Robot Battery Packs and BMS: Chemistry, Voltage and Protection

Cells, modules and packs

A cell is the basic electrochemical unit. Cells are connected in series to raise voltage and in parallel to raise capacity; a pack described as "14S2P" has fourteen groups in series, each of two cells in parallel. Cells come in cylindrical, prismatic and pouch formats. Cylindrical cells are standardised and robust; prismatic cells pack efficiently into larger batteries; pouch cells are light and flexible in shape but need external support.

Chemistries used in robots

NMC and NCA (nickel-based). High energy density, which means more runtime for the same mass. Used in legged robots, humanoids and drones where weight matters. They need careful thermal and protection design.

LFP (lithium iron phosphate). Lower energy density but longer cycle life and better thermal stability. Common in AMRs, service robots and stationary equipment where weight matters less than lifetime and safety.

High-rate LiPo. Pouch cells built for very high discharge currents, used where short bursts of power matter, such as dynamic legged robots and drones. Cycle life is usually shorter.

Nominal voltages are typical values; use the cell manufacturer's datasheet for design.

Choosing pack voltage

Pack voltage is set by the motors and drives. Many mobile and legged robots run on nominal 24 V or 48 V systems; higher-power robots use higher voltages to reduce current and cable size. Remember that a lithium pack's voltage varies with state of charge, so drives must operate across the full range from fully charged to empty, and must handle regenerative braking energy without exceeding the pack's charge limits.

What the BMS does

The battery management system protects the cells and reports their state.

Protection. Over-voltage, under-voltage, over-current, short-circuit and over-temperature protection, usually by switching MOSFETs or a contactor.

Balancing. Equalising the charge of series groups so that no group is over-charged or over-discharged. Passive balancing bleeds excess energy as heat; active balancing moves it between groups.

State estimation. Estimating state of charge and state of health from voltage, current and temperature.

Communication. Reporting data to the robot over CAN, RS-485, SMBus or similar, so that the robot can plan missions, warn the operator and shut down safely.

Pre-charge and power-up. Limiting inrush current when the robot's drives connect, which prevents damage to contacts and capacitors.

Mechanical and thermal design

Cells must be held firmly, protected from crushing and puncture, and kept within their temperature range during charge and discharge. Legged robots experience shock on every step, so cell fixing, busbar design and connectors must withstand vibration. Swappable packs need latches and connectors rated for repeated mating and for the full current.

Charging

Define the charger voltage and current, the charging temperature range and the charging connector. Docking stations for autonomous robots must align contacts reliably and must not short them. Fast charging shortens downtime but raises cell temperature and may reduce cycle life; the cell supplier's limits apply.

Certification and shipping

Lithium batteries are dangerous goods in transport, and every battery shipped needs a UN 38.3 test summary. Depending on your market and application, customers may also request test reports to safety standards for portable or industrial lithium batteries, such as IEC 62133-2 or IEC 62619. Ask which reports exist for the exact pack, not only for the cells.

Checklist for a battery pack inquiry

1. Nominal voltage, capacity and maximum continuous and peak current.
2. Size, mass and mounting constraints.
3. Chemistry preference and target cycle life.
4. BMS functions and communication protocol.
5. Charger, charging connector and charging time.
6. Operating and charging temperature ranges.
7. Shipping documents and test reports required.

Sources

• EVE Energy product information
• Grepow product information
• IATA: lithium batteries