Bosch Charger Flashing Red — Battery Temperature Fault

What Does This Error Mean?

A flashing red LED on a Bosch charger means the battery temperature is outside the safe charging range — too hot or too cold.
Take the battery out, let it sit at room temperature for 20–30 minutes, then put it back on the charger.
Most cases come down to a battery used hard in summer or stored in a cold garage.
If the LED stays flashing red after cool-down, the battery's temperature sensor has failed.

Affected Models

Bosch GAL 18V Charger
Bosch AL 1820 CV
Bosch GAL 18V-160
Bosch 18V GBA Battery System
Bosch Professional 18V

Common Causes

Battery temperature above 45°C from heavy use (most common in summer)
Battery temperature below 0°C from cold storage
Battery temperature sensor failed — reads wrong value
Charger temperature sensor fault (uncommon)
Battery cell damage causing internal heating

How to Fix It

Remove the battery from the charger.

Pull the battery out and feel it.
If it's noticeably warm, you've been working it hard — that's normal.
If it's hot to the touch, that's beyond the safe charging window.
Let it cool to room temperature.

Set the battery aside on a flat, dry surface — not in direct sunlight or a hot vehicle.
20–30 minutes is usually enough.
Don't try to speed it up by putting it in a fridge or freezer — sudden temperature changes can cause condensation inside the battery, which is worse than waiting.
Or warm a cold battery.

If the battery came from a cold garage or van, it might be below 0°C.
Bring it inside for 30–45 minutes before charging.
Lithium batteries don't accept charge well below freezing — Bosch's protection circuit is correct to refuse.
Re-dock the battery.

Once at room temperature, slide the battery back onto the charger.
The LED should change to a steady green or a slow flashing green (charging).
If the LED returns to flashing red, the battery's internal temperature sensor has failed — and that battery needs replacing.

Frequently Asked Questions

Can I leave Bosch batteries on the charger overnight?

Yes — Bosch chargers have automatic cutoff when the battery is full.
The battery won't overcharge and isn't damaged by being on the charger long-term.
For long-term storage (months), Bosch recommends 40–60% charge state, not 100%.

What's the difference between flashing red and steady red?

Flashing red = temperature out of range, no charging happening.
Steady red = battery defective, won't take charge at all.
Flashing red usually clears with a cool-down; steady red typically means the battery is at end of life and needs replacing.