We present the design and application of a novel high-power energy storage battery system specifically engineered for on-orbit maintenance. This lithium-ion battery pack is characterized by high specific energy and substantial discharge capability, making it ideal for long-term manned platforms such as space stations. Our design integrates a 60 Ah battery pack with a power controller and a cold plate, facilitating modular replacement by astronauts. The system has been successfully operating in orbit for over three years, demonstrating stable performance and providing a crucial technical reference for future manned space missions, including lunar research stations.
1. Introduction
Over the past decade, lithium-ion batteries have become increasingly prevalent in spacecraft applications due to their high specific energy, low self-discharge rate, high charging efficiency, and absence of memory effects. These batteries are utilized for both platform and payload applications. Platform batteries typically require low discharge power over extended periods, while payload batteries demand high-power, short-duration discharges. The payload system we addressed requires a power output exceeding 9.3 kW for more than 3 minutes, a level unattainable by the primary platform power supply. Therefore, an energy storage battery system was developed to function as an energy reservoir, meeting the short-duration, high-power demands of the payload. Unlike satellite applications, a space station is a long-term human habitat, imposing stringent requirements on safety, ergonomics, and space medicine. This paper details the design and application of an on-orbit maintainable, high-power energy storage battery pack.
2. Overall System Design
Our energy storage battery is designed for the China Space Station’s dedicated payload. The system comprises three primary components: a lithium-ion battery pack, a power controller, and a cold plate assembly. The entire unit is housed within a standard cabin interior equipment rack and is designed for in-orbit installation and maintenance. To achieve this, we adopted an integrated design of the battery pack and the power controller.
Table 1 details the components of our energy storage battery system.
| Assembly | Component | Quantity | Description |
|---|---|---|---|
| Lithium-ion Battery Pack | 30 Ah Lithium-ion Cells | 46 | 2 parallel cells x 23 series cells = 60 Ah pack |
| Cell Modules | 23 | Each module has 2 parallel cells | |
| Power Controller | Charge Regulator Module | 3 | Battery pack charging management |
| Lower Computer Module | 1 | Telecommand/telemetry, RS422 bus communication | |
| Sampling and Equalization Module | 1 | Cell voltage monitoring, balancing management | |
| Power Transfer Module | 1 | Bus protection, power bus routing, secondary power, commands, and telemetry | |
| Cold Plate Assembly | Cold Plate | 1 | Heat dissipation cold plate (with quick disconnects) |
| Coolant Hoses | Set | Circulate 36% ethylene glycol coolant |
3. Detailed Design
3.1 Power Controller Design
The power controller is the brain of our energy storage battery, responsible for acquiring voltage, temperature, current, and switch status, as well as managing charging and discharging. It consists of seven modules: three Charge Regulator Modules, one Lower Computer Module, one Sampling and Equalization Module, and one Power Transfer Module.
3.1.1 Lower Computer Module
This module acts as the control center. It incorporates logic control circuits, battery equalization algorithm (BEA) circuits, signal processing circuits, and power supply circuits. A key innovation is the “drawer-type” program download module, which allows for software modifications even when the unit is fully assembled, eliminating the need to open the chassis.
3.1.2 Charge Regulator Module
We designed three Charge Regulator Modules using a phase-shifted full-bridge topology with isolation. This ensures high-efficiency, high-power charging while providing necessary electrical isolation from the space station’s main bus. Under normal operation, two modules are active, each handling 600 W. If one fails, a third module acts as a redundant backup. The efficiency of this module is greater than 92%. The charging power (with two modules) is no less than 1200 W.
The control block diagram is described by the following functional relationship:
$$ P_{charge} = V_{bus} \cdot I_{charge} \cdot \eta $$
The charging module’s output voltage and current are regulated by a constant voltage/constant current (CV/CC) control loop. The CC/CV transition is automatic.
$$ V_{out} = \begin{cases} V_{float} & \text{if } I_{charge} < I_{threshold} \\ V_{batt} & \text{if } I_{charge} \geq I_{threshold} \end{cases} $$
An input overcurrent protection circuit is integrated, with a trip point set at \(13 \pm 1\) A to prevent damage to the station’s power bus in case of a short circuit.
3.1.3 Sampling and Equalization Module
This module performs cell voltage sampling, small-group voltage sampling, and total pack voltage sampling for the 2-parallel, 23-series battery configuration. It also provides 23 channels of shunt-based balancing and a “normally on” function for discharging the battery pack to a safe voltage during maintenance.
