If your battery controller has a red flashing light, it indicates a problem with the system. There are four possible reasons for this: high temperature, battery under voltage, battery over voltage, or a broken charger. To troubleshoot, first check the wiring using a multimeter to ensure there are no issues. Confirm that the positive and negative wires are connected correctly. Then, assess the battery by measuring its voltage with the multimeter. Depending on the voltage range, you may need to address high or low voltage conditions. Additionally, consider the ambient temperature, as extreme heat can affect the charger's operation. If the issue persists, provide specific data for further assistance.