Explanation 1: Reverse polarity.
Test: Make sure that the + cable/clamp is connected to the + terminal and the – cable/clamp is connected to the – terminal.
Explanation 2: Loads are connected to the battery which requires more power than the charger can provide.
Test: Turn off or disconnect the loads when charging.
Explanation 3: The charger is not connected to a 12V battery.
Test: Test with a fresh 12V battery.
Explanation 4: The charge cycle is interrupted in step 1.
Test: Test to restart the charge cycle, if it still fails in step 1 the battery is sulphated and needs to be replaced.
Explanation 5: The charge cycle is interrupted in step 4.
Test: Test to restart the charge cycle, if it still fails in step 4 the battery is can’t be charged and needs to be replaced.