Yes, portable chargers are allowed
Every major NPB stadium and J-League venue allows portable chargers. They don't trigger bag check issues. Security staff won't look twice at your power bank. Bring one.
Why you'll need it
A 3-hour baseball game drains about 30-40% of a typical smartphone battery. If you're taking photos, recording videos, using Google Maps, checking scores, and posting to social media, that number goes higher. Running out of battery at the stadium means no Google Maps for the trip home and no way to display your digital train pass.
Recommended capacity
10,000mAh is the sweet spot. It charges most phones twice and weighs about 200g. Anything larger is overkill for a single game. If you're out all day sightseeing before the game, 20,000mAh gives more runway but weighs twice as much.
Charging inside the stadium
Don't count on it. Japanese stadiums don't have USB charging stations or outlets accessible to fans. A few newer venues (ES CON Field) may have limited charging spots in premium areas, but for general admission, your portable charger is the only option.
Wi-Fi vs mobile data
Stadium Wi-Fi exists but struggles under load. 30,000-46,000 people on the same network means slow speeds. Use your mobile data for anything time-sensitive. An eSIM with unlimited data (available through travel platforms for $15-25/week) is the most reliable option.
Other electronics
Cameras are allowed (no flash during play). Laptops and tablets are technically allowed but impractical. Drones are prohibited. Laser pointers are prohibited.