A red and green alternating blink on a Milwaukee charger almost always means the charger is communicating with the pack but refusing to charge it. It's a fault signal, not a "charger broken" signal. In most cases the problem is the battery's condition or its temperature, and it's fixable.
Here's how to work through it in order.
What You'll Need
- The charger and the suspect pack
- A known-good pack (to isolate the fault)
- A clean, dry cloth and a soft brush
- Isopropyl alcohol
- A multimeter
Step 1 — Let the Pack Reach Room Temperature
This is the single most common cause. Milwaukee chargers won't charge a pack that's too hot or too cold. A pack straight off an impact wrench, or one that sat in a freezing truck, will blink a fault rather than risk the cells.
Set the pack aside until it's roughly room temperature — usually 30 to 60 minutes depending on how extreme it was. Then retry. In winter and peak summer, this resolves the majority of red/green blinking.
Step 2 — Clean the Terminals on Both Sides
Look at the charger contacts and the battery terminals. Dust, metal debris, or corrosion in a shop environment breaks the communication handshake, and the charger faults out.
Unplug the charger. Wipe the contacts dry, then with a cloth lightly dampened with isopropyl alcohol. Do the battery side the same way. Let both dry completely. The full method is in our guide to cleaning battery terminals.
Step 3 — Check the Pack's Voltage
Set your multimeter to DC voltage and read across the pack's main terminals. A healthy M18 pack should read near its nominal 18V, typically a bit above when charged.
- Reads near nominal: the pack itself is probably fine — the fault is temperature or contact related.
- Reads far below nominal (e.g. under 12V): the pack is deeply discharged and the BMS may have locked it out. This is often unrecoverable without specialist tools.
- Reads zero: the BMS has shut the pack down, or a cell group has failed.
For the full walkthrough, see our guide to testing a battery with a multimeter.
Step 4 — Try a Different Pack and a Different Charger
Cross-test both ways.
- Good pack into the suspect charger: if it charges normally, the charger is healthy.
- Suspect pack into a known-good charger: if it blinks the same fault, the pack is the issue.
This tells you which part to replace in under five minutes, instead of guessing.
Step 5 — Check for a Deep Discharge Lockout
If a pack has been left flat for months, its BMS may have entered a protection state where the charger refuses to bring it back. In some cases inserting and removing the pack a few times, or a gentle top-up on a different compatible charger, can wake it. If it won't respond and reads well under nominal voltage, the pack is effectively done.
⚠️ Don't attempt to force-charge a locked-out pack by bypassing the BMS. That's how packs vent.
| What you check | Normal | Points to a fault |
|---|---|---|
| Pack temperature | Roughly room temperature | Straight off a hot tool, or cold from a truck overnight |
| Terminals on both sides | Clean and dry | Dust, metal debris, or corrosion |
| Pack voltage | Near 18V, a little above when charged | Under 12V |
| Cross-test | Good pack charges in the suspect charger | Suspect pack faults in a known-good charger |
Common Mistakes to Avoid
- Charging a hot pack. Let it cool — this is the cause more often than anything else.
- Ignoring a swelling pack. If the housing is bulging, stop. It's not a charging problem, it's a failing pack.
- Mixing pack families in one charger. Use an M18 charger for M18 packs.
- Assuming the charger is dead. Test a known-good pack first.
When to Replace the Battery
Replace the pack if it swells, smells sweet, gets abnormally hot, or reads deeply under nominal voltage and won't recover. A healthy M18 replacement restores full runtime — our Milwaukee M18 12.0Ah replacement battery covers high-capacity, high-draw work where runtime matters most.
Frequently Asked Questions
What does flashing red and green on a Milwaukee charger mean?
It signals a fault on the pack or a communication problem — most often a pack that's too hot or too cold, or dirty terminals. Let the pack reach room temperature and clean the contacts, then retry.
How do I fix a Milwaukee battery that won't charge?
First let it reach room temperature, then clean the terminals on both the pack and charger, then test the pack's voltage with a multimeter. If it reads well below nominal and won't recover, the pack has failed.
Can a deeply discharged Milwaukee battery be revived?
Sometimes, if the discharge wasn't prolonged. Inserting and removing the pack a few times, or a gentle top-up on a compatible charger, may wake it. If it reads far under nominal and won't respond, it's usually done.
Is it safe to use a Milwaukee battery that's swelling?
No. Stop using it immediately and retire it. Swelling means a cell has failed internally, and continuing to charge or use it risks a fire.
Why does my Milwaukee battery blink red and green only sometimes?
Intermittent faults usually point to temperature or contact issues — a pack charged right after heavy use, or terminals that need cleaning. Consistent faults on every attempt suggest a failing pack or charger.