Battery Technology
Introduction
Batteries are the core energy storage element of a robot. Different battery chemistries exhibit significant differences in energy density, discharge capability, safety, and cost. Correctly selecting the battery type and properly designing the battery pack are fundamental to robot runtime and reliability.
Battery Fundamentals
Key Parameters
| Parameter | Symbol | Unit | Description |
|---|---|---|---|
| Nominal voltage | \(V_{nom}\) | V | Rated operating voltage of the battery |
| Capacity | \(C\) | Ah / mAh | Charge stored in the battery |
| Energy | \(E\) | Wh | Total energy stored in the battery |
| C-rate | C-rate | C | Charge/discharge rate relative to capacity |
| Internal resistance | \(R_{int}\) | mohm | Battery internal resistance |
| Cycle life | — | cycles | Number of charge/discharge cycles at 80% capacity |
Energy Calculation
Total energy stored in a battery:
\[E = V_{nom} \times C_{Ah}\]
For example, a 3.7V / 5000 mAh battery:
\[E = 3.7V \times 5.0Ah = 18.5Wh\]
Runtime Estimation
\[t_{runtime} = \frac{E_{battery}}{P_{total}} = \frac{V_{nom} \times C_{Ah}}{P_{total}}\]
If the robot's total power consumption is 30W using the above battery:
\[t = \frac{18.5Wh}{30W} \approx 0.62h \approx 37min\]
C-Rate
The C-rate expresses the discharge rate relative to capacity. For a 5000 mAh battery:
- 1C discharge = 5A, lasts 1 hour
- 2C discharge = 10A, lasts 30 minutes
- 10C discharge = 50A, lasts 6 minutes
\[I_{discharge} = C_{rate} \times C_{Ah}\]
LiPo (Lithium Polymer Battery)
LiPo is the most popular battery type in RC models and small robots, known for high discharge capability and light weight.
Specifications
| Parameter | Value |
|---|---|
| Nominal voltage (per cell) | 3.7V |
| Full charge voltage | 4.2V |
| Cutoff voltage | 3.0V (recommended >= 3.3V) |
| Energy density | 150–250 Wh/kg |
| Discharge C-rate | 20C–100C (pulsed even higher) |
| Cycle life | 300–500 cycles |
Battery Pack Labeling
LiPo batteries are typically labeled as "XS YYYYmAh ZZC":
- XS: Number of cells in series, determines voltage. E.g., 3S = \(3 \times 3.7V = 11.1V\)
- YYYYmAh: Capacity
- ZZC: Maximum continuous discharge rate
Common specifications:
| Specification | Voltage Range | Common Use |
|---|---|---|
| 2S (7.4V) | 6.0–8.4V | Small robots |
| 3S (11.1V) | 9.0–12.6V | Medium wheeled robots |
| 4S (14.8V) | 12.0–16.8V | Drones, quadrupeds |
| 6S (22.2V) | 18.0–25.2V | Large drones, robots |
Pros and Cons
- Pros: Lightweight, high discharge rate, flexible form factor, moderate cost
- Cons: Swelling risk, requires careful storage, sensitive to overcharge/overdischarge, shorter lifespan
18650 Lithium-Ion Batteries
The 18650 (18 mm diameter, 65 mm length) is the most mature cylindrical lithium cell standard, widely used in laptops, power tools, and robots.
