impact driver seems weak, it struggles to drive 3 inch screws into pine and the battery dies super fast.

Diagnosis

The DCF787 is a compact, lightweight impact driver designed for overhead work and tight spaces. When it struggles with 3-inch screws in pine and drains the battery rapidly, the root cause is almost always "voltage sag" resulting from an insufficient discharge rate. If you are using the 1.3Ah or 2.0Ah batteries that often come in the C1 kit, these cells cannot provide the high current (amperage) required to keep the brushless motor spinning under a heavy load. As the motor demands more power to drive the screw, the small battery's internal resistance causes the voltage to drop sharply. This leads to a loss of torque (the "weak" feeling) and forces the battery to work harder, which generates heat and exhausts the remaining capacity much faster than normal. You can confirm this by noticing if the battery feels hot to the touch after a few screws or if the tool slows down significantly right before the screw head seats.

How to Fix It

To restore the full power of the DCF787 and extend your runtime, you must upgrade to a high-capacity battery pack. This increases the "fuel tank" size and allows for a more stable current flow.

1. Remove your current low-capacity battery by pressing the silver release tabs on the sides of the battery base.
2. Acquire a DeWalt 20V MAX 5.0Ah or 6.0Ah battery pack (e.g., DCB205). These packs contain more cells wired in parallel, which significantly reduces voltage sag.
3. Slide the high-capacity battery into the rail until you hear a distinct click. Ensure the battery is fully seated; a loose connection can cause intermittent power loss.
4. Check your driver bits. Ensure you are using a high-quality T25 or Square drive bit that fits the screw head perfectly. A slipping bit creates friction and makes the tool feel weaker than it actually is.
5. Apply the trigger gradually. The DCF787 uses a variable speed trigger; ramping up the speed allows the impact mechanism to engage more effectively without stalling the motor.

If That Didn't Work

• Check for Mechanical Binding: If the tool still feels weak with a 5.0Ah battery, the internal hammer and anvil mechanism may be clogged with sawdust or metal shavings. Use a canister of compressed air to blow out the vents and the nose of the tool to ensure the internal striking mechanism can move freely.
• Inspect the Battery Terminals: Examine the copper contact plates on both the tool and the battery. If they are charred or covered in debris, use a cotton swab with 90% isopropyl alcohol to clean them. Poor contact creates electrical resistance, which mimics a weak battery.
• Verify Screw Material and Length: Ensure you are using "Construction" or "Deck" screws rather than general-purpose wood screws. 3-inch screws in pine can create significant friction; if the pine is particularly dense or wet, the tool may be hitting its torque limit. Try pre-drilling a small pilot hole (1/8" bit) to reduce the load on the driver.
• Test with a Different Battery Brand/Age: If your 5.0Ah battery is several years old, the cells may have degraded. Test the tool with a brand new 20V Max pack to determine if the issue lies within the tool's motor or the battery's health.

Heads Up

• Avoid Overheating: Even with a large battery, the DCF787 is compact. If you are driving dozens of 3-inch screws in rapid succession, give the tool a 5-minute break to prevent the brushless controller from entering thermal protection mode.
• Battery Storage: Never store your 20V Max batteries in a freezing garage during winter. Cold cells have higher internal resistance and will exhibit the same "weak" performance as a low-capacity battery.
• Bit Selection: Always use "Impact Rated" bits. Standard bits will shear off under the torque of the DCF787, and the resulting slippage can lead you to believe the tool is losing power.
• Charging Cycle: Always charge your batteries to 100% before a heavy project. DeWalt batteries exhibit a noticeable drop in torque once they fall below 25% charge.