my dewalt battery pack gets really hot when charging, is that safe?

Diagnosis

Warmth during the charging cycle of the DeWalt DCD771C2 battery pack is expected due to the internal chemical reaction of the Lithium-Ion cells and the energy loss that occurs during the conversion of AC power to DC. However, if the battery is too hot to touch or smells of ozone or burning plastic, you are experiencing a thermal runaway or a failure in the charging circuit. This typically happens when the internal resistance of the battery cells has increased due to age, deep discharge, or physical damage, forcing the charger to push more current to overcome the resistance, which generates excessive heat. Another common cause is a failure in the charger's thermal management system. The DeWalt charger uses a series of vents to dissipate heat from the transformer and the battery cells. If these vents are obstructed or if the ambient temperature of the room exceeds 104°F (40°C), the battery cannot shed heat fast enough. You can confirm a problematic heat level by checking if the charger's LED indicator is flashing red; this is the built-in "Hot/Cold Delay" feature, which prevents the battery from charging until it reaches a safe internal temperature.

How to Fix It

To resolve overheating and ensure a safe charge, follow these precise environmental and mechanical steps:

1. Disconnect the battery from the charger and allow it to cool down to room temperature (approximately 68°F to 75°F) for at least 30 minutes before attempting to charge again.
2. Clear a 12-inch radius around the charger. Place the unit on a non-conductive, hard surface such as a wooden workbench or a concrete floor. Do not place it on cardboard, towels, or plastic sheeting, as these materials insulate the bottom of the charger and trap heat.
3. Inspect the battery terminals (the metal contact points) for any debris, sawdust, or oxidation. Use a cotton swab dipped in 90% isopropyl alcohol to clean the contacts on both the battery pack and the charger pins to ensure a low-resistance connection.
4. Verify the battery is fully locked into the charger. Push the battery down firmly until you hear a distinct "click." If the battery is loose, it creates an arc at the contact point, which generates localized heat that can melt the plastic casing.
5. Move the charging station away from direct sunlight or heat-generating equipment like space heaters or compressors.

If That Didn't Work

• Test with a different battery pack: If a second 20V Max battery charges without overheating, the original battery has likely developed a "dead cell" or high internal resistance. In this case, the battery is chemically compromised and must be replaced to avoid a fire hazard.
• Swap the charger: If multiple batteries are overheating on the same charger, the internal voltage regulator or the cooling fan (if applicable to your specific charger model) has failed. Replace the charger unit immediately, as an unstable voltage output can permanently damage your battery cells.
• Check for physical swelling: Inspect the plastic casing of the battery pack. If the seams are bulging or the battery does not slide into the drill smoothly, the cells have "swelled." This is a critical failure; stop using the battery immediately and dispose of it at a hazardous waste facility.
• Avoid "Opportunity Charging": If you are repeatedly putting a 90% charged battery back on the charger for short periods, you can cause heat buildup. Let the battery drop to about 20-30% before initiating a full charge cycle.

Heads Up

• Never attempt to charge the battery in an environment colder than 32°F (0°C) or hotter than 104°F (40°C), as this can permanently degrade the lithium chemistry.
• Avoid using third-party "knock-off" chargers. Non-genuine chargers often lack the critical thermal cutoff switches that prevent the DCD771C2 batteries from overheating.
• Store your batteries in a cool, dry place. Extreme heat during storage can increase the internal resistance, making the battery run hotter during the next charge cycle.
• If you notice the charger's LED flashing red, do not force the charge. This is a safety feature; wait for the battery to reach a safe temperature automatically.