Hi John, I'm glad to assist you. You can turn on the headlights while the engine is off. If the headlights appear bright, it suggests that the battery has a sufficient charge. Dim or flickering headlights may indicate a lower charge. Also, when you attempt to start the engine, pay attention to the cranking sound. If the engine cranks slowly, it may indicate a low battery charge. Lastly, If you hear rapid clicking sounds when you turn the key to start the engine, it may indicate that the battery charge is insufficient to turn the engine over. I hope this helps! https://www.carparts.com/battery-charger/battery-tender/btt0220150dlwh?can=cpje1043_CSQA_20231209