Hubble's electrical power comes from its Solar Arrays, which also recharge its six 125 pound nickel hydrogen (NiH2) batteries. Each battery contains 22 individual cells wired together in series. The batteries reside in Support System Module (SSM) Equipment Section Bays 2 and 3. The six batteries are contained in a pair of 3-battery modules, each weighing 460 pounds (209 kg) and measures 36 by 32 by 11 in. (91 x 81 x 28 cm).
Hubble electronics run on battery power during three periods: during the portion of orbit in earth's shadow, when demand exceeds the Solar Array capability and during safemode entry.
Hubble's original NiH2 batteries have lasted more than 13 years longer than their design orbital life which is owed to very careful management on a daily basis by Electrical Power System engineers at GSFC. Due to aging and cycling,the batteries are showing a slow loss in capacity, a normal and expected trend and, if not replaced, they will eventually be unable to support Hubble's science mission.
The replacement batteries that will be install during SM4 are superior to the old ones in several ways. A process called "wet slurry" is now used to create the batteries, which results in batteries that are physically stronger and better performing. Each new battery also has the added safety feature of a battery isolation switch that electrically dead-faces each connector, creating a safer environment for astronauts installing the battery modules.