When you slide a Milwaukee battery onto its charger, a solid red light means everything is working correctly. Milwaukee battery flashing red and green lights signals a common error. This issue frequently happens with M18 tools like lawn mowers and compact M12 power tools, and it can usually be fixed. This guide explains what causes the problem and gives simple step‑by‑step solutions to get your battery charging again.
Why Is My Milwaukee Battery Flashing Red and Green?
Why is my Milwaukee battery flashing red and green? When your charger blinks red and green, it’s letting you know something is wrong. The charger won’t start charging safely. This built‑in safety feature keeps your battery and charger from getting damaged. Sometimes it points to a big internal issue, but more often it’s caused by simple problems you can fix.
- Bad Connection: The metal pins on the battery and charger need to line up and touch properly. If the battery isn’t pushed all the way in, or dirt, grease and rust cover the contacts, the charger can’t make a good link and throws an error.
- Battery Too Hot or Cold: Lithium batteries don’t handle extreme temperatures well. If the battery gets really hot after heavy work such as mowing the whole yard, or it freezes overnight in your car during winter, its built‑in protection stops charging. Charging at the wrong temperature can permanently ruin the battery.
- Battery Drained Too Far: When a battery runs completely dead and sits unused for a long while, its voltage drops too low. Smart chargers can’t wake up this deeply drained battery. They treat it as broken so they won’t charge an unstable battery pack.
- Broken Battery or Charger: Occasionally, the blinking lights mean serious trouble. One of the battery cells might fail, an internal wire could break, or the circuit board inside the battery or charger stops working.
How to Fix Milwaukee Battery Flashing Red And Green?
Before you jump into harder fixes, try these easy, low‑risk steps first. They fix this issue most of the time.
Step 1: Push the Battery in Firmly
Milwaukee batteries don’t always make a clear “click” when they sit right. You might think it’s locked in, but it’s not really connected.
Take the battery off the charger. Push it back in nice and steady with some force. A quick firm nudge, even a light tap, can help all the metal pins make good contact. If you get a steady red light, you’re all set.
Step 2: Clean the Metal Contact Points
Dust, sawdust, or light rust can build up on the metal pins of your battery and charger over time.
Unplug the charger from the wall. Dab a cotton swab with rubbing alcohol and wipe down all the metal contacts on the battery and inside the charger slot. Wait about one minute for the alcohol to fully dry. Plug the charger back in and test it again.
Step 3: Let the Battery Warm Up or Cool Down to Room Temperature
If you just used the battery hard on a tool, or it sat somewhere freezing cold, temperature is probably causing the error.
Take the battery off the charger and bring it inside. Leave it out for 30‑45 minutes so it gets to normal room temperature (roughly 65‑75°F / 18‑24°C). Don’t use a heater or freezer to speed this up — you could damage the battery. Once it’s settled to room temperature, put it back on the charger.
Step 4: Figure Out If It’s the Battery or the Charger
If nothing above works, you need to check which piece is broken.
- Test your charger: Put in another Milwaukee battery that you know works. If it charges normally, your charger is okay, and your original battery is the problem.
- Test your bad battery: If you have another matching charger, try charging the faulty battery on that one.
- Reset the charger: Sometimes the charger’s brain just needs a restart. Unplug it for at least two minutes, plug it back in, and give it another go.
How to Reset a Milwaukee Battery?
If you think your battery is totally drained, there are two ways to try and wake it up. One is fairly safe, but the other has big dangers.
Method 1: Wake It Up with Quick On‑And‑Off Tries
This trick uses the charger’s built‑in safety system to send tiny bursts of power. It can raise the battery voltage high enough for normal charging.
- Slide the battery onto the charger. As soon as the red and green lights start blinking, pull the battery right off. Wait a few seconds, then slide it back on again.
- Repeat this 5 to 10 times. These short connections can get the charger to send just enough power to fix the low‑voltage error. If it works, the light will turn solid red and regular charging will start.
Method 2: Jump‑Start the Battery (Use Extreme Care)
This uses a good, fully charged battery to add a little power to the dead one. Lots of online videos show this trick, but it turns off the battery’s safety protections and can be very dangerous.
Warning: You might get electric sparks, break the battery’s internal control board, or cause overheating. In the worst cases, the battery can catch fire or explode. Only try this if you fully understand and accept these risks.
- What you need: A fully charged Milwaukee battery (the donor battery) and simple jumper wire, like speaker wire fitted with spade connectors.
- Find the terminals: Locate the main positive (+) and negative (-) contacts on both batteries.
- Hook them up: Connect positive on the good battery to positive on the dead battery. Then link negative to negative. A small spark may appear when you make the connection.
- Wait a few minutes: Keep them connected for 5‑10 minutes. This adds a small surface charge.
- Unplug and test: Carefully remove the jumper wires and put the weak battery straight onto the charger. If the jump worked, it should charge normally.
Milwaukee M12 Battery Flashing Red and Green
Milwaukee M12 batteries power smaller handheld tools, and they use the same basic design as M18 batteries. That means they run into the very same charging problems. If your M12 battery blinks red and green on the charger, the reasons and fixes are exactly the same.
How to Fix a Milwaukee M12 Battery Flashing Red and Green?
Since they work the same way, you troubleshoot them the same way too. M12 batteries are smaller, so they often don’t lock in properly. This happens even more when the charger is hung vertically on a wall.
- Push the battery in all the way: Slide your M12 battery firmly into the charger until it sits tight and makes full contact.
- Clean the metal contacts: Grab a cotton swab and rubbing alcohol to wipe off the terminals.
- Check the battery temperature: Let it cool or warm up until it reaches normal room temperature.
- Try waking it up: Use the quick plug‑in and pull‑out trick we covered earlier to send tiny bursts of charge.
Milwaukee M18 Battery Flashing Red and Green Fix
If you use M18 batteries on power‑hungry tools such as lawn mowers or heavy‑duty impact wrenches, stopping problems before they start beats fixing them later. That blinking red‑and‑green warning usually comes from bad connections or extreme temperatures, but draining your battery all the way over and over will wear it out faster. Use these easy tips to keep your M18 batteries in good shape and avoid that annoying flashing error:
- Avoid Complete Draining. Stop using your tool once power starts fading or only one charge bar remains. That’s the right time to recharge it.
- Store batteries correctly during long breaks, like winter. Leave them about half charged, between 40% and 60%. Leaving them fully dead or fully charged for months hurts battery performance.
- Keep batteries and your charger clean. Wipe them regularly, and pay extra attention to the metal contacts. Dirt and moisture build‑up causes charging troubles.
Final Thoughts
That flashing red‑and‑green light on your Milwaukee battery happens a lot, but you can often fix it on your own. Follow our troubleshooting tips, and most issues get resolved. For cheap, reliable aftermarket replacement parts that fit many models, check out FridayParts.