The cell voltage sampling circuit uses precision instrumentation amplifiers (AD620) with high input impedance (\(10^9 \Omega\)) to mitigate errors from source impedance imbalances. A multi-channel analog multiplexer (HS1-0546RH-Q) and a 12-bit ADC (AD574) are used for digitization. The total pack voltage is measured by three independent circuits for redundancy, with the final value being the average of valid readings. The measurement accuracy is:
$$ \text{Cell voltage error} \leq \pm 10 \text{ mV} $$
$$ \text{Pack voltage error} \leq \pm 270 \text{ mV} $$
The balancing circuit for each cell uses a Darlington pair of transistors, controlled by an optocoupler for isolation. The two-transistor series design prevents accidental shunting due to a single component failure. The design can be represented by the following logic:
$$ \text{Balancing State} = \begin{cases} \text{ON} & \text{if } V_{cell} > V_{balance\_threshold} \text{ AND } \text{Command ON} \\ \text{OFF} & \text{otherwise} \end{cases} $$
3.1.4 Power Transfer Module
This module provides platform bus input protection, filtering, and power routing. It includes a high-power DC contactor, current Hall sensors, and secondary power supplies. The design uses a six-cavity integrated blind-mate connector for easy on-orbit installation without visual alignment.
3.2 Lithium-ion Battery Pack Design
The core of our energy storage battery is designed for a 5-year low-Earth orbit (LEO) life, a 2C to 3C discharge rate, and high safety standards for manned platforms.
3.2.1 Battery Pack Configuration
The battery pack is composed of 46 cylindrical lithium-ion cells configured in a 2-parallel by 23-series arrangement, yielding a nominal capacity of 60 Ah. Based on a single cell voltage range of 3.4 V to 4.1 V and a 1 V internal drop, the series count was calculated:
$$ N_{series} = \frac{V_{pack\_max} + V_{drop}}{V_{cell\_max}} \approx \frac{96V + 1V}{4.2V} \approx 23 $$
$$ V_{pack\_min} = (V_{cell\_min} \times N) – V_{drop} = (3.4V \times (23-1)) – 1V = 73.8V $$
$$ V_{pack\_max} = (V_{cell\_max} \times N) – V_{drop} = (4.2V \times 23) – 1V = 95.6V $$
The resulting operating voltage range of 73.8 to 95.6 V meets the required 72 to 96 V specification. The use of a cylindrical cell structure provides excellent mechanical stability and tolerance to launch vibrations.
3.2.2 Cell Material System Selection
We evaluated various cathode materials (NCA-1, NCA-2, NCA-3, NCM). The selection was based on life cycle tests. Table 2 summarizes the results.
| Material System | Working Voltage (V) | Life Cycle Performance |
|---|---|---|
| NCA-1 / Graphite | 4.1 – 4.2 | Average |
| NCA-2 / Graphite | 4.2 – 4.3 | Excellent |
| NCA-3 / Graphite | 4.2 | Average |
| NCM / Graphite | 4.3 | Poor |
The NCA-2/Graphite system was chosen due to its superior cycle life, capable of supporting 29,200 cycles (5 years in LEO) at a 2C discharge rate.
3.2.3 Safety Performance Comparison
To enhance safety, we optimized the separator thickness and cell can wall thickness compared to traditional LEO batteries. A comparison is shown in Table 3.
| Parameter | Our Requirement | Our Measurement | Traditional LEO Battery |
|---|---|---|---|
| Capacity (Ah) | 30 | 33 | 50 |
| Specific Energy (Wh/kg) | 150 | 153 | 170 |
| Discharge Rate | 2 C | 3 C | 1 C |
| Separator Thickness (μm) | 40 | – | 25 |
| Can Wall Thickness (mm) | 1.00 | 0.97-1.03 | 0.60 |
| Overcharge Test | No fire/explosion | Max temp 70°C, no fire/leak | No explosion/leak |
| Short Circuit Test | No fire/explosion | Max temp 91°C, no fire/explosion | No explosion/leak |
During an external short circuit test on a 2-parallel cell module, the peak temperature reached 91°C after 9 minutes, and the battery experienced no fire, explosion, or leakage, demonstrating excellent safety characteristics.
