I would go ahead and replace the battery, anyway. The battery could be shorting internally, or the contacts inside the battery could be detaching. When you replace the batteries, check the metal contacts on the helmet and make sure they are making good solid contact with the battery.
If that doesn't work, then check that the light sensors are clean and not blocked by anything... if none of that is the problem, then it's time for a new helmet... or a call to the manufacturer and/or retailer.
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