I recently saw some complaints from a few different users about their Milwaukee m12 battery not charging, all while a red and green light is flashing on the charger.
Are you one of the people encountering this? If your answer is yes, join me for this article as I’ll be going through the reasons for this. That should help you narrow down the problem and handle it.
Related: Common Milwaukee M12 Hammer Drill Problems Troubleshooting
What Does The Flashing Red and Green Light Mean?
I have made a video about what this means and you can watch it below.
To fix this problem, you need another similar healthy battery with the same specs. Connect the positive and negative terminals of the 2 batteries using two wires.
This will jump start the battery and reset it. Then, leave it for around 15 seconds and disconnect it. Finally, try to recharge it on the Milwaukee charger.
Milwaukee battery chargers come with light indicators. These serve as a signal that informs you about the current affairs of whatever battery you’re currently charging.
Different types of light mean different things. But, what do the flashing red and green lights mean specifically, and why are you seeing them right now? Well, you’re not going to like the answer.
However, this is still better than if your Milwaukee M12 charger isn’t working and it also shows no light.
And in case you’re curious about the meaning of each light indicator on a Milwaukee M12 battery I also have a video about it. You can watch it below.
You could see 4 different light indicators on a Milwaukee M12 battery when you charge it. A solid red means it’s still charging while a solid green means it’s already fully charged.
Related: How I Solved My Milwaukee M12 Battery Shows Green but Not Charging to Full Problem
If it displays a flashing red and green it means it’s already a bad battery. But if it displays only a flashing red it indicates a hot/cold relay.
This specific type of light is a warning from the Milwaukee battery charger that you’re trying to power up a dead battery. The battery isn’t charging, as there’s no power flowing through.
While that’s what this indicator means, that doesn’t need to be the case every time.
There are two or three other possibilities that may be the reason why your Milwaukee M12 battery is not charging while red and green lights are flashing. So, don’t lose all hope before you read more!
Read: Common Milwaukee M12 Impact Wrench Problems Troubleshooting
4 Ways To Deal With A Milwaukee M12 Battery Not Charging
- Push Down The Battery Harder
Even though the red and green lights signal a dead battery, there’s a more common cause for them. This cause is a result of a user error that’s so very common, especially among new users.
So many people don’t know how hard they actually have to press down the Milwaukee 12 battery on the charger so that it sits properly. If you don’t use a lot of force, that means it’s still loose.
There will be a distinct clicking sound, and the battery will slide all the way in. So, do as I’ve done in this image and push your Milwaukee M12 battery hard into the charger until it’s fully seated.
Some people don’t push their battery hard enough for it to fit on the prongs. This stops the charger from sending power into the battery, which is why it’s not charging and there’s also no light on its indicator LED.
Before doing anything else, I suggest applying force on the Milwaukee M12 battery while fitting it into the charger, just to rule out whether or not this common cause is the reason for your worries.
- Debris Can Affect The Connection
I told you how the previous issue stops power from flowing through the Milwaukee M12 battery and charger. Another thing with similar effects is when there’s debris on the prongs.
It doesn’t matter where the dirt or debris came from. As long as it’s there, it might get in the way of your Milwaukee M12 battery and its charger, which means it won’t charge.
Ruling this out only requires cleaning the prongs on the charger.
Remove the battery and unplug the charger. Then use a cloth on the prongs, which I’ve highlighted in the photo below so you know what I mean.
Using a blower might also help since some tinier, more stubborn pieces of debris are hard to get out with a cloth. Once you do this, anything that was there should be gone.
Clean the part of your Milwaukee M12 battery that connects to the charger as well. There are similar prongs there that might be dirty.
While you’re doing all this, I advise you to check to see if the prongs on your Milwaukee M12 battery or charger are bent or have any other complications, as that can stop power flow too.
- Too Cold or Overheating
If you look at the lights on your Milwaukee battery charger, you’ll note that it has a different type of indicator for overheating. So, why am I listing this here as a possible reason for your issue?
Well, overheating, or even “over-cooling”, are both things that stop proper flow of power within your Milwaukee M12 battery. That’s why there are suggested temperature ranges in the tool’s manual.
I already mentioned how a bad flow of power can make the red and green lights flash since the charger thinks it’s trying to give power to a dead battery.
So, sometimes, the red and green lights flash instead of the overheating ones. This is especially the case if your Milwaukee M12 is too cold, or even wet, possibly.
You’ll be able to tell if it’s just normal overheating with the indicator lights on your charger. You can even see them on my own Milwaukee battery charger in the image down here.
In any case, you need to bring the battery back to a normal temperature. If it’s wet or has vapor on it of any kind, immediately cut off power to the battery and charger. Then, set them away.
Don’t use either the battery or the charger until you can tell for sure that their temperature is back to normal, and they aren’t cold, wet, or too hot in any way.
- The Battery Really Is Dead
We’re back to square one if none of the things I mentioned so far were of any help. I told you at the start that the red and green lights signal a dead battery, and that’s likely what you have.
If your Milwaukee M12 is a few years old or has been put through a lot of use, it’s not unlikely for it to die. But, you can revive even a dead battery as long as you have the right things.
What you’ll need is another Milwaukee M12 battery. You can try to borrow one from a friend, a local mechanic, or anyone else willing to lend you one.
Then, use the healthy battery and connect its positive terminals to the positive terminals of your dead battery. Do the same thing for the negative terminals too.
I have a video which can help you know more about the battery terminals of the Milwaukee M12 battery. You can watch it below.
You can see on the video that there are 5 terminals on a Milwaukee M12 battery. I also showed there a diagram of those terminals.
Those 5 terminals are the negative terminal – (C1-), positive terminal – (C3+), contact point between cell C3 and C2 – (C2+), contact point between cell C1 and C2 – (C1+) thermistor, and (T) for temperature monitoring and overheating protection.
Leave it like this for a few minutes, and your Milwaukee M12 battery will start charging again. There won’t be any green and red flashing lights to bother you either!
