Guides / BMS & Charging Faults
Charger Flashing Lights and the BMS
What a flashing charger is usually telling you, and what the battery management board does inside the pack.
A flashing charger light isn't random. It's the charger, and often the battery's own protection board, telling you why charging hasn't started. The exact codes are different for every brand, so the manual is always the final word.
1. What the lights usually mean
Most brands use some version of these states. Check your manual for the exact pattern.
- Charging: steady or slow flashing light.
- Fully charged: a different colour, a steady light or lights off.
- Hot or cold delay: the pack is outside the charging temperature range. The charger waits until it cools down or warms up.
- Fault or defective pack: the charger has detected something it won't charge, such as very low voltage, a damaged cell or a communication fault.
2. What the BMS does
Many tool packs have a battery management system (BMS), a small circuit board that looks after the cells. Depending on the design, it may:
- Stop charging when the cells reach full voltage.
- Switch off the output when the cells run too low, to protect them.
- Cut out if the current is too high, for example when a tool jams.
- Stop charging or discharging when the pack is too hot or cold.
- Balance the cells so they stay at similar charge levels.
- Talk to the charger or tool, on some brands.
Not every system works the same way. On some brands, part of the protection is in the tool or charger rather than the pack.
3. Common reasons a charger refuses a pack
- The pack is hot from heavy use. Let it cool.
- The pack has been stored flat for a long time and has dropped below the charger's minimum voltage.
- The cells are out of balance, so one group is much lower than the others.
- Dirty or damaged terminals.
- A failed cell or board.
4. What not to do
- Don't bypass or remove the BMS. It's there to stop overcharging, over-discharging and overheating.
- Don't force charge a pack the charger has rejected, using another battery or a bench supply.
- Don't use a charger from a different brand or system.
If the pack is warm, swollen, smells odd or has been damaged, stop and see Lithium Battery Storage and Disposal. To find where the fault is, work through Why Won't My Cordless Battery Charge?
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