Stationary battery energy storage systems are widely used for uninterruptible power supply systems. Furthermore, they are able to provide grid services. This leads to rising installed power and capacity. Lead-acid batteries provide one of the most mature and economically feasible solutions. Although this technology has been known and studied for more than 100 years, a further understanding of the different operating conditions is still obligatory for an efficient utilization of resources. The area of deep discharge has so far been mostly neglected in published research apart from fundamental material investigations. However, this condition will become more dominant in storage applications for renewable energy sources, UPS and off grid applications. The underlying study has been conducted to obtain a better understanding of deep discharge behavior of lead acid batteries. The results have been implemented in a semi-empiric battery model.