dewalt 20v battery won't charge, red light just blinks once and stops on my charger

Diagnosis

The single red blink followed by a complete stop on a DeWalt DCB115 or similar charger for the DCD771C2 indicates a "Hot/Cold Pack Delay" or a critical communication failure between the battery's internal Thermistor and the charger's logic board. The charger is designed to protect the Lithium-Ion cells from permanent damage; if the internal temperature of the pack is above 113°F (45°C) or below 32°F (0°C), the charger will refuse to initiate the current flow to prevent thermal runaway or plating of the lithium. In many cases, this isn't actually a temperature issue but a "dirty contact" problem. Because the DCD771C2 is often used in high-dust environments (drywall, wood, masonry), fine particulate matter builds up on the battery terminals. This creates a high-resistance bridge that tricks the charger into thinking the battery is malfunctioning or thermally unstable, triggering the single-blink error code instead of the steady charging pulse.

How to Fix It

To resolve the Hot/Cold Pack Delay and restore charging functionality, follow these precise steps:

1. Remove the battery pack from the charger and move it to a climate-controlled environment (approximately 68°F to 72°F). If the battery was just used for heavy drilling, let it sit for at least 30 minutes.
2. Inspect the battery terminals (the metal slides on the top of the pack) and the charger pins. Look for any blackened carbon buildup, sawdust, or oxidation.
3. Use a cotton swab dipped in 90% Isopropyl Alcohol to scrub the positive and negative terminals on the battery. Ensure you remove all residue.
4. Using a can of compressed air, blow out the charger's terminal slots to ensure no debris is preventing the pins from making full contact with the battery's thermistor pin (the center pin).
5. Insert the battery firmly into the charger. You should feel a distinct "click" as the rails lock.
6. Observe the LED. A successful start will show a blinking red light that continues at a steady rhythm, indicating the charging cycle has begun.

If That Didn't Work

• Perform a Hard Reset of the Battery: Some DeWalt 20V packs enter a "sleep mode" if the voltage drops below a critical threshold (deep discharge). Try inserting the battery into the charger and removing it quickly 3-5 times in a row. This can sometimes "wake up" the onboard Battery Management System (BMS) and allow the charger to recognize the pack.
• Test with a Secondary Power Source: Plug the charger into a different circuit in a different room. If you are using a surge protector or an extension cord, bypass it and plug directly into a wall outlet. Voltage sags in the charging circuit can cause the charger to trip the error light prematurely.
• Check for Terminal Deformation: Inspect the metal slides on the battery pack. If the slides are slightly bent or pushed inward, they won't engage the charger pins fully. Use a small flat-head screwdriver to gently nudge any bent terminals back into alignment, ensuring they are flush and parallel.
• Verify the Charger's Health: Test a different known-working 20V Max battery in the same charger. If the second battery also produces a single red blink, the fault lies within the charger's internal capacitors or the charging logic board, and the charger unit must be replaced.

Heads Up

• Never attempt to "jumpstart" a DeWalt Lithium-Ion battery by connecting it to another power source; this will likely blow the internal fuse or cause a cell rupture.
• Avoid storing your batteries in an unheated garage during winter months; temperatures below freezing can cause the chemistry to degrade, leading to more frequent "Cold Pack" errors.
• Keep the battery terminals clean by wiping them with a dry cloth every few uses to prevent the buildup of conductive dust.
• If a battery has been left completely discharged for more than six months, the cells may have dropped below the recovery voltage, rendering the pack permanently unchargeable for safety reasons.