Design of an On-Orbit Maintainable High-Power Energy Storage Cell System

I am part of the team that developed a high-power energy storage cell system designed for on‑orbit maintenance on the Chinese Space Station. This system provides peak power exceeding 9.3 kW for durations longer than 3 minutes, enabling payloads that demand short‑duration, high‑current discharge. The product integrates a lithium‑ion battery pack, a power controller, and a liquid‑cooled cold plate into a single unit that is installed inside a standard equipment rack. Over three years of in‑orbit operation, the system has demonstrated stable performance and fullfils all safety, ergonomic, and medical requirements for long‑term manned platforms.

1. Introduction

Lithium‑ion batteries have become the preferred energy storage solution for spacecraft due to their high specific energy, low self‑discharge, high charge efficiency, and absence of memory effect. In manned platforms such as the International Space Station (ISS) and the Chinese Space Station, energy storage cells serve both platform and payload applications. Platform batteries typically operate at low discharge rates over long periods, while payload batteries must deliver short, high‑power bursts. Our system is designed to act as an energy buffer, storing energy during the orbital day and releasing it during the brief periods when the payload demands up to 9.3 kW. Unlike previous manned‑spacecraft batteries that operate at moderate discharge rates (≤1 C), our system discharges at 2 C to 3 C, requiring a careful balance between high power, long life, and safety.

The design philosophy emphasises maintainability: astronauts must be able to replace the entire unit within 2 hours. This imposes unique constraints on the mechanical, electrical, and thermal interfaces, which we addressed through a blind‑mate connector, guided rails, and captive fasteners.

2. Analysis of Existing Products

The ISS originally used Ni‑H₂ batteries (81 Ah, 38 battery assemblies) with a 6.5‑year life at 30 % depth of discharge. Starting in 2017, these were replaced by lithium‑ion batteries (134 Ah cells, 30s10p, 15 kWh, 10‑year life) operating at 1 C discharge and a 20 % depth of discharge. The Chinese Space Station platform batteries use 30 Ah energy‑type cells (3p22s, 90 Ah, 1 C discharge). All these systems are designed for low‑rate platform power, have lower specific energy, and separate the power controller from the battery assembly.

Our system is the first manned‑spacecraft energy storage cell that combines a high‑specific‑energy lithium‑ion battery (≥153 Wh/kg at cell level) with a 3 C discharge capability and an integrated power controller and cold plate. This integration reduces mass and volume while simplifying on‑orbit replacement.

3. Overall System Design

The energy storage cell system comprises three major assemblies: the lithium‑ion battery pack, the power controller, and the cold plate assembly. Table 1 lists the components and their quantities.

The system is housed in a 19‑inch rack‑mounted enclosure. The battery pack is electrically isolated from the structure by two 200 kΩ resistors connected in parallel between the pack negative and the chassis.

3.1 Power Controller Design

The power controller manages charging, discharging, cell balancing, and system protection. It consists of four functional blocks.

3.1.1 DSP Module

The DSP module is the control centre. It implements:
– Logic control (RS‑422 communication, microcontroller minimum system)
– Battery charging control (BEA – Battery Energy Management)
– Signal conditioning and telemetry acquisition
– Internal power supply

A “drawer‑style” programming connector allows in‑orbit firmware updates without opening the sealed unit.

3.1.2 Charge Regulation Module (CRM)

Three identical CRMs use a phase‑shifted full‑bridge topology with zero‑voltage switching (ZVS) to achieve high efficiency (>92 %) and galvanic isolation between the platform bus and the battery. Each module handles 600 W; under normal conditions two modules operate while the third serves as a cold spare. The charging profile is constant‑current / constant‑voltage (CC/CV). The transfer function of the closed‑loop control can be expressed as:

$$
V_{\text{batt}} = V_{\text{ref}} – K_p \cdot (I_{\text{batt}} – I_{\text{ref}})
$$

but the actual implementation uses a PI compensator. Over‑current protection is set at (13 ± 1) A per module.

