Quick answer
You drop an M18 pack on the charger expecting the steady red charging light, and instead you get a blinking red light that never settles into a charge. The pack stays at the same fuel level, the tool won't run, and you're stuck. Before you write off the battery, read the light. On Milwaukee's M18 chargers the blink pattern points to a specific cause, and most of them aren't a dead pack.
What the light is actually telling you
Milwaukee's M18 chargers use a simple legend, and getting it right saves you from replacing a good battery.
- Steady red is normal charging. Leave it alone.
- Steady green means the pack is full and ready.
- Red and green flashing together (alternating) means the pack is too hot or too cold to charge safely. The charger is waiting, not failing. This is the most common "won't charge" call, and it's normal behavior, not a defect.
- Red flashing on its own means the charger has flagged the pack as faulty and won't push current into it.
That temperature lockout matters more than people expect. Lithium-ion won't accept a charge below roughly 32F (0C), and a charger sitting in a cold garage or a pack pulled straight off a hard cut that's still hot will both trip the wait. This is a built-in safety interlock on every M18 pack, not a sign anything's broken.
Work the fixes in order
Start with the cheapest, most likely cause and move down.
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Let the pack reach room temperature. If you just finished a heavy cut or the garage is cold, set the battery on a bench at 50 to 80F (10 to 27C) for 20 to 30 minutes, then reseat it. If the flashing was the red/green temperature pattern, it'll often start charging on its own once the temperature normalizes.
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Reseat the pack, firmly, a few times. Slide it fully onto the charger until it clicks, pull it, and repeat two or three times. A weak or partial seat reads as a fault. Hard-seating clears it surprisingly often.
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Clean the contacts. Pull the pack and look at the metal terminals on both the battery and the charger rail. Sawdust, grit, or corrosion break the connection. Wipe them with a soft brush and a little isopropyl alcohol, let it dry, and try again.
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Try a known-good pack on the same charger. If a second M18 battery charges fine, the charger is good and the original pack is the problem. If nothing charges, the charger or its outlet is the issue. Plug the charger straight into a wall outlet, not a strip, to rule that out.
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Break a deep-discharge lockout. A pack run flat and left sitting can drop into a protective sleep the charger reads as defective. Leave it on a genuine charger for about 10 minutes, then remove and reseat it three times. That sequence sometimes wakes a pack the BMS had shut down.
When the pack is genuinely dead
If the red-only flashing keeps coming back after the pack is at room temperature, the contacts are clean, and you've reseated it, the fault is internal. A cell has dropped out of balance or failed, and the battery management system won't let it charge. There's no home fix for that. It's not a contact or a temperature issue at that point.
The good news is M18 packs carry a multi-year warranty, often five years on the battery. Check the date code stamped on the pack. If it's in range, take it to a Milwaukee service center or the retailer for an exchange rather than buying a new one.
One thing to keep straight: M18 and M12 don't cross. An M12 pack won't charge or run on M18 gear and the reverse is also true, even though the rapid charger handles both on separate rails. If you grabbed the wrong battery in a hurry, that's the whole problem, and the fix is just reaching for the right pack.
Most flashing-red M18 batteries come back to life on the temperature wait or a contact clean. Save the replacement for the ones that fail the known-good test.