I gave all my dead batteries away today, free of charge.
Why this joke works
A Free Offer with No Power
Free of charge normally means that no money is required. Dead batteries suggest another kind of charge, their ability to supply electrical energy. The sentence initially sounds like a small act of generosity, but the closing phrase also explains why the items are not especially useful. Gave away establishes the price meaning before dead batteries activates the electrical one, so the same expression both advertises the offer and quietly admits the drawback.
A closer look
A Closer Look
The comma creates a natural point for a brief pause, but the line should still sound like a casual household update. Do not replace dead with spare: spare batteries may work perfectly, which removes the second meaning. Read it as a fictional giveaway announcement, keeping the punchline focused on the offer rather than adding disposal or repair advice.
Fact behind the joke
Fact Behind the Joke
The Department of Energy explains that batteries store energy chemically. During operation, electrons move through an external circuit while ions move through the electrolyte inside. A depleted battery is not simply an empty container with every electric charge removed. The joke uses the everyday sense of needing a charge rather than a literal account of its particles.