Symptoms
- Red light blinks continuously instead of turning solid or green
- Charging never completes even after hours
- Battery feels warm or hot in the charger
- Same charger works fine with other batteries
Likely causes
- CommonBattery is too degraded to accept a full chargeChargers with fault detection blink red when a battery can't reach a normal charge voltage.
- CommonWrong battery type or chemistry for that chargerUsing NiCd batteries in a NiMH-only charger, for example, can trigger a fault blink.
- PossibleOne bad cell in a multi-cell battery packA single weak cell can make the whole pack fail charger diagnostics.
- PossibleCharger contacts are dirty or misalignedPoor contact can be misread as a battery fault.
- Less commonCharger itself has failedTest with a known-good battery to rule this out.
What to do
- Confirm the battery type matches what the charger is designed for.
- Wipe the battery's contacts and the charger's contacts with a dry cloth to remove any grime or corrosion.
- Reseat the battery carefully to make sure it's making full contact.
- Try a different, known-good battery of the same type in the charger to see if it charges normally.
- If other batteries charge fine, the problem battery is likely worn out and should be replaced.
- If no battery charges normally, the charger itself has likely failed.
Tools and parts
- Dry cloth
Replacement rechargeable batterySearch: “rechargeable battery replacement”
Replacement chargerSearch: “battery charger replacement”
When to call a pro
No professional repair applies; replace whichever component (battery or charger) testing points to.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.