A battery can store electric charge so that it can be used to power a craft's systems such as radio transmissions, reaction wheels, and probe cores. While command pods do store some electric charge, most batteries store much more. Every variety of battery has the same charge per unit mass, 20 units of electric charge per kilogram.
Zaltonic Electronics manufactures all of the available batteries in unmodded Kerbal Space Program. (You might say they're a power player in the industry.)
|Image||Part||Radial size|| Cost
|Z-100 Rechargeable Battery Pack||Radial mounted||80||0.005||1 200||8||50||100|
|Z-200 Rechargeable Battery Bank||Tiny||360||0.01||2 000||8||50||200|
|Z-400 Rechargeable Battery||Radial mounted||550||0.02||1 200||8||50||400|
|Z-1k Rechargeable Battery Bank||Small||880||0.05||2 000||8||50||1 000|
|Z-4K Rechargeable Battery Bank||Large||4 500||0.2||2 000||8||50||4 000|
Purpose of a battery
The function of a battery is to buffer periods when use of electric charge exceeds generation, and to make use of periods where generation exceeds demand by storing charge. There are several circumstances under which demand and supply are not evenly matched.
The most commonly encountered scenario is when electrically powered vehicles which normally gather their power from solar panels are expected to operate at night. Without sufficient battery storage, such vehicles will need to stop until local dawn. Remember that night happens in space too!
Choosing a Battery
Since the amount of electric charge per unit mass is constant, the best battery to choose is based on cost, craft design and available space.
The most cost effective way to store power is to stack instances of the Z-100 Rechargeable Battery Pack since it has the lowest cost per unit charge; however, while this approach might serve the needs of large rockets (stuff them in service bays) or space stations, it may might have practical hindrances due to part count on very large constructions or mounting space on small ones. The (ten times larger) Z-1k_Rechargeable_Battery_Bank is only 10% more expensive per unit of charge.
For satellites or rovers, where mounting space is limited, figure out how much juice you want to have available and then pick the battery that fits your mounting needs and has the lowest unit cost.