Summer is harder on cordless batteries than winter is, and the damage is quieter. Cold makes a pack feel weak and then it recovers. Heat takes capacity permanently, a little at a time, and you only notice a year later when nothing holds a charge like it used to.

Where the heat actually comes from

Three sources, in order of how much damage they do.

Parked vehicles. A closed vehicle in the sun climbs well above outside air temperature within minutes and keeps climbing. This is the single worst place batteries commonly live, and it is where most people keep them because that is where the tools are.

Direct sun on a jobsite. A pack sitting on a roof deck, a concrete slab, or a truck bed rail in full sun absorbs a lot more than the ambient temperature suggests. Dark plastic in sun gets hot fast.

Sealed containers. A tote or a closed case with packs and a running charger inside traps heat with nowhere to go.

What heat does to the pack

Lithium cells age faster at high temperatures whether they are being used or not. That aging shows up as lost capacity: the pack still charges to full and still reads full, but the amount of work it does before dying keeps shrinking.

Heat also affects the short term. Battery and tool electronics monitor temperature and will reduce output or shut down to protect the cells. A tool that keeps cutting out on a hot afternoon is often doing exactly what it should.

And chargers refuse hot packs. If a battery sits on the charger with a temperature indicator lit and nothing happening, that is the charger waiting for the cells to come down to a safe range.

The summer routine

None of this requires spending money.

  • Keep a small insulated cooler in the vehicle for packs. No ice, no cold packs, just insulation and shade. It levels out the worst of the swings.
  • Store spare packs in the cab footwell rather than the bed, and in shade rather than on the dash.
  • At the end of the day, bring batteries inside. Indoors at room temperature is the ideal storage condition and it is free.
  • Give hot packs several minutes in shade before they go on a charger.
  • Charge indoors, on a hard surface, with space around the charger.

Storing for the off season

If tools go quiet for part of the year, store packs at a partial charge in a cool, dry indoor spot. Not the shed, not the attic, not the garage rafters in July. Check every couple of months and top up anything that has drifted low.

When a pack has gone too far

Stop using and safely recycle any battery that shows:

  • Swelling or a housing that no longer sits flat
  • Any leaking or residue at the seams
  • A burnt or chemical smell
  • Damage from being dropped hard, crushed, or submerged
  • Heat that continues well after the tool has been set down

Do not put lithium packs in household trash or curbside recycling. Damaged lithium cells can ignite. Use a battery recycling drop-off; many hardware retailers and municipal facilities accept them. If a pack is visibly damaged, keep it away from anything flammable while you arrange disposal.

The short version

Heat is the one battery variable you control for free. Shade, airflow, indoor overnight storage, and a few minutes of patience before charging will do more for your pack collection than any accessory you can buy.