Quick answer
On the Milwaukee 48-59-1812, continuous red is charging and continuous green is complete. Fast flashing red means the battery is too hot or too cold, and the other flashing-red pattern means a second pack is waiting its turn. Flashing red and green together means Milwaukee considers the pack damaged; the manual gives steps to try before you replace anything.
Try our charger light-code decoder for these patterns. Everything in the table below comes from Milwaukee's 48-59-1810 / 48-59-1812 operator's manual.
LED code decoder (48-59-1812)
| LED pattern | What it means (Milwaukee's manual) | Action |
|---|---|---|
| Continuous red | Charging. A fully discharged pack in the normal temperature range takes about 30 to 240 minutes | Wait |
| Continuous green | Charging is complete | Nothing needed |
| Green flashing | Maintenance charging; the charger keeps a pack left on it fully charged | Nothing needed |
| Fast flashing red | The battery is too hot or too cold; charging begins when it reaches the correct temperature | Wait. Normal charging starts automatically |
| Slow flashing red | Battery charge is pending; charging begins when the first pack is fully charged | Wait. This is the second pack waiting its turn |
| Flashing red and green | Damaged or faulty battery pack | Follow the steps below |
| No light indicator | Not defined as a state; the manual's troubleshooting covers an indicator that does not come on | Follow the steps below |
Source: Milwaukee 48-59-1810 / 48-59-1812 operator's manual, pages 3 to 5. We have this manual only for the 48-59-1812, so we don't list light codes for other Milwaukee chargers.
Flashing red and green, or no light
The manual's steps, in order:
- Check that the battery pack is fully seated in the bay.
- Remove the pack and reinsert it.
- If the indicator still does not come on, remove the pack(s) and unplug the charger for at least 2 minutes. After 2 minutes, plug the charger back in and insert the pack.
- If it still does not come on, contact a Milwaukee service facility.
If a different pack charges normally on the same charger, the original pack is the likely problem. If several packs fail on this charger but charge on another, suspect the charger.
Charging temperature
The manual gives the recommended ambient charging temperature as 40°F to 105°F (Fahrenheit only). Milwaukee's M18 battery instruction sheets (58-22-1815) give 5°C to 40°C (about 40°F to 104-105°F). The 40°F figure is Milwaukee's, not the 32°F that is often quoted.
A hot or cold pack shows fast flashing red and the charger waits until it is in range, then moves to continuous red by itself. Hot or cold packs may charge more slowly.
Fuel gauge diagnostics on the pack
Milwaukee's M18 battery instruction sheet lists two diagnostic patterns on the pack's fuel gauge:
- All four lights flashing quickly: the current draw is too high. Release the trigger and restart.
- Lights 1 and 3, then 2 and 4, flashing alternately: the battery temperature is too high. Allow it to cool.
The sheet also says not to store a pack above 120°F (50°C), because long storage above that temperature can permanently reduce capacity. The 48-59-1812 manual makes the same point.
M12 vs M18 compatibility
Milwaukee pairs M12 tools with M12 batteries and M18 tools with M18 batteries. The 48-59-1812 is a multi-voltage charger for M12 and M18 packs. Milwaukee's manual says packs charge one after another: the second pack begins charging when the first is fully charged. The M12-only charger 48-59-2401 charges M12 packs only. See our M12 vs M18 fit guide and the battery compatibility checker.
If it still isn't working
Warranty terms depend on the product and the pack model, so read Milwaukee's current warranty page and the terms on your pack's product page. If a pack is swollen, cracked or leaking, stop using it.
For a replacement pack, see our genuine vs third-party M18 battery guide.
View Milwaukee M18 REDLITHIUM XC 5.0Ah (48-11-1850) on Amazon (paid link)
FAQ
Why is my Milwaukee charger flashing red if the battery is fine?
Fast flashing red means the battery is too hot or too cold, and charging begins when it reaches the correct temperature. Slow flashing red means a second pack is waiting for the first to finish. Neither is a fault.
What does flashing red and green mean on a Milwaukee charger?
Milwaukee's manual calls it a damaged or faulty battery pack. It says to check the pack is fully seated, remove and reinsert it, unplug the charger for at least 2 minutes, and then contact a Milwaukee service facility if it still does not work.
Can I use an M12 charger for my M18 battery?
No. Milwaukee pairs M12 batteries with M12 chargers and tools, and M18 with M18. The 48-59-1812 is a multi-voltage charger that takes both.
Does the 48-59-1812 charge two packs at once?
No. Milwaukee's manual says the second pack begins charging when the first pack is fully charged.
What is Milwaukee's charging temperature range?
The 48-59-1812 manual says 40°F to 105°F, and Milwaukee's M18 battery sheets say 5°C to 40°C (about 40°F to 104-105°F).