Issue: The light cannot be turned on. OR the light flashes once then turn off.
First step is to check if the buyer purchased a correct battery with the light. If not, most likely his battery is not compatible with the light. There are many variations of the replies. Different stores might require different answers. The goal is always the same.
1. Explain to the buyer that what the issue is and what kind of the battery works in the C1.
2. Persuading the buyer or showing the buyer an easy way to get this battery.
Below is a sample reply. Do not copy it without modification.
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Both the product manual and the listing page state that the light requires an 18650 with higher than 8A output to activate the light.
From what you described, you are using an 18650 with lower than 8A output. If you reinstall the battery and double click the switch, it might enter the low setting. But if you click it once, the light tries to enter a regular high setting but could not because your battery does not supply sufficient current.
If you have multiple 18650s you might want to test a different battery since their outputs vary. If you have an IMR 18650s, it shall work for sure. For the best result, we recommend using an 18650 with a high drain current as the manufacturer recommended. Please check the link below for one that fits the requirement.