Extra batteries are the most underrated accessory. Modern mirrorless cameras are powerful, but they are also power‑hungry. Having at least one or two spare batteries is essential. Starting a photo session with an empty battery is almost as annoying as starting it with a full memory card. In addition, cold weather drains batteries quickly, so extra batteries can keep you going longer. Most battery grips can store an extra battery, so by using that option the camera body practically has access to two batteries at the same time. Long shoots and video recordings also drain battery life.
A dead battery is the fastest way to end a photo session. Don’t let it happen. USB‑C power banks can run many modern cameras directly, and that can be handy if your batteries are depleted and you have a long day out taking pictures.