If the light on your Nikon D3500 battery charger is flashing rapidly (about 8 times a second) during charging, it's a warning of an error. The first thing to check is that the ambient temperature is within the recommended range for battery charging (5 °C–35 °C (41 °F–95 °F)). If that's not the issue, disconnect the power, remove the battery, and either seek a replacement from your retailer or service center or buy a new battery.