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The battery is generally nominal 3.6V 4000mAh, which is converted to wh is 3.6*4wh, and the theoretical charging time is 14.4wh divided by 15w, about 1 hour full. The actual situation can be more than an hour, because of various conversions, coupled with consideration of battery life will have constant voltage charging process.
A 15w charge for an hour is equivalent to 15,000 mah