Power Tool Battery Not Charging? Here's How to Diagnose and Fix It
You plug your battery into the charger, expecting that satisfying green light. Instead — nothing. No lights, no blinking, no charge. Maybe you've even tried another charger, another outlet, and still the same problem.
I've been there myself, standing in the middle of a job site with a dead battery and a deadline. Before you toss that battery in the trash and drop another $80 on a replacement, let me walk you through what's actually going wrong and how to fix it. Nine times out of ten, the problem is simpler than you think.
Step 1: Rule Out the Obvious (You'd Be Surprised)
Start with the basics before you break out any tools.
Check the charger first. Is the charger itself getting power? Plug it into a different outlet. If it's connected to an extension cord or power strip, try plugging it directly into a wall outlet. Some chargers have a small LED that lights up even without a battery — if that's dead, your charger might be the problem, not the battery.
Check the contacts. Look at the metal terminals on both the battery and the charger. Are they clean? Dirt, grease, sawdust, and corrosion can block the connection. Wipe them with a dry cloth first. If there's visible corrosion (white or green crust), a bit of isopropyl alcohol on a cotton swab will clean them right up.
Temperature check. Lithium-ion batteries have built-in temperature protection. If your battery is hot from heavy use or cold from sitting in a freezing garage overnight, the charger won't send power to it. Let a hot battery cool down for 30-60 minutes. If it's cold, bring it inside and let it warm up to room temperature before trying again.
Real-world tip: I once spent 20 minutes troubleshooting a "dead" Makita battery only to realize the charger was plugged into a switched outlet that someone had turned off. Always check the outlet first.
Step 2: Measure the Voltage with a Multimeter
This is where you actually figure out what's going on inside the battery. Every contractor should own a basic multimeter — they're cheap ($15-20) and save you from guessing.
Set your multimeter to DC voltage (20V range for most power tool batteries). Touch the red probe to the positive terminal and the black probe to the negative terminal. Here's what the readings mean:
- 18V-21V (for an 18V/20V Max battery): Battery is healthy. The issue is likely the charger.
- 12V-17V: Battery is partially discharged but should still charge. May respond to a "jump start" (Step 4).
- Under 10V: The battery management system (BMS) has cut off the output to protect the cells. This is the most common "won't charge" scenario and often fixable.
- 0V or near zero: Could be a broken connection inside the battery or the BMS has fully locked it out. Still worth trying Step 4 before giving up.
Pro tip: Some batteries have extra "communication" terminals between the main power terminals. Don't confuse those with the positive/negative terminals. Look for the (+) and (-) markings on the battery casing.
Step 3: Check for a Tripped BMS (Battery Management System)
This is the #1 reason lithium-ion power tool batteries stop charging. Every lithium battery has a small circuit board called a BMS that protects the cells from:
- Over-discharge (draining too low)
- Over-charge
- Short circuits
- Overheating
When your tool shuts off because the battery is "dead," the BMS actually cuts power while there's still a small reserve in the cells. That's normal. But if you store the battery for months without charging, or if you run a tool until it absolutely stalls, the voltage can drop low enough that the BMS goes into full protection mode.
The good news? The BMS isn't permanently locked — it just needs to "see" enough voltage to wake up and allow charging.
Step 4: Jump-Start a Deeply Discharged Battery (The "Zombie" Fix)
⚠️ Safety first: Only attempt this if you're comfortable with basic electrical work. Work on a non-flammable surface, wear safety glasses, and never leave a jump-started battery unattended while charging.
Here's the method I've used dozens of times:
- Find a known-good battery of the same voltage (ideally the same brand).
- Briefly connect the good battery to the dead battery using short jumper wires — positive to positive, negative to negative. Hold the connection for only 5-10 seconds.
- Remove the jump wires and check the dead battery's voltage. It should have climbed above 10V.
- Put the dead battery on its original charger immediately.
- The metal terminal strips
- The solder points where terminals meet the PCB
- The charger's contact pins (use a pencil eraser for gentle abrasion)
- Battery case is swollen or cracked → Replace immediately. Swollen cells are a safety hazard.
- Voltage reads 0V after jump-start attempt → The BMS has permanently failed, or internal connections are broken.
