I'm a caregiver for my Aunt, who's on oxygen

We just don’t know what to do…my Aunt is on oxygen concentrator of 4liters, when she walks she’s on 4 litters with Imogen portable oxygen, mind you…it’s a used machine that only gives you less than 2 hrs of oxygen on 4 liters, I get it you have to plug it in to ur car charger, I understand. What happens when ur in target, shop rite, costco, or even a casino and can not find an outlet to plug in to immediately??? What are you suppose to do? A backup battery last another 2 hrs, and maybe you want to spend the day out in to the late night hours? What is one suppose to do? ANY SUGGESTIONS PLEASE??