Short answer: a lithium-ion power tool battery is designed not to. It contains protection against exactly the conditions that cause fires. But "designed not to" and "cannot" are different statements, and it's worth understanding which failures are real, how they start, and how to make them effectively impossible in your shop.
I've seen a swollen pack and I've smelled a vented cell. Both times the cause traced back to something predictable — physical damage, a bad charger, or a pack that was already failing and kept being used.
What Thermal Runaway Actually Is
The scary word is thermal runaway. It's not spontaneous combustion. It's a chain reaction that needs a trigger.
Inside a cell, a separator keeps the positive and negative materials apart. When that separator is damaged or the cell overheats past a threshold, the two sides meet, current flows where it shouldn't, and the cell heats up. Heat damages neighboring cells, they join in, and the pack can vent hot gas or flame.
Every one of those steps can be prevented. And the protection systems in a quality pack — the BMS and the cell design — exist to interrupt the chain at the earliest link.
The Real Triggers, Ranked
Here's what actually starts battery incidents, in the order I've seen them.
- Physical damage. A pack dropped off a ladder, crushed under a truck toolbox, or punctured by a tool bit. Internal damage isn't always visible from the outside.
- Wrong or faulty charger. A charger that overcharges, or a generic charger not matched to the pack's voltage and protocol, pushes cells past safe limits.
- Continuing to use a failing pack. A pack that swells, gets hot, or won't hold charge is already telling you a cell group is compromised. Using it anyway is the most common path to a real incident.
- Water and contamination. Submerged packs, or packs with corroded terminals creating resistance and heat.
- Extreme heat. A pack left in a closed truck cab in summer, or on a dashboard, can be pushed toward its limits before it's even used.
Notice that "the battery just exploded for no reason" isn't on the list. It doesn't happen to a healthy pack.
| Trigger | How it starts | What prevents it |
|---|---|---|
| Physical damage | A drop, crush, or puncture damages the separator inside a cell | Retire any pack that took a hard hit, even if it still works |
| Mismatched charger | The wrong charge profile pushes a cell past its limit | Use a charger rated for that pack family |
| Extreme heat | A pack charged or stored in direct sun or a hot cab | Charge and store at room temperature |
| Over-discharge | A pack left flat for months sits below its safe voltage | Recharge before it sits empty, and check it if it does |
What the Protection Systems Do
A quality pack and charger work together to prevent the triggers above.
- Overcharge cutoff. The charger stops at the correct voltage, and the BMS independently cuts off if it doesn't.
- Over-discharge cutoff. The pack stops delivering current before cells drop below safe voltage — deep discharge damages the cell structure.
- Over-current and short protection. A dead short across the terminals trips protection instead of dumping full current.
- Temperature monitoring. Chargers stop if a pack is too hot or too cold to charge safely.
- Cell balancing. Keeps cell groups even so one group doesn't get driven out of range.
This is why "just use the charger that came with it" is real advice and not manufacturer lock-in. The handshake between pack and charger is part of the safety design.
The Real-World Odds
For context: millions of lithium packs run on job sites daily, and incidents are rare. When they happen, they're overwhelmingly tied to damage, a mismatched charger, or a pack that was already showing failure signs. The tool/battery combination is engineered to be far safer than the gasoline and propane that also live in a work truck.
The practical conclusion is not "be afraid." It's "don't ignore the signals."
Making It Effectively Impossible in Your Shop
These habits remove essentially all the realistic risk.
- Retire any pack that swells, smells sweet, gets abnormally hot, or takes a sudden hit. Don't "use it up."
- Charge only with a charger matched to the pack's voltage and brand family. Never force a pack into a charger it doesn't belong in.
- Don't charge on combustible surfaces — no charging on a stack of rags or cardboard. Charge on concrete or metal.
- Unplug chargers when not in use, and don't leave packs cooking for days.
- Store packs at room temperature, away from direct sun. A hot truck cab is not storage.
- Keep metal objects away from terminals so nothing can bridge them.
- Have a Class D or lithium-rated extinguisher approach in mind — and know that for a single pack, the priority is clearing people away and letting it burn out in a safe location.
Our fuller lithium battery safety guide covers storage, charging, and disposal in detail. If your pack has already stopped holding charge, that's a warning worth acting on — our piece on why batteries stop holding charge walks through the diagnosis.
Chargers Are Part of the Safety System
If you're running a mixed-brand shop, use a charger rated for each pack family rather than one universal unit that "fits" everything. A Dewalt 12V-20V Max fast charger is matched to Dewalt packs and enforces the right charge profile. That matching is the point.
Frequently Asked Questions
Can a power tool battery explode on its own?
A healthy lithium pack will not spontaneously explode. Incidents require a trigger: physical damage, a faulty or mismatched charger, extreme heat, or continued use of an already-failing pack. Remove those and the risk is negligible.
How do I know if my battery is about to fail?
Watch for swelling, a sweet chemical smell, abnormal heat during use or charging, a sudden drop in runtime, or visible corrosion on the terminals. Any of these means retire the pack.
Is it dangerous to leave a battery on the charger overnight?
For a quality pack and matched charger, occasional overnight charging is fine — both stop at full charge. But it adds unnecessary heat and stress, and it's bad practice if the pack or charger is damaged. Unplug when you can.
What should I do if a battery starts smoking?
Don't handle it. Move people away, and if it's safe to do so move the pack outside onto concrete or bare ground away from anything combustible. Let it burn out. For larger incidents, evacuate and call the fire service.
Do third-party batteries increase fire risk?
Only the unverified ones. A third-party pack with quality cells, a real BMS, and UN38.3 documentation carries the same protections as an OEM pack. A cheap pack missing those protections is where risk rises.