Milwaukee Batteries Not Charging – How To Revive Dead M12 And M18

ByJim Boslice Metalworking & Fabrication

If your Milwaukee battery isn’t charging, it is likely “tripped” into a sleep mode due to over-discharge or dirty terminals. You can often fix this by cleaning the metal contacts with rubbing alcohol or “jump-starting” the dead battery using a fully charged one and two jumper wires to boost the voltage back to a recognizable level.

A flashing red and green light on your charger usually indicates the battery is too cold, too hot, or the voltage has dropped below the charger’s minimum threshold for safety. Most of the time, this does not mean the battery is permanently broken.

Nothing kills the momentum of a Saturday morning project like grabbing a fresh pack from the shelf only to realize your milwaukee batteries not charging properly. You see that dreaded flashing red and green light on the charger, and suddenly, your impact driver is nothing more than a paperweight. I’ve been there myself, standing in a cold garage with a half-finished workbench, wondering why a premium tool system is giving me the “Christmas tree” treatment.

The good news is that these lithium-ion packs are smarter than we often give them credit for, and that intelligence is usually what’s stopping the charge. Whether you are a weekend woodworker or a professional fabricator, understanding how the Milwaukee RedLithium system communicates with its charger is the first step to saving a hundred dollars on a replacement pack.

In this guide, I’m going to walk you through the exact steps I use in my own workshop to diagnose and revive these batteries. We’ll cover everything from simple cleaning to the famous “jump-start” method that brings “dead” cells back to life. By the time we’re done, you’ll have a clear plan of action to get your tools back in the game safely and efficiently.

Understanding Why Your Milwaukee Batteries Are Not Charging

When you encounter a situation where you find your milwaukee batteries not charging, it is rarely a total hardware failure. Most often, the Battery Management System (BMS) inside the pack has triggered a safety lockout. This happens when the voltage of the individual cells drops below a certain threshold, usually around 2.0 to 2.5 volts per cell.

The charger is programmed to refuse a charge if the voltage is too low because forcing high current into a deeply depleted lithium-ion cell can be unstable. This “sleep mode” is a protective measure designed to prevent fires or cell damage. However, for a DIYer, it looks like a broken battery that needs to be tossed in the recycling bin.

Other common culprits include dirty terminals, extreme temperatures, or a faulty charger base. If you’ve been working in a freezing garage or leaving your tools in a hot truck, the internal thermistor will tell the charger to wait until the chemistry stabilizes. Understanding these triggers is the key to choosing the right fix.

What Those Flashing Lights Really Mean

Before you start tearing things apart, you need to decode the language of your Milwaukee charger. These lights are your first line of diagnostic data, and they tell a specific story about the health of your M12 or M18 packs.

  • Solid Red Light: The battery is currently charging normally. No action is required.
  • Solid Green Light: The battery is fully charged and ready for use.
  • Flashing Red Light: The battery is either too hot or too cold. The charger will wait for the battery to reach room temperature before starting.
  • Flashing Red and Green Light: This is the “damaged or defective” signal. It usually means the voltage is too low for the charger to recognize, or there is a communication error between the BMS and the charger.

If you see the red and green flashing, don’t panic. It doesn’t always mean the battery is “junk.” It simply means the charger is refusing to talk to it. In the next sections, we will look at how to force that communication back open.

The “Jump-Start” Method for Reviving Dead Lithium-Ion Cells

This is the most common fix for milwaukee batteries not charging after they have been left in a tool for too long. When a battery sits, the small parasitic draw from the tool or the internal BMS can pull the voltage down past the “point of no return” for the standard charger. We can fix this by “jumping” the dead battery with a healthy one.

Tools You Will Need

To perform this safely, you’ll need a fully charged Milwaukee battery of the same voltage (M18 to M18, or M12 to M12), and two short lengths of 12-gauge or 14-gauge copper wire. I prefer using solid-core wire because it stays in the terminals better than stranded wire.

Step-by-Step Jump Procedure

  1. Identify the Terminals: Look at the top of your battery packs. You will see a positive (+) and a negative (-) symbol near the slots.
  2. Connect Positive to Positive: Insert one end of your first wire into the (+) terminal of the good battery and the other end into the (+) terminal of the dead battery.
  3. Connect Negative to Negative: Insert the second wire into the (-) terminal of the good battery and then into the (-) terminal of the dead battery.
  4. Wait 30 to 60 Seconds: You are essentially equalizing the charge. The good battery will dump a small amount of voltage into the dead one, hopefully raising it above the 2.5V-per-cell threshold.
  5. Test the Charge: Remove the wires and immediately place the “dead” battery on the official Milwaukee charger. If the light stays solid red, you’ve successfully revived it.

Safety Warning: Do not leave the batteries connected for more than a minute. If the dead battery starts to feel hot to the touch, disconnect immediately. Always wear safety glasses when working with live electrical components in the workshop.

Cleaning and Maintenance: Keeping Your Terminals Conductive

Sometimes the reason for milwaukee batteries not charging is much simpler than a voltage drop. In a woodworking or masonry environment, fine dust and debris can coat the metal contacts. Even a thin layer of sawdust or oxidation can increase resistance enough to disrupt the charging cycle.

