Quick answer
- Red blinking on a Milwaukee M18 charger (like the 48-59-1812 dual-bay) means the battery pack temperature is outside the acceptable charging range. The charger is waiting. This is a recoverable state — not a fault.
- Red steady = charging in progress (normal).
- Green steady = fully charged.
- Red and green alternating = the charger has detected a defective pack. Try a different battery to confirm.
- Milwaukee's published charging range is 40°F to 105°F (5°C to 40°C) — not the 32°F floor that generic Li-ion sources sometimes cite.
Symptoms
You slide an M18 REDLITHIUM pack into the charger and the red LED starts blinking. The charger doesn't settle into the steady-red charging state. Maybe the pack feels cold from sitting in the truck overnight, or it just came off the tool after a hard session.
The blinking-red pattern on Milwaukee chargers is specifically the temperature-out-of-range signal. It looks alarming but it's the charger doing its job: protecting the battery cells from damage by refusing to charge until conditions are right.
Quick checks
- Touch the battery. Cold to the touch after sitting outside or in a vehicle? That's the cause. The 48-59-1812 dual-bay charger manual specifies a minimum charging temperature of 40°F (5°C) — charge won't start below that.
- Did the pack just come off a tool? Hot packs trigger the same blinking-red hold. Milwaukee's upper limit is 105°F (40°C). Let the pack cool for 20–30 minutes.
- Try a second battery. If a different pack charges normally (settles into steady red), the original pack is the issue. If the second pack also blinks red in the same port, suspect the charger port or the charger itself.
- Check the contacts. Wipe both the battery terminals and the charger contacts with a dry cloth. Debris can interfere with the pack-temperature reading.
- Switch charger ports. If you're using the dual-bay 48-59-1812, try the other bay. A single port fault can look like a battery problem.
Step-by-step fix
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Remove the battery and let both rest. Set the pack and charger on a bench for 5 minutes before troubleshooting further.
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Identify the temperature direction. Cold battery from overnight storage? Hot battery from work? The fix is opposite in each case.
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For a cold pack: Move it indoors and let it reach room temperature — at least 40°F (5°C), ideally 65°F (18°C) or warmer. This can take 30–60 minutes if the pack was very cold. Don't try to speed it up with a heat gun or oven.
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For a hot pack: Set the battery on a workbench in a cool, well-ventilated area. 20–30 minutes is usually enough. Avoid placing it in direct sunlight or back in the tool bag.
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Re-insert the battery and watch the LED. It should transition to steady red (charging) within a few seconds of achieving the right temperature. If it goes back to blinking red, the pack hasn't reached range yet — give it more time.
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If you see red and green alternating: This is the Milwaukee defective-pack signal. Test a known-good battery in the same charger. If the known-good pack charges normally (steady red → steady green), the original pack is defective. If the alternating pattern appears with a good pack too, suspect the charger.
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If it still isn't working
If a pack stays in the blinking-red hold for more than an hour at room temperature — and temperature is clearly not the issue — the battery may have an internal fault that the charger can't clear.
Milwaukee covers M18 REDLITHIUM batteries under a 3-year limited warranty and M18 tools under a 5-year limited warranty. If the pack is within that window, contact Milwaukee Tool at milwaukeetool.com/support or call their service line. Bring the pack to an authorized service center for assessment — don't attempt to open or modify the battery pack.
Storage note from the 48-59-1812 manual: Milwaukee specifies a maximum storage temperature of 120°F (50°C). Packs left in vehicles in summer can exceed this easily. A pack that's been baked in a hot truck may have permanent capacity loss that the blinking-red symptom won't fully diagnose.
FAQ
Does red blinking on a Milwaukee M18 charger mean the battery is ruined?
No. Red blinking is the temperature-out-of-range hold, not a fault state. The charger is waiting for the pack to come into the 40°F–105°F (5°C–40°C) charging range. Once the pack temperature is in range, charging resumes automatically. The pack ruined state is the alternating red-green pattern.
Why does my Milwaukee charger blink red on one battery but charge another fine?
The battery triggering the blink is likely outside the temperature range, while the other pack is in range. It can also indicate the blinking pack has a weak or deeply discharged cell — the charger may detect the temperature differential from a partially shorted cell. If the blink stays through multiple temperature-conditioning attempts, have the pack tested at a Milwaukee service center.
What does alternating red and green mean on a Milwaukee charger?
Alternating red and green is the defective-pack signal on Milwaukee M18/M12 chargers (including the 48-59-1812 dual-bay). The charger has determined it cannot charge the pack. Verify by testing a known-good battery in the same charger bay. If the alternating continues with any battery, the charger is the issue.
Can I use an M18 charger to charge M12 batteries?
On dual-bay chargers like the 48-59-1812, yes — one bay charges M18 packs and the other charges M12 packs. Single-bay M18 chargers charge M18 only. The key rule: M12 batteries never fit M18 tools and vice versa, but a dual-bay M18/M12 charger handles both on the charger side.
What's the minimum charging temperature for Milwaukee M18 batteries?
Per the 48-59-1812 dual-bay charger manual, Milwaukee's published minimum charging temperature is 40°F (5°C). Below this, the charger holds in the blinking-red state until the pack warms. Many generic sources cite 32°F (0°C) as the Li-ion minimum — Milwaukee's own specification is higher.