If you want a purpose-built battery that doesn't add much weight and brings a ton of function, we'd say the $99 MagSafe Battery Pack fits the bill.
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Apple's official MagSafe Battery Pack returns to 2023 low with rare discount to $84 (Reg. $99)
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Apple MagSafe Battery Pack
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Snap. Start. Recharge! On the go wireless charging that provides 2x iPhone battery life in a snap! DOUBLE POWER Full iPhone charge in your pocket!
Pink MagSafe Battery Power Pack