From tech spec pages:

iPhone SE (3rd gen) (https://www.apple.com/iphone-se/specs)

Video playback:

    Up to 15 hours

Video playback (streamed):

    Up to 10 hours

Audio playback:

    Up to 50 hours

iPhone SE (2nd gen) (https://support.apple.com/kb/SP820?locale=en_US)

Video playback:

    Up to 13 hours

Video playback (streamed):

    Up to 8 hours

Audio playback:

    Up to 40 hours

So looking like a ~25% improvement. For some reason Apple likes to hide the mAh... so not sure how much is the chip vs possibly a bigger battery?