You tell them that and offer a regular cart to lean on or, if store has a bench, a place to wait until one opens up. Then drag dead cart back. I've left carts unplugged because my old store had about no elderly and disabled shoppers (college town & store down the street was where the elderly shopped).  Also I worked late shift, so no one was around to really chastise me about it. "oh ma'am sorry, none are working or charged. I can't leave my post, our manager is over there. No I'm sorry I don't have a radio. You'll have to walk or wait."