Every January the same conversation happens at the truck. The pack that ran all day in October now dies halfway through a task, and somebody decides the battery is shot. Usually it is not. It is cold, and cold does something specific and mostly temporary.

Why runtime drops

A lithium-ion cell works by moving ions through an electrolyte between two electrodes. Cold thickens that electrolyte and slows the chemistry down, which raises internal resistance. Higher resistance means voltage sags harder under load, and the tool's protection electronics see that sag and shut down to protect the pack.

The energy is still in there. The pack cannot deliver it fast enough at that temperature. Warm the same pack up and most of the capacity comes back. This is why a battery that quit on a cold roof will run fine an hour after it sits in the cab with the heat on.

The part that causes real damage

Discharging in the cold is inconvenient. Charging in the cold is harmful.

When a lithium cell is charged below freezing, lithium can deposit as metal on the anode surface instead of intercalating into it. That deposit does not come back out. It permanently reduces capacity, and in enough quantity it creates internal structures that raise the risk of a short.

Modern tool chargers generally guard against this. Most will read pack temperature and refuse to start, showing a fault light while the pack sits and warms. That flashing light is the system working, not a broken charger.

The failure case is the older charger, the off-brand charger, or the one that charges a cold pack like a normal pack. Not worth finding out which one you own the hard way.

Practical winter routine

  • Bring packs inside at the end of the day. A tote by the door works.
  • If packs did stay in the cold, let them sit at room temperature before charging. An hour is usually plenty for a compact pack, longer for a large one.
  • Do not speed the process with a heater, a defroster vent aimed directly at the pack, or anything else that heats one side fast.
  • Keep a pack in an inside coat pocket on brutal days if you only need one at a time. Body heat is enough to keep it working.
  • Charge indoors, on a hard non-combustible surface, with clearance around the charger.

Long-term storage

For packs going into a corner until spring:

  • Store at a partial charge rather than full or empty.
  • Store cool and dry, but above freezing where possible.
  • Take the packs off the tools so nothing can be triggered and nothing slowly drains.
  • Check them every couple of months and top up any that have drifted low. A cell that sits fully discharged for a long stretch can drop below the voltage a charger is willing to recover.

Heat is the other half

Winter gets the attention, but sustained heat does more cumulative damage to lithium cells than cold does. A pack that lives on a dashboard in July or in a metal truck box in August ages faster in ways that do not reverse.

The storage advice ends up the same in both directions. Packs are happiest where a person would be comfortable. If the space is miserable to stand in, it is a bad place to keep batteries.

When it really is the battery

A pack that runs short even after warming up, that gets hot quickly on light work, that will not take a charge, or that has any swelling has reached the end of its service life. At that point it is a recycling item, not a repair item, and it should go to a proper battery drop-off rather than a trash can.