A rechargeable battery system can include features to facilitate determination of the charge state of a battery or a battery power unit. A rechargeable battery system can include a battery case that can hold a plurality of batteries. The batteries held by the battery case can include a tab located on a portion of the battery, such as, the terminal. The tab can include indicia of the charge state of the battery, and can be detached when the battery is at least partially discharged. The tab can facilitate determination of the charge state of the battery by allowing identification of batteries with the tab attached and batteries from which the tab has been detached.