Battery #1 terminal lights red and stays red.
Disconnect the - lead from that battery, Repower the charger. The LED should be green. If it stays RED, charger defective.
Reconnect the - lead to the battery, measure voltage when charger is on. Should be between 11.5 and 13 volts. If OK, replace the battery. If lower than 11.5, check for charger cable connections OR for a connected load ( power drawing ) of more than 10 amps.
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