Most cams with batteries tend to claim an hours usage running off the battery but you can usually only get around 20 minutes, I wouldn't bother getting photos, just let it continue to film as you slowly walk round the scene, That way if an "offender" doesn't want you to take his photo, he wont know you'll already have his face on cam if you're discretely recording (or her, ya never know), and you can then just take screenshots as you review the footage on the computer.
You could also keep a charged powerbank handy in the car too, which would actually probably work with a capacitor based cam away from the car. One of my cams is a battery cam but due to its age the cam literally only lasts a few seconds on the battery now