I recommend cleaning your terminals every few months, especially if you work with metal grinding or concrete. Use a cotton swab dipped in 90% isopropyl alcohol to wipe down the copper contacts on both the battery and the charger. If you see visible corrosion (green or black buildup), you can use a small piece of fine-grit sandpaper or a contact cleaning spray like DeoxIT.

Make sure the metal “leaves” inside the battery slots aren’t bent or pushed too far back. If they aren’t making firm contact with the charger pins, the BMS won’t be able to send the “okay” signal to start the flow of electricity. A small pick can be used to gently bend them back into place, provided the battery is handled carefully.

Charger Troubleshooting: Is the Problem the Battery or the Base?

We often blame the battery because it’s the part we handle most, but the charger itself can fail. Milwaukee chargers have internal fuses and capacitors that can blow during a power surge. If you have multiple batteries and none of them are charging, the issue is likely the base.

Check the power cord for any nicks or frays. In a busy shop, cords get stepped on or pinched by heavy machinery. If the cord is fine, try plugging the charger into a different outlet on a different circuit. Occasionally, a GFCI outlet in a garage can trip, providing just enough phantom voltage to light an LED but not enough to charge a high-capacity M18 pack.

If the charger smells like burnt electronics (that distinct acrid ozone smell), it’s time to replace it. These units are generally not serviceable for the average DIYer due to the complexity of the circuit boards and the risk of high-voltage shock.

Advanced Fixes: Checking Individual Cell Voltage

If the jump-start didn’t work, you might have a single “bad” cell within the pack. Milwaukee batteries are made up of several 18650 or 21700 lithium-ion cells wired in series and parallel. If one cell group fails, the whole pack shuts down.

To diagnose this, you will need to open the case using Security Torx (T10) bits. Once inside, use a multimeter set to DC voltage to test each cell group. A healthy cell should read between 3.0V and 4.2V. If you find one group reading 0V or significantly lower than the others, that specific cell has likely failed.

While it is possible to replace individual cells, I generally don’t recommend it for beginners. It requires a spot welder—never solder directly to lithium cells, as the heat can cause an explosion. If you find a dead cell group, it’s usually a sign that the pack has reached the end of its functional life.

Best Practices for Milwaukee Battery Longevity

To avoid the headache of milwaukee batteries not charging in the future, a little preventative care goes a long way. Lithium-ion chemistry is different from the old NiCad batteries we used to use; they don’t have a “memory,” but they do have “stressors.”

  • Avoid Deep Discharge: Stop using the tool as soon as you notice a drop in power. Don’t pull the trigger repeatedly to try and squeeze out one last screw.
  • Store at Room Temperature: Don’t leave your batteries in the van during a blizzard or a heatwave. Extreme temperatures degrade the electrolyte inside the cells.
  • Partial Charges are Fine: You don’t need to wait for the battery to be empty to charge it. In fact, keeping them between 20% and 80% charge is the “sweet spot” for long-term health.
  • Use Them Regularly: Batteries that sit for a year without use are much more likely to fall into “sleep mode.” Rotate your packs so they all get a workout.

Frequently Asked Questions About Milwaukee Batteries Not Charging

Why is my Milwaukee charger flashing red and green even with a new battery?

This is often due to the battery being in “shipping mode” or the terminals having a protective coating from the factory. Try sliding the battery in and out of the charger 5-10 times rapidly to “scrub” the contacts, or check if the battery is too cold from being in a delivery truck.

Can I fix a Milwaukee battery that got wet?

If a battery gets wet, do not put it on the charger. Open the case if possible and let it dry out for at least 48 hours in a warm, dry area. Use compressed air to blow out moisture. If you see signs of corrosion on the circuit board, it may be unsafe to use.

Is the “freezer trick” real for Milwaukee batteries?

The “freezer trick” was more effective for old NiCad batteries. For modern Lithium-Ion Milwaukee packs, putting them in the freezer is generally not recommended and can actually cause moisture to condense inside the electronics, leading to a short circuit.

How do I know if my Milwaukee battery is under warranty?

Most Milwaukee RedLithium batteries carry a 2-year or 3-year warranty from the date of purchase or the date of manufacture (encoded in the serial number). If your milwaukee batteries not charging despite your best efforts, check the Milwaukee website to see if you qualify for a free replacement.

Reviving Your Power Tools for the Long Haul

Dealing with a battery that won’t take a charge is a rite of passage for any serious DIYer or tradesman. While it’s frustrating, most of the time it’s just the battery’s way of asking for a little maintenance. By understanding the light codes, keeping your contacts clean, and knowing how to safely jump-start a sleeping pack, you can save yourself a lot of money and downtime.

Remember, safety should always be your priority. If a battery is physically cracked, leaking fluid, or smelling like sweet chemicals, do not attempt to charge or fix it. Take it to a local hardware store for proper recycling. But for those packs that are just “tired,” the steps we discussed today will usually get them back into the rotation.

Keep your tools clean, your batteries at room temperature, and don’t be afraid to break out the multimeter when things go sideways. Your workshop is only as good as the power behind your tools—so keep those RedLithium packs healthy and ready for the next project!

Jim Boslice
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