1. The charge you carry into battle

Joe Haldeman’s The Forever War is full of soldiers returning from relativistic missions to find that everything has changed except the need to keep their equipment powered. The desktop does not fight battles, but it does carry its own power reserve into every eclipse, and the size of that reserve determines what it can do in the dark.

Entry 239 chose triple-junction GaAs for the deployable wings and silicon for body-mounted backup. This entry sizes the battery and picks the power distribution voltage.

2. Battery capacity

From entry 238, the desktop’s average demand is about 200 W. A LEO eclipse lasts roughly 35 minutes, so the minimum energy needed per eclipse is:

200 W × 0.58 h ≈ 116 Wh.

This is the absolute minimum. A practical battery must also cover:

  • Peak loads that exceed average demand during eclipse.
  • Efficiency losses in the battery and distribution system.
  • A margin for degraded solar array output.
  • A depth-of-discharge limit that preserves cycle life.

With a 50% depth of discharge and 20% margin, the required nameplate capacity is roughly:

116 Wh × 1.2 / 0.5 ≈ 280 Wh.

A reasonable battery size for the first desktop is therefore 300–400 Wh.

3. Cell chemistry

Lithium-ion is the standard for modern spacecraft. Within lithium-ion, the common space-qualified variants are:

  • Lithium-ion cobalt oxide (LiCoO₂): high energy density, but more sensitive to overcharge and thermal runaway.
  • Lithium-ion iron phosphate (LiFePO₄): lower energy density, but better thermal stability and longer cycle life.
  • Lithium-ion nickel-manganese-cobalt (NMC): a compromise between energy density and stability.

For the desktop, NMC or LiFePO₄ are preferable to LiCoO₂. The desktop’s mass budget is not so tight that it needs the absolute highest energy density, and the safety margins of NMC or LiFePO₄ reduce the risk of thermal runaway in a densely packed platform.

4. Bus voltage

The choice of bus voltage is a trade between current, component availability, and safety.

  • 12 V: common in CubeSats and small spacecraft. Easy to source components, but high current for a 200 W load (17 A). Copper losses and connector sizing become significant.
  • 28 V: the historical spacecraft standard. At 200 W, current is about 7 A. Many space-qualified components are available at 28 V.
  • 50 V or 100 V: used on larger spacecraft. Lower current and lower losses, but components are more expensive and safety requirements stricter.

For the desktop, 28 V is the right choice. It keeps currents manageable, uses a well-supplied space component ecosystem, and is compatible with most payload expectations. Customer payloads that need other voltages can use local DC-DC converters.

5. Distribution architecture

The desktop should use a regulated 28 V main bus with switched distribution to each attachment.

Each attachment receives its own fused or circuit-breaker-protected feed. Critical functions such as command and telemetry, battery heaters, and attitude control have unswitched or essential-bus feeds. Non-critical loads such as compute, communications, imaging, and robotics are switched and can be shed if the battery state of charge falls too low.

A small number of standard voltages are derived from the main bus:

  • 28 V for motors, heaters, and high-power payloads.
  • 12 V for computing, storage, and communications.
  • 5 V and 3.3 V for digital electronics.

These conversions happen inside each attachment or in a central power converter unit. The attachment interface contract specifies the input voltage range and maximum inrush current.

6. Battery management

The battery needs a management system that monitors:

  • State of charge.
  • Cell voltage balance.
  • Temperature per cell or module.
  • Charge and discharge current.
  • Cycle count and health trends.

The management system must also protect against overcharge, over-discharge, and overtemperature. In a thermal emergency, it should be able to disconnect the battery from the bus independently of the main computer.

7. Physical packaging

A 300–400 Wh lithium-ion battery at 28 V corresponds to roughly 11–14 Ah. The cells can be packaged as a single block or as two parallel strings for redundancy. The block should be thermally coupled to the desktop’s radiator path so it does not overheat during charge, and it should have heaters for cold eclipses.

Battery mass for space-qualified lithium-ion is roughly 3–5 kg per kWh. A 350 Wh battery therefore masses about 1.0–1.8 kg.

What this changes

  • The desktop carries a 300–400 Wh lithium-ion battery, sized for eclipse plus margin at 50% depth of discharge.
  • The main power bus is regulated 28 V.
  • Distribution is switched per attachment, with an essential bus for critical functions.
  • Standard derived voltages are 12 V, 5 V, and 3.3 V.
  • Battery management includes monitoring, balancing, and independent safety disconnect.
  • The next entry can close the power attachment arc and move to thermal control.