Quick answer
- The Makita DC18RD dual-port charger has a granular high-temperature warning that other Makita LXT chargers (DC18RC, DC18RF) summarize less clearly:
- TWO charging lights flashing red = the pack is OVER ~70°C (158°F) — the pack is too hot to charge safely; the charger will not start a charge cycle until the pack cools.
- ONE charging light flashing red = the pack is approximately 50-70°C (122-158°F) — the pack is hot, charging will start at a SLOWER rate to protect the cells.
- Neither indicator is a fault. Both are documented thermal-management states from the DC18RD manual. The pack is fine; the charger is enforcing a safe-temperature window.
- Remove the pack, let it cool for 15-30 minutes in ambient air (NOT in front of an AC vent or fan), then reinsert.
- The same logic applies to the DC18RF rapid charger.
Symptoms
When the DC18RD detects a hot pack, you'll see one of these two patterns on the front panel of the affected bay:
- Two red charging lights flashing in sync — the pack is too hot (over ~70°C / 158°F). Common after running a high-demand tool (impact driver under continuous load, circular saw cutting hardwood, angle grinder) and immediately mounting the pack to charge.
- One red charging light flashing — pack is hot but in the working range (50-70°C / 122-158°F). The charger will start a slower-than-normal charge cycle to keep the pack from getting hotter during charging.
Both patterns are accompanied by:
- No green LED illuminated (green = charging complete; not relevant during the warning state).
- No "charging in progress" red-solid state — that only appears when the pack is in the safe temperature range and charging is actively ramping current.
- Charger fan running (the DC18RD has cooling — fan helps the charger dissipate heat from its own electronics, but it doesn't actively cool the pack itself).
Quick checks
Before assuming the pack is damaged, verify:
- Did you just stop using the pack on a high-demand tool? That's the most common cause. After heavy use, packs can easily reach 60-80°C internally even if the case feels only "warm" to the touch.
- Was the pack in a hot environment? Truck cab in summer, direct sunlight, near a heat source — these can also push pack temp above the safe charging threshold.
- Charger model and bay. The two-flashing-red pattern is documented for the DC18RD (dual-port) and behaves the same on the DC18RF rapid charger. The DC18RC single-port charger uses a yellow warning LED instead — different visual pattern, same underlying thermal-management logic.
- Pack model. This pattern is for genuine Makita 18V LXT packs (BL14xx + BL18xx series). Third-party packs may not communicate temperature data correctly to the charger, producing different LED behavior.
Step-by-step fix
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Remove the pack from the charger immediately. Don't leave it inserted while waiting to cool — the charger continues drawing standby current and the pack contacts can transfer additional heat.
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Place the pack on a flat, ventilated surface away from direct sunlight, hot vehicles, or other heat sources. A workshop bench or kitchen counter is fine.
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Wait 15-30 minutes for the pack to cool. Larger packs (BL1850B 5.0Ah, BL1860B 6.0Ah) take longer than compact packs (BL1815N 1.5Ah). The pack should feel near-room-temperature on all surfaces (case, contacts, vents).
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Do NOT use a fan, AC vent, ice, or refrigerator to accelerate cooling. Rapid cooling can cause condensation inside the pack which damages cells. Ambient cooling only.
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Reinsert the pack into the DC18RD. The two flashing red lights should clear within seconds. If pack temp is now in the 50-70°C range (still hot but within working range), you'll see ONE flashing red and a slower-than-normal charge cycle will begin. If pack is at room temperature, you'll see normal RED SOLID (charging in progress).
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For frequent thermal triggers, plan your workflow. If you're running a high-demand tool through multiple battery swaps, rotate three packs instead of two — that gives each pack adequate cool-down time before the charger sees it again.
For a 3-pack rotation: View Makita 18V LXT 5.0Ah (BL1850B) on Amazon (paid link) View Makita 18V LXT 5.0Ah (BL1850B) on Home Depot (paid link)
If it still isn't working
If the pack has cooled to ambient temperature for 30+ minutes and the DC18RD still shows two flashing red lights:
- Check the pack's temperature sensor. A pack with a failed internal thermistor will report (incorrectly) high temperatures to the charger even when cool. There's no consumer-accessible test for this — try the same pack on a different DC18RD or DC18RC; if the second charger also reports hot pack on a known-cool pack, the pack's thermistor is faulty.
- Check the charger's bay sensors. The DC18RD has independent sensors per bay. Try the same pack in the OTHER bay of the same DC18RD. If the other bay accepts the pack normally (red solid = charging), the original bay's sensor is faulty.
- Try a different (known-good) Makita 18V pack. If a different pack also triggers the two-red-flashing pattern in the same bay, the bay's sensor is the problem. If the different pack works normally, the original pack is the problem.
- Check pack age. Heavily used packs (1000+ charge cycles) develop higher internal resistance, which causes them to heat up more than new packs during normal use. A pack that flags "hot" on charge after only mild use is showing its age — capacity is degraded, replacement is appropriate.
Makita's standard warranty: 3 years on 18V LXT packs and chargers. If a pack genuinely fails the temperature test after troubleshooting, take it to a Makita Authorized Service Center with proof of purchase.
FAQ
Is "two flashing red lights" actually a fault, or is the pack damaged? Per the DC18RD manual, two flashing red lights indicates the pack is over 70°C (158°F) — that's a thermal management state, not a fault. The pack is fine; it's just too hot to charge safely right now. Lithium-Ion cells charged at high temperature degrade faster (loss of capacity, increased internal resistance), so the charger refuses to engage until the pack cools. There's no damage as long as you let it cool naturally.
Why does the DC18RD show two flashing reds when the pack feels only mildly warm? Pack temperature internally can be much higher than the case temperature, especially right after heavy use. The Makita BMS measures cell-level temperature via internal thermistors and reports that to the charger — not the case temperature. A pack that's been driving an impact wrench under load may have cells at 75-80°C while the case feels only "warm."
How is this different from the DC18RC's "yellow warning LED"? The DC18RC is a single-port charger and uses a single yellow LED for any temperature warning (hot OR cold pack). The DC18RD has more granular feedback — separate patterns for "too hot" (two red flash) vs "hot but in working range" (one red flash). Same underlying thermal-management logic, more detailed indicator on the dual-port unit.
Can I cool the pack faster by putting it in the fridge or freezer? No, and this can damage the pack. Rapid temperature changes cause condensation to form inside the pack as it warms back up, which can short-circuit cells or trigger BMS protection lockouts. Always cool packs at ambient room temperature, even if it takes longer.
What if the pack is HOT all the time, even when not being used hard? That's a symptom of failing cells (high internal resistance) — the pack heats up just sitting in standby because more energy is lost as heat instead of stored. A pack that runs hot during light use is at end-of-life. Replace it; trying to keep using it stresses the charger and shortens the charger's life as well.