ER1 (Error Code 1) appears when:
Battery clamps are connected incorrectly.
The correct connection is Red Clamp to Positive, Black Clamp to Negative.
The charger is powered on before being connected to the battery (connect first, then power on).
The battery is completely flat. The charger requires at least 2V to add charge; below 2V, it will not charge.
ER2 (Error Code 2) appears when:
A defective battery is connected.
The battery voltage does not match the selected charge mode (e.g., a 6V battery with a 12V setting).