5.7 Rechargeable Power Sources

Secondary Batteries

Secondary batteries are by far the most common energy storage option for spacecraft. Secondary batteries are rechargeable and commonly paired with a power generation source, like solar cells. The most common batteries currently used in space flight are nick-cadmium (NI-Cad) and with the rise in CubeSats, lithium polymer (Li-pol) and lithium-ion (Li-ion) batteries [NASA, Knap]. A battery module is pictured in the figure above, composed of many individual battery cells connected in series to achieve a defined power, energy capacity, and voltage.

Secondary batteries are evaluated and selected by their energy density, discharge rate, allowable depth of discharge, cycle life, and temperature limits.

• To achieve the necessary power storage requirements for the whole mission lifetime, we need to pay attention to energy density or total energy storage capacity, dictated by the choice of material and the number of cells. Energy density degrades so be sure to add margin when sizing the batteries.
• A battery’s required discharge rate is dictated by the maximum instantaneous power draw of the spacecraft bus during its most demanding phase, a quantity derived from a power budget and profiling.
• A battery’s depth of discharge is dictated by the maximum sustained energy consumption of the spacecraft bus during the phase that draws the most power over the longest period (power * time), a quantity also derived from a power budget and profiling.
• A battery’s cycle life is dictated by the concept of operations and mission objectives by answering the question: how many orbital periods, associated with charging and discharging cycles, are proposed in the concept of operations to fulfill the mission objectives?
• As batteries have the strictest operational temperature limits, batteries typically create a thermal requirement that the structures and thermal subsystem leads must design around. The EPS lead can help these other subsystem leads by considering batteries with the widest temperature range so that the other subsystem leads need to adhere to a more forgiving requirement.

Energy density degrades over time due to several factors: internal physical changes or corrosion, fast discharging, overcharging, environmental conditions (vibration, shock, temperature), and storage. Degradation will always exist but best practices may minimize the amount of total degradation.

• Batteries naturally discharge when not in use, called self-discharge. Secondary batteries typically self-discharge faster than primary batteries. To mitigate the effects of self-discharge, we can store batteries in refrigerators or freezers, slowing the uncontrollable internal processes.

• To mitigate degradation due to fast discharging, we can size our battery module such that the necessary power draw is limited in discharge rate.

• Environmental conditions may be regulated with a structural dampening or thermal control to minimize the vibration, shock, or extreme temperature exposure.

A battery’s state of charge, or percent of total capacity discharged, can be estimated by measuring the voltage of the cell. We want to predict the state of charge of a battery to ensure that we do not fully discharge the battery and impart permanent damage to the battery cell. The voltage may be read by Analog-to-Digital converters on the onboard computer or battery board. For lithium-ion batteries, the voltage for a significant portion while discharging can read the same voltage level, which makes determining the true capacity difficult and potentially inaccurate. The danger for lithium batteries is not protecting the battery from discharging until the rapid fall in cell voltage upon complete discharge of the battery, dramatically shortening the cycle life of the battery. Other methods for estimating the state of charge are based upon current, internal impedance, specific gravity, etc. Estimating the state of charge is very difficult due to but not limited to nonlinearity to voltage, hysteresis, or memory effects due to the previous cycling, variation to temperature, and on. Today, algorithms for estimating the state of charge are state-of-the-art research, utilizing advanced techniques such as machine learning techniques [Hannah et al.].

Battery Sizing

To size the battery, we must consider the total capacity, total system load, duration, DOD, and average voltage. To calculate the total capacity of the battery, the formula follows:

The load and duration are determined from the spacecraft system power budget and profile. The DOD and average voltage are taken from the battery specifications.

Artemis Kit Specific

• To achieve the necessary power storage requirements for the whole mission lifetime, we need to pay attention to energy density or total energy storage capacity, dictated by the choice of material and the number of cells. Energy density degrades so be sure to add margin when sizing the batteries.
• A battery’s required discharge rate is dictated by the maximum instantaneous power draw of the spacecraft bus during its most demanding phase, a quantity derived from a power budget and profiling.
  • Talk about the Initial startup, takes 16.05 W but how long?
• A battery’s depth of discharge is dictated by the maximum sustained energy consumption of the spacecraft bus during the phase that draws the most power over the longest period (power * time), a quantity also derived from a power budget and profiling.
  • Talk about the Data Transmit mode, which takes 3.29 W for 4 minutes
• A battery’s cycle life is dictated by the concept of operations and mission objectives by answering the question: how many orbital periods, associated with charging and discharging cycles, are proposed in the concept of operations to fulfill the mission objectives?
• As batteries have the strictest operational temperature limits, batteries typically create a thermal requirement that the structures and thermal subsystem leads must design around. The EPS lead can help these other subsystem leads by considering batteries with the widest temperature range so that the other subsystem leads need to adhere to a more forgiving requirement.

Artemis Kit Specific

Flywheel Energy Storage

Flywheels are rotors that rotate at a very high speed and maintain energy as mechanical, rotational energy. The kinetic energy of the rotor can be converted into electrical energy in which the rotor’s speed decreases but surplus electrical energy can be stored back into the rotor as mechanical energy for further use. The rotors are suspended by magnetic bearings and encased in a vacuum to reduce energy loss to friction. State-of-the-art research involves cooling superconductors and levitating magnet rotors as a way to achieve a frictionless bearing to reduce bearing losses and the complexity of traditional flywheel storage systems [Andrare].