Battery Modules. The six batteries currently on board the observatory are all original equipment. After more than 17 years of continuous operation the batteries are degrading. In the current condition, the Hubble electrical power system requires careful management of system state of charge to assure adequate power margins for all operational scenarios. This will become more difficult, and will start to constrain operations. The replacement of the two Hubble battery modules (each containing three batteries) will rejuvenate the electrical power system. This, combined with the power system enhancements made in Servicing Mission 3B, will result in ample power margins for the remainder of Hubble's lifetime.