Lithium Tool Batteries in the Cold
Why this matters
On a 20 F morning a pack that drove a full day of fasteners in September quits before lunch, the charger on the tailgate blinks and refuses it, and somebody decides the batteries are shot and orders a new set. Most of the time nothing is wrong with them. A cold lithium-ion pack delivers less and sags harder under load, and that is mostly temporary. The part that is not temporary is charging a cell while it is below freezing, which can do permanent damage. Knowing which is which saves a battery budget and keeps the crew working.
What cold does inside the pack
A lithium-ion cell moves lithium ions through a liquid electrolyte between two electrodes. Cold thickens that electrolyte and slows the reactions at the electrode surfaces, and the result shows up as higher internal resistance.
Less runtime. At low temperature the cell cannot release all the energy it holds at the rate the tool demands, so a cold pack gives you fewer fasteners, fewer cuts or fewer feet of drilling than the same pack warm. Most of that energy is still there. It comes back when the cell warms.
Voltage sag under load. Higher internal resistance means the voltage drops further when the tool pulls current. A heavy-draw tool (a circular saw in wet lumber, a large hole saw, an impact on lag screws) pulls the pack voltage down to the point where the tool's or the pack's protection circuit cuts out. That cutoff is protecting the cells from being run too low. The pack can then show a reasonable charge on its fuel gauge at rest and still stall the tool the moment the trigger is pulled.
A light-draw tool hides the problem. The same cold pack may run a work light or a drill driver on small screws for a long time, because light current produces little sag. A crew that says "it works in the drill but not the saw" has described voltage sag exactly.
Heat helps itself, a little. Current flowing through internal resistance warms the cells, so a pack in continuous use warms itself somewhat. A pack that sits idle on frozen ground between uses gets no benefit and starts every cut cold.
Why a pack that died at noon may be fine
The pack that quit at noon and recovers after an hour in a warm truck cab is showing the reversible part of the cold effect. To tell a cold pack from a worn one, bring it to room temperature, charge it fully, and use it on a warm day for the same kind of work. If it delivers close to what it used to, it was cold, not dead.
A pack that is genuinely worn loses runtime warm as well, may get unusually hot in use, and tends to sag even at room temperature. Cold makes a worn pack look terrible sooner, so a set of older packs is the first to fail on a cold morning, which is part of why "all our old batteries died the same day" usually means cold plus age, not a sudden failure.
Charging below freezing is the one that does damage
When a lithium-ion cell is charged, lithium ions move into the anode, usually graphite. In the cold, they cannot move in fast enough, and some lithium deposits as metal on the anode surface instead. That is lithium plating. It permanently reduces capacity, and plated lithium can grow into structures that raise the risk of an internal short.
The general rule cell makers publish is not to charge lithium-ion cells below 32 F (0 C), and many specify a higher minimum. That limit is about the temperature of the cells, not of the air around the charger. A pack that sat in a 15 F truck box overnight is still well below freezing inside for a long time after it comes into a warm room.
The charger refusing a cold pack is doing its job. Most current tool chargers read a temperature sensor in the pack and will not charge outside their allowed range, often signaling it with a distinct light or a "hot/cold pack" indication. The charger usually starts by itself once the pack warms. Do not defeat this. Warming a pack on a heater, in an oven, with a heat gun or on a dashboard defroster vent is the wrong fix: it heats the case and the outer cells unevenly, can damage the case and seals, and can overheat cells nearest the heat source. Let the pack warm at room temperature, out of its case if it lives in one, and let the charger decide when it is ready.
Condensation comes with the warm-up. A cold pack brought into a warm, humid room collects moisture on its terminals and inside the vents. Let it warm with the terminals dry and wipe them before it goes on the charger. A pack that took on snow or water gets dried and inspected before charging, per the tool maker's instructions.
Storage on a winter job
- Keep packs in the cab, not the truck box. The heated cab during the day and the house or shop overnight keep packs near usable temperature. A soft insulated bag in the cab slows their cooling during the day.
- Rotate packs. Work one or two, keep the rest warm, swap. A warm pack in reserve is worth more than a full pack that is frozen.
- Do not leave packs on the charger in an unheated building overnight if the building can drop below the charger's range; the charger will not charge them, and they will sit cold.
