Apple has always tried to increase the battery life in newer models, but some users may argue giving an example that iPhone 6 features 1810 mAh battery, and iPhone 6s comes with 1715 mAh battery only. This article is the reply to such argument, you may have noticed battery power, but not system optimization. iPhone 6s with less mAh runs much longer compared to iPhone 6.
iPhone 7 may feature even more powerful battery according to a leaked image. The leaked battery Photo shows a 7.04 WHr (watt-hours) battery that is expected to use in iPhone 7. Just to remind, iPhone 6 features a 7.01 WHr battery, and iPhone 6s comes with 6.61 WHr battery. The charging capacity of iPhone 7 battery is still unknown, but this leaked image suggests that Apple would add more battery life to iPhone 7.
iPhone 7 has been rumored to be thinner compared to existing models. Apple would not use headphone port in iPhone 7 and iPhone 7 Plus, and it may give them more space to add little larger battery. Rumor about dual-rear camera for iPhone 7 has been rejected now, and such camera setup would be exclusively for iPhone 7 Plus ( or iPhone Pro). Apart from rear camera, Display and Battery, rest of the specs of both models would remain same.