It’s always the same story. You’re halfway through hanging a shelf or, worse, ripping a 2x4, and your drill just dies. You swap in the spare. Nothing. You slap it on the charger, and instead of that steady, rhythmic red blink, you get the dreaded "fast blink" or, even more frustrating, total silence. A dead DeWalt battery not charging is enough to make anyone want to chuck the whole kit into the driveway.
But honestly? Most of the time, the battery isn't even "broken."
Lithium-ion tech is fickle. These 20V Max (which are actually 18V nominal, let's be real) and FlexVolt packs have brains of their own. Sometimes they just need a little nudge to wake up. Before you drop $150 on a new 5.0Ah stack at the big-box store, you need to understand why the charger is giving you the cold shoulder. It usually comes down to heat, deep discharge, or just plain old dust.
The "False Start" and the Fast Blink Mystery
When you see that red light flashing like a strobe light at a 90s warehouse rave, the charger is trying to tell you something. Usually, it’s saying "Hot/Cold Delay." If you’ve been pushing your circular saw through wet pressure-treated lumber, the cells inside that plastic casing are cooking.
DeWalt’s internal sensors won't let the charger touch the cells if they are over a certain temperature—usually around 104°F (40°C). It’s a safety thing. If it charged while hot, the battery might literally vent or catch fire. If you see that fast blink, just walk away. Put the battery on a cool concrete floor for twenty minutes. Don't put it in the freezer; condensation is a silent killer for circuit boards.
Why your charger thinks the battery is dead (but it isn't)
Sometimes the charger doesn't blink at all. You slide the battery in, and... nothing. This often happens because of a "deep discharge."
Every DeWalt 20V battery has a Printed Circuit Board (PCB) inside. This board needs a tiny bit of voltage to "talk" to the charger. If you used the tool until it completely stopped moving, and then let that battery sit in a cold garage for three months, the voltage might have dropped below the "low-voltage threshold."
Basically, the battery is so dead it can't even tell the charger it exists. The charger sees 0 volts and assumes there’s no battery there at all. It’s a classic tech catch-22.
The Jumpstart Trick: Reviving a "Brick"
If your DeWalt battery is not charging because the voltage is too low, you can sometimes trick it back to life. You’ll need a second battery—one that is fully charged and has the same voltage (e.g., both 20V Max).
You essentially "jump" the dead one using a couple of pieces of speaker wire or even paperclips. You connect positive to positive (B+) and negative to negative (B-). Hold them there for about 30 to 60 seconds. What you’re doing is transferring just enough juice into the dead cells to kick the PCB into gear.
Wait. Be careful. Seriously. If you cross those wires, you’re going to see sparks, and you might fry the protection circuit. This isn't an official DeWalt-sanctioned repair, but ask any contractor who’s been in the game for ten years, and they’ve likely done this to save a $200 FlexVolt pack. Once you’ve boosted the voltage, slide it back onto the official charger. Usually, the red light will start its slow, happy pulse.
The "Ohmic" Resistance Factor
Sometimes the issue is physical. Take a look at the terminals on the battery and the charger. See that black gunk? That’s a mix of sawdust, moisture, and "arcing" residue. Even a thin layer of oxidation can increase the resistance enough that the charger thinks the battery is faulty.
Grab a Q-tip and some high-percentage Isopropyl Alcohol. Scrub those copper contacts until they shine. It's a simple fix, but it's the one most people skip before they give up and buy a replacement.
When the Cells Actually Fail
Look, batteries don't last forever. Most lithium cells are rated for about 300 to 500 charge cycles. If you’re a pro using these every single day, you’re going to hit that wall in about two or three years.
Inside a standard DCB205 (the 5.0Ah workhorse), there are ten 18650 cells arranged in two parallel rows of five. If just one of those cells goes "open" or shorts out, the whole pack is basically toast for the average user.
You can check this with a multimeter.
- Set your meter to DC Volts.
- Touch the probes to the outer terminals (B+ and B-).
- A healthy "dead" battery should still read around 15V to 16V.
- If you see 4V or 6V, you have a dead cell.
At that point, unless you’re comfortable soldering high-drain 18650s (which is dangerous if you don't know what you're doing), it’s time to recycle the pack.
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The Third-Party Charger Problem
We’ve all seen them on Amazon. The $25 "Yellow-Adjacent" chargers that look exactly like the DeWalt DCB115 but cost a fraction of the price.
Don't do it.
Genuine DeWalt chargers use a specific communication protocol to monitor the balance of the cells. Cheap knockoffs often just "dumb charge" by shoving current into the pack. This leads to unbalanced cells, where some are sitting at 4.2V and others are at 3.8V. Eventually, the battery’s internal brain realizes things are lopsided and shuts down the pack for safety. If you’ve been using a cheap charger and your DeWalt battery is not charging now, the charger might have actually "killed" the battery’s balance.
Is it the charger or the battery?
Before you blame the battery, check the charger’s cord. These things get stepped on, pinched in toolboxes, and chewed by jobsite pests. If the cord is frayed, the charger might be getting intermittent power. Also, try a different outlet. It sounds stupid, but a tripped GFCI on a jobsite has fooled many a seasoned foreman.
Action Steps to Save Your Battery
Don't just stare at the blinking light. Follow this sequence to diagnose and hopefully fix the issue without spending a dime.
- The Temperature Reset: If the battery was just in a high-draw tool (grinder, saw, vacuum), let it sit in a room-temperature environment for at least an hour.
- The "Stage 1" Clean: Use compressed air to blow out the charger ports and the battery terminals. Follow up with alcohol on a cotton swab.
- The Reseat Maneuver: Sometimes the battery isn't clicked in all the way. Slam it home. Not enough to break the plastic, but enough to ensure the terminals are fully mated.
- The Jumpstart (Advanced): If the battery is showing 0V on a multimeter, use a donor battery and jumper wires to bridge the B+ and B- terminals for 30 seconds.
- Check the Fuse: Some older DeWalt chargers have an internal fuse. If the charger has no lights at all when plugged in, it’s likely a blown fuse or a dead transformer in the base.
- Warranty Check: DeWalt generally offers a 3-year limited warranty on batteries. If your date code (stamped on the top or bottom) is within that window, call 1-800-4-DEWALT. They are surprisingly good about sending replacements if you haven't clearly abused the pack.
If you’ve tried all of this and the pack is still dead, it’s likely a hardware failure of the internal BMS (Battery Management System). At this stage, the most responsible move is to take it to a local Home Depot or Lowes for their battery recycling program. Lithium batteries in the trash are a major fire hazard for garbage trucks.
To prevent this from happening again, stop running your batteries until the tool completely dies. When you feel the power start to sag, swap the battery. Lithium-ion hates being drained to zero. Store them in a dry place, and never leave them on the charger for weeks at a time during the off-season. Keep the cells happy, and they’ll keep your tools spinning.