- Charge before the cold, not during it. Packs charged indoors the night before go out full and warm.
Summer is the other limit
Heat is worse for long-term life than cold. A closed truck cab in summer sun gets far hotter than a pack's storage range allows, and stored heat ages the cells permanently in a way the cold does not. A pack left on a dashboard is the summer version of a pack charged frozen. Some chargers will also refuse a pack that is too hot, which is the same protection working at the other end. In summer, packs ride shaded and ventilated, and a hot pack cools in shade before it goes on the charger.
Worked example: lag screws on a 20 F day
A deck crew is setting ledger lag screws on a 20 F day. They have four 5.0 Ah packs and one impact driver. From a previous cold job, the crew lead knows the shop's own numbers for this driver and lag size, and they are the kind of counts every shop should keep for its own tools.
- A warm, full pack drives about 120 lags.
- A full pack that has been in the truck box overnight and used outside at 20 F stalls at about 70 lags.
- A stalled cold pack, put in the warm cab for 45 minutes, drives about 30 more before stalling again.
These are this crew's counts on this tool, not a property of any battery. Another tool, fastener or pack size gives other counts. The job needs 300 lags.
Plan A, the way it usually goes. All four packs ride in the truck box overnight and start the day cold. Four packs at 70 lags each is 280 lags. The crew is 20 lags short of 300 with every pack stalled, and the charger on the tailgate refuses all four because they are below its charging range. The only way to finish is the cab: one stalled pack warms for 45 minutes and gives about 30 more, so the last 20 lags wait three quarters of an hour on a warm-up while the crew stands around.
Plan B, the same packs managed. The packs come home and charge indoors overnight, then ride in the cab in an insulated bag. The crew works one pack at a time and keeps the other three in the cab. A pack that starts warm and works at 20 F still cools as it works and stalls early, so the crew lead assumes something between the warm and cold counts for each first session. Say 90 lags per pack. Four packs at 90 is 360 lags, already past 300. Check from the other side: 300 divided by 90 is 3.3, so four packs cover it with most of the fourth left over, and the stalled packs never need a second warm-up session at all.
The difference between the plans is 280 and 360 lags from the same four packs, 80 lags, and none of it came from buying anything. The 90 is an estimate the crew lead should replace with a real count the first time the crew works this way.
At the end of the day. The stalled packs go into the cab, then indoors. The crew lead does not put them on the charger the moment they come through the door. They sit at room temperature until the charger accepts them, which for a large pack that has been cold all day takes a few hours. On the next morning the crew lead checks each pack's gauge and notes any that did not fully charge; a pack that refused to charge even after warming overnight is taken out of service and checked.
The damaged pack is not a cold pack
Cold makes the plastic case of a pack more brittle, so a pack dropped on frozen ground or concrete can crack. A cracked case, a swollen pack, a pack that is hot when it has not been used, or one that hisses, smells sweet or chemical, or smokes is a damaged pack, not a cold one, and the response is different. Stop using it. If it is smoking or venting, get people back from it and away from combustibles, keep it outdoors if it is outdoors, and call 911 if it ignites; a lithium-ion fire can reignite after it appears to be out. A damaged pack that is not venting goes outside, away from the truck and anything that burns, until it can be disposed of through a battery recycler. Damaged-pack handling, transport and disposal are covered in Lithium-Ion Batteries in Tools, Vans and Customer Equipment.
What changes the answer
- Packs with built-in heating. Some packs warm their own cells before charging or in use. They change the timing, not the rule: the pack's own instructions say when it can charge.
- Very low temperatures. Well below 0 F, even a warm pack cools fast enough that the cab rotation shortens to minutes. Corded tools or a generator-fed charger in a heated space may be the better call.
- Battery-powered equipment that charges itself. Some outdoor equipment and vehicles charge from their own system. Their charge control handles temperature; read their manual before leaving one plugged in outside in a freeze.
References
- Lithium-ion cell manufacturers' published charging temperature limits (no charging below 0 C, 32 F, with many specifying higher)
- Tool and charger manufacturers' instructions for temperature limits, charger indications and pack storage
- See related: Lithium-Ion Batteries in Tools, Vans and Customer Equipment
- See related: How to Keep Battery Tools Running on a Cold Day
- See related: Cold Stress, Frostbite, and Hypothermia