Synths store energy within batteries or battery tissue. To increase the amount of energy they can store, either more batteries or battery tissues must be added. Since the most common way for Synths to store energy is through the use of battery tissue, the most common method is to simply increase the size of battery tissue areas. This can be done through the use of the Synth's nanites, or through the installation of modules that already have a higher amount of battery tissue on them. Battery tissue is most typically found on hips, bellies, thighs and breasts, so adding larger thighs or breasts will typically also result in an extended battery life. It is soft to the touch and based on solid state battery technology within small, soft cells.
Battery life is something Synths are aware of, but typically don't have to spend too much time thinking about, since they are usually close to a charging point. Battery extensions are mostly used for aesthetic reasons and are a part of Synth fashion, but can be useful to Synths in certain professions that require them to operate far from charging points, such as scouts or pioneers.