3.1.3 Sampling & Equalisation Module

This module measures the voltage of each of the 23 series groups (cell groups), three full‑pack voltages, and five temperature points. Passive balancing dissipates excess energy from over‑charged cells via resistors. The balancing circuit uses two series transistors for redundancy; both must be turned on to activate the bypass path. The sampling accuracy is ±10 mV per cell group and ±270 mV for the total pack. The full‑pack voltage is measured by three independent channels, and a voting algorithm selects the median or alerts the ground crew if all three disagree.

The differential amplifier used is an AD620 with an input impedance of 10⁹ Ω. The multiplexer is an HS1‑0546RH‑Q, and the ADC is an AD574 with 12‑bit resolution, giving a quantization step of:

$$
\Delta V_{\text{LSB}} = \frac{10\ \text{V}}{2^{12}} \approx 2.44\ \text{mV}
$$

3.1.4 Power Routing Module

This module contains input filtering, a large DC contactor for the discharge switch, current sensors (Hall effect), and the secondary power supply. It also houses the power busbars that carry the full charge/discharge current. The module uses a six‑cavity blind‑mate connector to simplify on‑orbit installation.

4. Battery Pack Design

The energy storage cell pack must deliver 9.3 kW for ≥3 minutes over a 5‑year low‑Earth‑orbit (LEO) life, which translates to about 29 200 charge/discharge cycles at 60 Ah capacity and a 2 C–3 C discharge rate.

4.1 Series/Parallel Configuration

The required output voltage is 72 V–96 V. Accounting for a 1 V internal drop, the number of cells in series is determined by:

$$
N_{\text{series}} = \frac{V_{\text{max}} + 1}{V_{\text{cell,max}}} = \frac{96 + 1}{4.2} \approx 23
$$

With 23 cells, the maximum voltage is:

$$
V_{\text{max}} = 4.2 \times 23 – 1 = 95.6\ \text{V}
$$

and the minimum voltage at end‑of‑life (assuming 3.4 V minimum per cell except one degraded cell) is:

$$
V_{\text{min}} = 3.4 \times (23 – 1) – 1 = 73.8\ \text{V}
$$

Both values lie within the 72 V–96 V window. The capacity requirement is met by using two cells in parallel for each series group (2p23s), giving 60 Ah nominal capacity.

4.2 Cell Chemistry Selection

Four cathode/anode systems were evaluated: NCA‑1, NCA‑2, NCA‑3, and NCM. Table 2 summarises the key results.

The NCA‑2/graphite system was chosen because it demonstrated the best cycle life (exceeding 29 200 cycles at 2 C discharge). The cell energy density is 153 Wh/kg, meeting the >150 Wh/kg requirement.

4.3 Safety Enhancements

Compared to the previous generation of low‑orbit platform batteries, we increased the separator thickness from 25 µm to 40 µm and the cell can wall thickness from 0.60 mm to 1.00 mm. Table 3 compares the safety performance.

The short‑circuit test on a 2‑cell parallel module (total loop impedance 9.8 mΩ) showed the voltage dropping below 0.1 V within 10 minutes and a peak temperature of 91 °C. No fire or explosion occurred. The thicker separator and can improve internal short‑circuit resistance and pressure tolerance.

5. Cold Plate Assembly

The cold plate is a vacuum‑brazed aluminium structure with a high‑density folded‑fin micro‑channel design. It is filled with a 36 % ethylene‑glycol water solution. The cooling capacity exceeds 300 W at a coolant flow rate of 100 L/h and inlet temperature of 27.5 °C. The design ensures that the energy storage cell modules remain within the operating temperature range of 10 °C to 40 °C even during the 3‑minute high‑power discharge. The quick‑disconnect fittings and flexible hoses allow the coolant loop to be broken and reconnected without leakage when the unit is replaced.

6. Operational Modes

The system operates in three modes: charging, discharging, and idle. Mode transitions are commanded via the RS‑422 bus. Automatic protection functions (over‑charge, over‑discharge, over‑temperature) force a transition to idle (discharge disabled). The state machine is shown in the diagram below (conceptually):

– Charging → Discharging (upon receiving a “discharge” command)
– Discharging → Charging (upon “charge” command or after the load is removed)
– Any mode → Idle (if a protection threshold is exceeded)

The DSP continuously calculates the state of charge (SOC) using Coulomb counting and voltage correction. The equalisation algorithm is activated when any cell group voltage deviates more than 30 mV from the average.