- Battery charges but dies in 5 minutes → The cells have reached end of life. Lithium batteries typically last 300-500 charge cycles.
- Battery gets hot during charging → Stop using it. Internal short circuit risk.
What's happening: You're giving the BMS enough voltage to "wake up" and recognize that charging is safe. Once the charger detects a voltage it can work with, it takes over normally.
Alternative method: If you have a bench power supply, set it to the battery's nominal voltage (18V for 18V batteries) with a 1-amp current limit. Connect for 30-60 seconds until the voltage rises above 10V.
⚠️ Common mistake to avoid: Don't use a car battery charger on power tool batteries. Car chargers push too much current and can damage the cells or cause overheating. Stick to the jumper method or a proper bench supply.
Step 5: Clean and Reseat the Terminal Connections
Even if the terminals look clean to the naked eye, corrosion can build up inside the plastic housing. Pop the battery casing open (carefully — some use clips, others need a small flathead screwdriver) and inspect where the metal terminals connect to the circuit board.
Use contact cleaner or isopropyl alcohol to clean:
I've brought back more than a few Milwaukee M18 and Dewalt batteries that just needed their contacts cleaned after sitting in a dusty workshop for months.
Step 6: Replace a Single Failed Cell (Advanced)
If you've measured the overall battery voltage and it reads healthy (above 15V for an 18V pack), but individual 18650 cells inside are at different voltages, you might have a single bad cell dragging the whole pack down.
Open the battery case and measure the voltage of each cell group. On most Dewalt and Makita batteries, this means measuring across each pair of nickel strips. If one cell group reads significantly lower than the others (more than 0.5V difference), that cell is failing.
Can you replace it? If you've got a spot welder and some experience with lithium cells, yes. But honestly, for most people, it's safer to check our guide on signs your battery needs replacing and consider a quality replacement battery. Single-cell replacement involves soldering near lithium cells, which carries fire risk if done incorrectly.
Common Myths and Mistakes Guys Make
Myth: "Tapping the battery on the bench fixes it."
Nope. That might temporarily reseat a loose connection, but it doesn't fix the underlying problem. And you can damage the cells inside.
Myth: "Putting a dead battery in the freezer revives it."
This is an old trick from the NiCad days. For lithium-ion, freezing can actually cause condensation inside the battery, leading to shorts and corrosion.
Myth: "Leave it on the charger overnight and it will eventually wake up."
If the BMS has tripped, the charger won't push any current at all. Leaving it on the charger for days won't help and could strain the charger.
Biggest mistake: Forcing the battery onto a charger. If it doesn't click in easily, check the alignment. Don't jam it — you can bend the charger pins or crack the battery housing.
When to Give Up and Get a Replacement
Not every battery can be saved. After you've gone through the steps above:
If your battery falls into any of these categories, check out our lithium battery safety guide for proper disposal and replacement options.
FAQ
Can I use a higher AH battery in my drill after fixing the old one?
Yes. Higher AH (amp-hour) batteries have more capacity and will run longer. They're physically larger but work fine in any tool designed for that voltage. Just make sure the voltage matches.
Why is my Milwaukee battery flashing red and green?
That's the charger telling you something's wrong. Red and green alternating usually means a bad cell or a temperature issue. Let the battery cool down and try again. If it persists, test the cells with a multimeter.
How do I reset a Dewalt battery charger?
Unplug the charger for 5 minutes, plug it back in, insert the battery firmly. If the charger still won't recognize the battery, the issue is likely with the battery's BMS, not the charger.
Is it bad to leave lithium batteries on the charger all the time?
Modern chargers stop charging once the battery is full, so it's generally safe. But for long-term storage (weeks or months), keep the battery at 40-60% charge in a cool, dry place — not fully charged on the charger.
How many charge cycles does a lithium power tool battery last?
Most quality lithium-ion batteries last 300-500 full charge cycles before capacity noticeably drops. After that, they still work but won't hold as much charge.
*Got a battery you're trying to revive? Drop the model and voltage in the comments and I'll point you to the right fix.*
If your packs keep draining fast or the charger is acting up, we’ve rounded up the best replacement battery chargers for Dewalt, Makita and Milwaukee, plus a quick guide to how long lithium power tool batteries really last.