Portable Bottle Warmer

so I'm in an outdoor wedding next month and my husband will be taking care of our daughter for most of the day. I'm looking for a portable bottle warmer or a battery operated bottle warmer for him to heat up my breastmilk to feed our baby. she won't take a bottle cold or at room temp. we thought about getting a car adapter to just use a regular plug in Warner but my husband doesn't think the car battery will have enough voltage to support the warmer. hes thinking it'd kill our battery every time. anyone have any recommendations??