1. The mission that became a battery problem

Jim Lovell and Jeffrey Kluger’s Lost Moon returns as the recalled work because after the explosion on Apollo 13 the spacecraft’s electrical life was reduced to a few amp-hours in the command module batteries and the need to keep the lunar module’s systems alive. A LEO desktop has more predictable eclipses, but the same question: when the Sun is gone, what is stored and how safely can it be used?

This entry reads about batteries and power conversion.

2. Energy storage: lithium-ion dominates

NASA’s power chapter states that secondary lithium-ion and lithium-polymer batteries are now the standard for small spacecraft, replacing nickel-cadmium and nickel-hydrogen. Li-ion is attractive because of its high energy density and reversible chemistry. Li-polymer offers more flexibility in form factor at some cost in cycle life and energy density.

A NASA document on COTS lithium-ion cells for LEO highlights that LEO missions are high-cycle, high-rate environments. The battery must survive thousands of charge-discharge cycles with only modest depth of discharge. Common COTS cylindrical formats such as 18650 and 21700 cells have extensive flight heritage, including Mars Helicopter, MarCO, and PhoneSat.

The key battery design parameters are:

  • capacity and energy density;
  • allowable depth of discharge, which trades cycle life against usable capacity;
  • charge and discharge rate capability;
  • temperature range and heater requirements;
  • safety and thermal runaway containment.

3. Power architectures: MPPT versus DET

A SciELO paper on small-spacecraft EPS architectures explains that power is transferred from the solar array to the battery and loads through either Maximum Power Point Tracking (MPPT) or Direct Energy Transfer (DET).

  • MPPT uses a switching converter to hold the solar array at its maximum-power voltage as illumination and temperature change. It extracts the most energy but adds converter losses and complexity.
  • DET connects the array more directly to the battery bus. It is simpler and more efficient in stable illumination, but it wastes power when the array maximum-power point differs from the battery voltage.

The choice depends on orbit, array temperature stability, and power level. LEO CubeSats often use MPPT because of the rapid changes in Sun angle and array temperature. Higher-orbit spacecraft with more stable illumination may favor DET.

4. Regulation and distribution

The battery bus voltage is not usually what the subsystems need. DC-DC converters step the voltage up or down to produce regulated rails such as 3.3 V, 5 V, and 12 V or 28 V. A battery charge regulator protects the battery from overvoltage and overcurrent. Power distribution units add fuses, load switches, and current monitoring. Efficiency matters: a converter that is 90% efficient turns 10% of the energy into heat that must be rejected.

5. What this changes

  • The desktop will almost certainly use Li-ion batteries, probably in 18650 or 21700 format, with a battery management system for balancing and protection.
  • The power architecture choice between MPPT and DET is mission-dependent, not automatically MPPT.
  • Converter efficiency and thermal dissipation are first-order design parameters.
  • The next entry will read about energy budgets and fault protection.