat destroys capacity permanently — a different mechanism with a different remedy, and the reason packs left in a van all summer die the following spring.

Difficulty: beginner
Time: 12 min
Read: 6 min
Safety: warning

Short answer

A hot pack will usually refuse to charge, which is the charger protecting it. That much is recoverable — let it cool. What is not recoverable is the capacity lost by storing lithium cells hot: leave packs in a sunlit van, shed or car boot all summer and they will hold measurably less charge for the rest of their life. Store them indoors at around half charge.

The companion guide on cold weather explains why a drill dies fast in January and comes back in the spring. Heat is the opposite case in every respect: the tool often works fine on the day, and the damage is invisible until months later. That is why people connect a dead battery to the winter and almost never to the summer that actually caused it.

Safety

Heat damage to a lithium battery is permanent and, unlike cold, it can be dangerous. Never charge a pack that is hot to the touch, and never charge or store a pack that is swollen, split, leaking or has been dropped hard — put it somewhere non-combustible away from the building and dispose of it at a facility that takes lithium batteries. Never put a lithium pack in a household or kerbside bin.

Step by step

Understand the difference from cold.Cold raises the internal resistance of the cells, so voltage sags under load and the tool cuts out with charge still in the pack — a temporary effect that reverses as it warms. Heat accelerates the chemical ageing of the cells, permanently reducing how much charge they can hold. Cold is a bad day; heat is a shortened life.
Never charge a hot pack.A pack that has just come off a hard job, or has been in the sun, is well above the temperature at which lithium cells charge safely. Most chargers detect this and refuse, flashing a fault — that is the charger doing its job, not a failure. Let the pack cool to roughly room temperature first, in the air, without help.
Never cool a pack artificially.Do not put a battery in a fridge, freezer, water or in front of an air line to speed it up. Rapid cooling causes condensation inside the pack, and moisture in a lithium battery is a genuine hazard. Time and shade are the only correct methods.
Get packs out of the van, car and shed.This is the single change that matters. The interior of a vehicle or a sealed shed in summer reaches temperatures far above the air outside, and lithium cells stored at those temperatures lose capacity fast. Packs come indoors at the end of the day, in both seasons but especially this one.
Store at part charge, not full.Lithium cells age fastest when held at full charge, and heat multiplies that effect. For anything being put away for weeks — a spare pack, or the mower battery at the end of the season — around half charge is the right storage state. A pack stored full in a hot shed is the worst combination there is.
Keep the pack out of the sun on the job.A battery left lying on a sunlit patio slab or on the parcel shelf between uses gets hotter than one in a shaded toolbox. Keep spares in the shade or in the bag.
Let a pack rest between heavy cuts.Hard work generates heat inside the pack itself, on top of the ambient temperature. Rotating two packs so each one cools while the other works keeps both of them cooler and extends the life of both.
Recognise a pack that is already damaged.Sharply reduced runtime, getting hot unusually fast, charging much quicker than it used to, refusing to take a full charge, or any swelling. Swelling is the one that matters — a swollen pack is not to be charged, used or kept indoors, and needs proper disposal.

Tips

The damage is cumulative and silent. A pack that spent last summer on a van dashboard will not fail that summer — it will underperform for the rest of its life.
If the van is the only realistic storage, an insulated cool box in the footwell, without ice, buffers the peak temperature considerably.
Charge indoors, on a hard non-combustible surface, and not overnight while everyone is asleep. That advice is about lithium in general, not about heat, but summer is when people charge in outbuildings.
The same rules apply to e-bike and scooter batteries, garden tool packs and power stations. Heat does not care what the battery is fitted to.
Buy packs from the tool manufacturer or a reputable source. Cheap third-party cells have less thermal headroom and less protection circuitry, which matters most in exactly these conditions.

Common mistakes

Leaving batteries in the van or car boot through the summer — It is the most common cause of premature capacity loss, and because the loss is gradual and invisible it is almost never connected to the cause.
Putting a hot pack straight on the charger — Charging lithium cells above their safe temperature range accelerates ageing and, in the worst case, is unsafe. A charger refusing a hot pack is protecting it.
Cooling a pack in the fridge or with water — It causes condensation inside the pack. Moisture in a lithium battery is a real hazard and the pack should simply be left in the shade.
Storing packs fully charged over a hot summer — Full charge and high temperature are the two things that age lithium cells fastest, and together they compound. Around half charge is the correct long-term storage state.

If it doesn't work

Charger flashes a fault and will not start

Cause: The pack is outside its safe charging temperature range, usually too hot — Fix: Leave it in the shade at room temperature until it is cool to the touch, then charge. Do not cool it artificially.

Tool cuts out after a short burst in hot weather

Cause: The pack's thermal protection is tripping under load in high ambient temperature — Fix: Rotate two packs so each cools, keep spares in shade, and let the tool rest between heavy cuts.

Runtime much shorter than last year

Cause: Permanent capacity loss from being stored hot, typically in a van, shed or car — Fix: Nothing recovers it. Store the replacement indoors at part charge and the loss will not repeat.

Pack is swollen or the casing has distorted

Cause: Internal cell failure, often after prolonged heat or physical damage — Fix: Stop using it immediately. Do not charge it. Move it somewhere non-combustible away from the building and take it to a facility that accepts lithium batteries.

Questions people ask

How hot is too hot for a tool battery?

Manufacturers publish a charging range and a wider storage range, and the charging range is the narrower of the two — typically somewhere around freezing to the low forties Celsius. The practical test is simpler: if the pack is warm enough that you notice it holding it, it is too warm to charge.

Is the damage reversible?

No. Unlike cold, which simply hides capacity until the pack warms up, heat causes permanent chemical ageing in the cells. A pack degraded by a hot summer does not recover in autumn.

Can I leave a battery on the charger in hot weather?

Leaving lithium packs on a charger indefinitely is poor practice in any weather, and worse in heat, because it holds the cells at full charge at elevated temperature. Take them off when charged and store them at part charge if they are not being used for a while.