1 volt = 1 joule per coulomb.
Current doesn't actually pass 'through' a battery. But if it did, then each coulomb would gain or lose 6 joules in traversing 6 volts, depending on its sign, and whether it climbed or fell.
Each coulomb of charge passing through a 6 V battery gains 6 joules of energy. This is derived from the relationship between voltage, energy, and charge. Therefore, the energy is calculated by the formula E = V × q, resulting in 6 J for 6 V and 1 C.
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