3.3 Cold Plate Design
The cold plate assembly uses a high-density, multi-layer micro-channel heat sink technology to dissipate heat generated by the energy storage battery during high-power discharges. It circulates a 36% ethylene glycol-water coolant. The cold plate is designed to support a load of over 80 kg and provides a heat transfer rate of up to 300 W/h at a coolant flow rate of 100 L/h and an inlet temperature of 27.5°C. The quick-disconnect couplings use a mature, reliable radial sealing mechanism to prevent coolant leakage during on-orbit replacement.
4. Modes of Operation
We defined three primary operating modes for our energy storage battery: Charge, Discharge, and Idle. Mode transitions are handled via bus commands or automatically triggered protection mechanisms (overcharge, over-discharge, over-temperature). The state transition logic is as follows:
$$ \text{Mode} = \begin{cases} \text{Discharge} & \text{if } \text{Command}_{\text{discharge}} = \text{TRUE} \; \text{OR} \; \text{Protection Event} \\ \text{Charge} & \text{if } \text{Command}_{\text{charge}} = \text{TRUE} \; \text{AND} \; \text{No Protection Event} \\ \text{Idle} & \text{otherwise} \end{cases} $$
The power controller’s embedded software manages autonomous protection functions including overcharge protection, over-discharge protection, over-temperature protection, and cell balancing. Key threshold parameters can be updated via wired or wireless commands to adapt to different battery aging states.
5. Structural Design
The energy storage battery features an integrated structure where the battery pack and power controller are mechanically linked and then mounted onto the cold plate. A T-shaped side plate, machined from a single piece of 2A12H112 aluminum alloy, enhances the overall rigidity and ensures accurate co-planarity with the rack interfaces. The critical stress values from our finite element analysis (FEA) are shown in Table 4.
| Direction | Max Stress (MPa) | Location |
|---|---|---|
| X | 64.31 | Power Controller Screw Hole |
| Y | 56.62 | Front Panel |
| Z | 57.56 | Long Screw Rod |
All stress values are well below the yield limit of the aluminum alloy (321 MPa), confirming the design’s structural integrity. The unit successfully passed both unit-level and system-level vibration tests.
6. Maintainability Design
Maintainability is a critical feature of our energy storage battery. To facilitate on-orbit replacement by astronauts, the design includes guide rails on both sides and a single, blind-mate backplane connector. This connector integrates all power and signal lines, simplifying the installation process. The unit is secured with captive screws and washers. The total time for removal and replacement (MTTR) was verified to be ≤ 2 hours during maintainability tests.
7. Ergonomic and Medical Design
As our energy storage battery is intended for a manned environment, we placed a high priority on ergonomic and medical design. Ergonomic tests were conducted at the China Manned Space Agency’s facilities.
Medical design requirements included strict limits on temperature, non-metal material selection, odor levels, flammability, mold resistance, acoustic noise, non-ionizing radiation, microbial control, and gas pollutants. The most stringent requirement was limiting the release of harmful gases to sub-ppm levels. We achieved this by minimizing the use of non-metallic materials, selecting low-outgassing silicone and fluorine-free wires, and implementing a three-stage outgassing process (raw materials, modules, and final assembly). The final product passed all 46 required gas emission tests.
8. On-orbit Performance Analysis
Our energy storage battery was launched in 2022 and has been operating on the China Space Station for over three years. Telemetry data confirms stable performance. Key operational parameters are summarized in Table 5.
| Parameter | Value |
|---|---|
| Maximum Charging Current | 11 A |
| Maximum Discharging Current | 99 A (~9.3 kW) |
| Operating Voltage Range | 84 – 93 V |
| Operating Temperature Range | 21 – 25 °C |
| Charging Efficiency | > 92% |
9. Conclusion
We have successfully developed and deployed the first on-orbit maintainable high-power energy storage battery system for manned spacecraft. This energy storage battery uniquely combines high power output with high specific energy, utilizing an NCA/Graphite chemistry. The integrated design of the battery pack, power controller, and cold plate simplifies maintenance and reduces system weight. Through rigorous process control, we minimized harmful gas emissions to meet strict space medicine standards. Having operated flawlessly for over three years in orbit, our energy storage battery provides a valuable technical blueprint for future manned spacecraft, space stations, and lunar research stations. The in-orbit performance continues to be monitored, and spare units are being produced to support potential future replacements.