I purchased these batteries: Aluratek Portable Battery Charger with Qualcomm Quick Charge 3.0, 20000mAh. They work on my Flex units. With the flashy status lights running, I get about 50 or so hours.
Each battery was about $33 with our state contract pricing. https://www.govconnection.com/product/aluratek-portable-battery-charger-with-qualcomm-quick-charge-3.0-20000mah/aspb20kf/36737696?cac=CartItem