7. Structural Design and Analysis

The mechanical structure integrates the battery pack, power controller, and cold plate as a single assembly. Two T‑shaped side rails, milled from aluminium alloy 2A12H112, provide stiffness and carry the loads from the front and rear panels. The entire unit is mounted in a standard 19‑inch rack using captive fasteners and guide pins. Finite‑element analysis (FEA) was performed to verify the structural integrity under sinusoidal vibration:

All stress values are well below the yield strength of aluminium (321 MPa). The unit passed both unit‑level and system‑level vibration tests without any deformation or loosening of fasteners.

8. Maintainability Design

To enable on‑orbit replacement by astronauts within 2 hours (MTTR ≤ 2 h), we incorporated:
– **Blind‑mate electrical connector**: A single 37‑pin hybrid connector on the rear panel carries all power and signal lines. Two guide pins ensure alignment before the connector is mated.
– **Guided insertion**: The side rails have a chamfered lead‑in that self‑aligns the unit as it slides into the rack.
– **Captive fasteners**: All mounting screws are of the “non‑captive” type that remain attached to the unit when loosened, preventing floating hardware.

The removal procedure is: disconnect the coolant quick‑disconnects → loosen the four captive screws → slide the unit out. Installation follows the reverse order. The design was verified with a full‑scale mockup and astronaut trials.

9. Human‑Factor (Ergonomics) Design

The system underwent a system‑level ergonomic evaluation at the China Manned Space Agency’s simulation facility. Key ergonomic features include:
– Hand‑holds and labelling clearly indicate the correct orientation.
– The mass (approximately 50 kg) is balanced such that a single astronaut can handle it with a wrist‑mounted tether.
– Connectors are colour‑coded and keyed to prevent mis‑mating.

10. Medical (Crew Health) Design

For a manned cabin, all materials must meet strict limits on off‑gassing, microbial growth, and odour. The most challenging requirement is the concentration of 46 volatile organic compounds (VOCs) – each must be below part‑per‑million levels. We minimised the use of organic materials; where unavoidable we selected low‑outgassing silicones and fluoropolymer‑insulated wires. A three‑stage bake‑out process (cell‑level, module‑level, and final assembly) in a vacuum oven at 60 °C for 48 hours reduced the total VOC emission rate to well below the permissible limits. The unit passed the final off‑gas test before delivery.

11. In‑Orbit Performance

The energy storage cell system was launched in 2022 and has been operating continuously for over three years. Telemetry data (sampled every 1 s) show:

– **Charging current**: stable at 11 A (two CRM modules active), switching to constant‑voltage when the pack reaches 93 V.
– **Discharge current**: peaks at 99 A (≈9 kW) during payload operation, lasting 180 s. One telemetry anomaly showed a single data point at 140 A, but the pack voltage was unaffected, confirming it was a measurement glitch.
– **Pack voltage**: ranges between 84 V and 93 V over the orbit.
– **Temperature**: measured at five locations on the battery pack; the maximum temperature during discharge rises to 25 °C from a baseline of 21 °C, well within the design limit.

The system has completed more than 15 000 charge/discharge cycles with no degradation in capacity or increase in internal resistance. All autonomous protection functions have operated correctly.

12. Conclusion

We have successfully designed and fielded the first on‑orbit maintainable high‑power energy storage cell system for a manned spacecraft. The system combines a NCA/graphite lithium‑ion battery pack (153 Wh/kg, 3 C discharge), an integrated power controller with ZVS charging, and a liquid‑cooled cold plate into a single unit that can be replaced by astronauts in under 2 hours. Safety enhancements (thicker separator and can) and rigorous off‑gassing controls ensure compatibility with the crewed environment. After three years of stable in‑orbit operation, the design has proven to be reliable and efficient. This technology provides a strong foundation for future space stations, lunar research stations, and other long‑duration manned missions that require high‑power energy storage with on‑orbit servicing.