Optimate III when connected to the battery, lit only the power LED, none of the four status lights.
Optimate III won’t even try charging a battery if the voltage left is < 2V (what’s called a deep discharge).
- Disconnect the battery from the bike (to prevent damage to the electronics)
- Hook the charger up to a working battery
- Quickly move the cables from the fresh battery to the “dead” one. If the charging light goes out before the charger gets reconnected, you’re too slow.
- Wait patiently