A flashing red and green light means your battery is too hot or too cold to charge safely
Milwaukee batteries flash red and green together when the internal temperature is outside the safe charging range. This is a protection feature built into the charger, not a sign the battery is broken. The charger detects the battery's temperature and stops charging until it returns to a safe zone — typically between 32°F and 104°F (0°C to 40°C), depending on your model.
The battery itself is fine. What you are seeing is the charger refusing to work until conditions improve. This can happen in winter, in a hot garage, or after heavy use when the battery is still warm from the last job.
Key Takeaways
- Red and green flashing together means temperature is out of range; move the battery to a warmer or cooler location and wait 15 to 30 minutes before trying again.
- Do not force the charger to work by removing and reinserting the battery repeatedly — this does not override the safety lock.
- If the battery flashes red and green even at room temperature after it has cooled completely, the charger or battery may have a fault that needs replacement.
- Milwaukee chargers have different temperature thresholds depending on the model, so check your manual for the exact range your charger uses.
Why Milwaukee batteries have temperature protection
Lithium-ion batteries charge through a chemical reaction that generates heat. If that reaction happens outside a safe temperature window, the battery can be damaged or fail to hold a charge. Charging a cold battery can cause permanent loss of capacity. Charging a hot battery can shorten its lifespan or cause the cells to swell.
Milwaukee's chargers include a thermistor — a temperature sensor — that reads the battery's internal heat before allowing current to flow. When the sensor detects a temperature outside the safe zone, it locks the charger and flashes red and green to tell you why it stopped.
What to do when your battery flashes red and green
First, remove the battery from the charger and let it sit at room temperature. If the battery is cold, move it to a warmer spot — inside the house, a heated garage, or a warm vehicle. If the battery is hot from recent use, move it to a cooler location and give it time to cool. Most batteries return to a chargeable temperature within 15 to 30 minutes.
Do not repeatedly remove and reinsert the battery trying to force the charger to work. The sensor will read the same temperature each time, and the charger will flash red and green again. Patience is the only fix here.
Once the battery has been at room temperature for at least 30 minutes, place it back in the charger. If the light changes to solid red or solid green, the charger has accepted it and charging is underway. If it flashes red and green again, the battery temperature is still out of range.
Solid red versus flashing red and green
A solid red light means the battery is charging normally. A flashing red light alone (not green) usually means the battery has a fault — a bad cell, internal short, or damaged circuit board — and should not be used. A solid green light means charging is complete.
The combination of red and green flashing together is specifically the temperature warning. No other Milwaukee charger light pattern means the same thing. If you see red and green flashing at the same time, temperature is the problem.
When temperature protection is a real problem
If you live in a climate where winter temperatures regularly drop below 32°F, you may find your batteries will not charge outdoors in cold months. The solution is to store and charge batteries indoors. Many contractors keep batteries in a heated shop or truck cab during winter for this reason.
In very hot climates, batteries that have just finished a heavy job may be too warm to charge when ready. Let them cool for 30 minutes before putting them back on the charger. This is normal and does not mean anything is wrong.
When the flashing does not stop after cooling
If your battery flashes red and green even after sitting at room temperature for an hour, the temperature sensor may be faulty. This is rare but does happen. The charger may also have a sensor problem that prevents it from reading the battery correctly.
In this case, the battery or charger likely needs replacement. Contact Milwaukee customer service with your model numbers and purchase date. They can tell you whether the battery or charger is at fault and what your options are. Do not continue trying to charge a battery that will not respond to temperature changes — you risk damaging it further.
Frequently Asked Questions
Can I charge my Milwaukee battery in the cold?
No. Most Milwaukee chargers will not charge below 32°F. If you need to use batteries in winter, charge them indoors in a heated space and bring them outside only when you are ready to work. The battery will hold its charge in cold weather; it just cannot be charged while cold.
Is my battery ruined if it flashes red and green?
No. The flashing is a protection feature that stops charging before damage occurs. Once the battery returns to a safe temperature, it will charge normally and work as before. The battery itself is not harmed by the temperature warning.
Why does my battery get hot after I use it?
Heavy use generates heat inside the battery as current flows through the cells. This is normal. The battery will cool down on its own within 30 minutes of stopping work. You do not need to do anything — just wait before charging.
Can I charge my battery in direct sunlight?
Not if the sun heats the battery above the safe charging temperature. If your battery is warm to the touch, move it to shade and let it cool before charging. Sunlight can raise battery temperature quickly, especially in summer.
What if only the red light flashes, not red and green together?
A flashing red light alone indicates a battery fault, not a temperature problem. This usually means the battery has an internal defect and should be replaced. Do not use the battery if it shows only a flashing red light.