Common Models
| Model | Manufacturer | Capacity | Max Continuous Discharge | Features |
|---|---|---|---|---|
| NCR18650B | Panasonic/Sanyo | 3400 mAh | 5A (1.5C) | High capacity |
| INR18650-25R | Samsung | 2500 mAh | 20A (8C) | High discharge |
| INR18650-30Q | Samsung | 3000 mAh | 15A (5C) | Balanced |
| INR18650-HG2 | LG | 3000 mAh | 20A (6.7C) | High discharge |
| INR18650-VTC6 | Sony | 3000 mAh | 30A (10C) | Very high discharge |
Nominal Parameters
| Parameter | Value |
|---|---|
| Nominal voltage | 3.6–3.7V |
| Full charge voltage | 4.2V |
| Cutoff voltage | 2.5V (recommended >= 3.0V) |
| Energy density | 200–270 Wh/kg |
| Cycle life | 500–1000 cycles |
Unitree Go2 Battery Pack Case Study
The Unitree Go2 quadruped robot uses 32 18650 cells to form its battery pack:
- Configuration: 8S4P (8 series, 4 parallel)
- Nominal voltage: \(8 \times 3.6V = 28.8V\)
- Capacity: \(4 \times 3.0Ah = 12.0Ah\) (assuming 30Q cells)
- Total energy: \(28.8V \times 12.0Ah = 345.6Wh\)
- Runtime: Approximately 1–2 hours (depending on locomotion mode)
Series and Parallel Design
Series: Increases voltage
\[V_{total} = n_s \times V_{cell}\]
Parallel: Increases capacity and discharge capability
\[C_{total} = n_p \times C_{cell}\]
\[I_{max} = n_p \times I_{cell\_max}\]
XS YP Configuration Examples:
| Configuration | Voltage | Capacity (3Ah/cell) | Cell Count |
|---|---|---|---|
| 3S1P | 11.1V | 3 Ah | 3 |
| 4S2P | 14.8V | 6 Ah | 8 |
| 6S3P | 22.2V | 9 Ah | 18 |
| 8S4P | 29.6V | 12 Ah | 32 |
Pros and Cons
- Pros: High consistency, mature supply chain, replaceable, long cycle life
- Cons: Cylindrical form not easily packed tightly, requires dedicated holders or spot welding, lower per-cell discharge capability than LiPo
LiFePO4 (Lithium Iron Phosphate Battery)
LiFePO4 (also known as LFP) is renowned for its safety and long cycle life, suitable for applications with high safety requirements.
Specifications
| Parameter | Value |
|---|---|
| Nominal voltage (per cell) | 3.2V |
| Full charge voltage | 3.65V |
| Cutoff voltage | 2.5V |
| Energy density | 90–160 Wh/kg |
| Discharge C-rate | 1–5C (some high-power types reach 10C) |
| Cycle life | 2000–5000 cycles |
Pros and Cons
- Pros: Extremely high safety (resistant to thermal runaway), very long cycle life, flat discharge curve, better cold-weather tolerance
- Cons: Low energy density (larger and heavier), low voltage platform (4S needed to approach 12V)
Suitable Scenarios
- Indoor service robots (safety first)
- AGV/AMR (Automated Guided Vehicles)
- Large outdoor robots (weight is tolerable)
- Scenarios requiring frequent charge/discharge cycles
Battery Type Comparison
| Feature | LiPo | 18650 Li-ion | LiFePO4 |
|---|---|---|---|
| Nominal voltage/cell | 3.7V | 3.6–3.7V | 3.2V |
| Energy density | 150–250 Wh/kg | 200–270 Wh/kg | 90–160 Wh/kg |
| Max discharge rate | 20–100C | 1.5–10C | 1–5C |
| Cycle life | 300–500 | 500–1000 | 2000–5000 |
| Safety | Moderate | Fairly high | Very high |
| Cost | Moderate | Lower | Moderate |
| Form factor flexibility | High (pouch) | Low (cylindrical) | Moderate |
| Typical application | Drones, RC robots | Quadrupeds, large robots | AGV, service robots |
Other Battery Technologies
NiMH (Nickel-Metal Hydride)
- Nominal voltage: 1.2V/cell
- No memory effect (modern types), good safety
- Low energy density (60–120 Wh/kg)
- Suitable for: Low-cost educational robots
Lead-Acid
- Nominal voltage: 2.0V/cell (12V = 6S)
- Very cheap but extremely heavy (30–50 Wh/kg)
- Suitable for: Large AGV chassis (where weight is not a concern)
Solid-State Batteries (Future)
- Uses solid electrolyte instead of liquid
- Theoretical energy density >400 Wh/kg
- Extremely high safety, still in early mass production stage
Battery Selection Decision Flow
graph TD
A[Determine power requirements] --> B{Safety requirements?}
B -->|Very high| C[LiFePO4]
B -->|Normal| D{Discharge rate needed?}
D -->|>10C| E[LiPo]
D -->|<10C| F{Energy density priority?}
F -->|Yes| G[18650 Li-ion]
F -->|No| H{Cost sensitive?}
H -->|Yes| I[NiMH / 18650]
H -->|No| G
Battery Safety Precautions
- Storage: Store in a cool, dry place; LiPo storage voltage 3.8V/cell
- Transportation: Comply with air transport regulations (IATA DGR)
- Charging: Use a matched charger; never overcharge
- Physical protection: Avoid compression, puncture, and short circuits
- Temperature: Charge at 0–45°C, discharge at -20–60°C
- Disposal: Do not discard casually; bring to designated recycling points
References
- Battery University: batteryuniversity.com
- 18650 Battery Database: lygte-info.dk
- Unitree Go2 teardown analysis
- Cell manufacturer datasheets (Samsung SDI, LG Energy, Panasonic)