Bring a portable charger. Period.
Tokyo Dome has zero public charging stations. No USB outlets at seats. No charging kiosks. If your phone dies during the game, you lose your e-ticket display, Google Maps for getting home, your Suica transit pass, and the ability to post the photos you spent 3 hours taking.
Charger rules at Tokyo Dome
Allowed: yes. Capacity restrictions: none. Security check issues: none. You can bring any portable charger of any size. Staff won't even look at it during the bag inspection.
How much battery you'll use
3-hour game with photos and social media: 30-40% battery drain. 3-hour game with video recording: 50-60% drain. 3-hour game with minimal phone use: 15-20% drain. If you arrived at the stadium with less than 50% battery (after a day of sightseeing), you're at risk.
What to buy
10,000mAh: charges most phones twice. Weighs about 200g. Costs Β₯2,000-3,000 ($13-20). This is the right choice for a single game. 20,000mAh: overkill for a game but useful if you're sightseeing all day before. 5,000mAh: risky. Might not fully charge a modern smartphone once.