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Li-ion battery

Li-ion battery – definition

A lithium-ion (Li-ion) battery is a rechargeable electrochemical energy-storage system based on the reversible movement of lithium ions between a negative electrode and a positive electrode. In mobile robotics, a Li-ion battery pack supplies electrical energy to propulsion motors, motor controllers, onboard computers, sensors, network equipment, and auxiliary payloads.

A battery pack is not equivalent to a single cell. A practical UGV pack typically includes multiple cells connected in series to increase voltage and, where required, in parallel to increase capacity and allowable current. It also includes conductors, fuses, connectors, enclosure components, temperature sensing, and usually a Battery Management System (BMS).

For a mobile robot, battery selection affects runtime, payload capacity, drive performance, peak-current availability, voltage stability, thermal behavior, and safety. It also affects the reliability of compute hardware. A Raspberry Pi-based computer, motor controller, LiDAR, depth camera, or GNSS receiver may reset or report incorrect data if the power rail drops below its specified operating range during acceleration or high-load payload operation.

Li-ion battery architecture in UGV platforms

Li-ion cells are assembled into packs using series and parallel connections. The electrical properties of the complete pack depend on both the cell chemistry and the electrical configuration. Cell-level specifications must not be used as pack-level specifications without accounting for the number of cells and the BMS limits.

The main pack relationships are shown below. These equations describe ideal electrical scaling and do not include voltage sag, BMS losses, cable resistance, temperature effects, or cell aging.

Pack nominal voltage = cells in series × cell nominal voltage

Pack capacity [Ah] = parallel cell groups × cell capacity [Ah]

Nominal energy [Wh] = nominal voltage [V] × capacity [Ah]

Runtime [h] ≈ usable battery energy [Wh] / average system power [W]

For example, a pack with higher nominal energy does not automatically provide a longer useful mission time. The usable energy depends on the permitted depth of discharge, BMS cutoff thresholds, current draw, temperature, and the battery’s state of health.

Key electrical and operational parameters should be verified in the battery and BMS documentation:

  • Nominal voltage [V] – a chemistry-dependent reference voltage used for pack classification and energy calculations.
  • Maximum charge voltage [V] – the highest permitted pack voltage during charging.
  • Discharge cutoff voltage [V] – the BMS or load-disconnect threshold intended to prevent excessive cell discharge.
  • Capacity [Ah] – stored charge under a stated discharge test condition.
  • Energy [Wh] – a more useful runtime metric than ampere-hours when comparing packs with different voltages.
  • Continuous and peak discharge current [A] – current limits defined by the cells, wiring, connectors, fuse, and BMS.
  • State of charge (SoC) [%] – an estimate of remaining charge, not a direct measurement of remaining runtime.
  • State of health (SoH) [%] – an estimate of remaining capacity or performance relative to the battery’s initial condition.

Battery Management System and protection

A BMS supervises a multi-cell battery pack. Its functions commonly include cell-voltage monitoring, overcurrent protection, overcharge protection, undervoltage protection, temperature monitoring, and cell balancing. The exact behavior is implementation-specific. Engineers should treat BMS cutoff values, balancing method, communication interface, and fault recovery procedure as hardware-specific parameters.

Cell balancing is important in series-connected packs because unequal cells can reach charge or discharge limits before the rest of the pack. A pack-level voltage alone may therefore be insufficient to identify a weak or unbalanced cell group. Where a BMS exposes per-cell telemetry, it should be logged during charge, idle, and loaded operation.

Li-ion battery packs must be charged with a charger compatible with the pack chemistry, series count, charge-voltage limit, and BMS requirements. A charger intended for a different battery type, such as lead-acid or nickel-metal hydride, must not be used unless the battery manufacturer explicitly specifies compatibility.

Battery monitoring in ROS 2

ROS 2 defines the sensor_msgs/msg/BatteryState message for battery telemetry. The message can represent voltage, current, temperature, charge, capacity, design capacity, percentage, power-supply status, health, technology, and cell-level voltage or temperature arrays. The POWER_SUPPLY_TECHNOLOGY_LION constant identifies lithium-ion battery technology.

A ROS 2 battery driver may obtain data from a smart BMS over CAN, UART, USB, I2C, or another vendor-specific interface. It may also estimate battery state from an analog voltage measurement. These approaches have different accuracy. Voltage-only SoC estimation is limited because terminal voltage changes with load, temperature, cell aging, and relaxation time.

ros2 interface show sensor_msgs/msg/BatteryState

ros2 topic echo /battery_state

ros2 topic hz /battery_state

The topic name is not fixed by the message definition. A robot may publish /battery_state, /power/battery_state, or a namespaced equivalent. Integrators should inspect the robot’s launch files and use ros2 topic list before configuring Nav2, diagnostics, or fleet-monitoring software.

Li-ion batteries on Leo Rover and Raph Rover

On Leo Rover, the battery system must supply the differential-drive base and the onboard Raspberry Pi compute unit. Additional sensors such as a 2D LiDAR, depth camera, IMU, GNSS receiver, or USB hub increase the electrical load. A navigation stack can remain computationally active while the robot is stationary, so mission planning should account for both drive energy and continuous compute-and-sensor power.

Leo Rover can be used with ROS 2; supported distributions depend on the installed software image and platform documentation. Autonomous operation is not provided by battery hardware alone. Battery telemetry can be integrated into a ROS 2 system to trigger warnings, stop a mission, return to a charging location, or shut down compute hardware before a low-voltage event. The threshold values must be derived from the installed battery pack and BMS documentation, not from generic ROS settings.

Raph Rover is intended for larger payload configurations. A higher-payload UGV can also have higher traction, compute, sensing, and payload-power requirements. When integrating LiDARs, NVIDIA Jetson computers, RTK receivers, radio links, or inspection payloads, engineers should create a power budget covering average consumption, startup inrush current, peak drive current, connector ratings, and safety margin.

Limitations and engineering trade-offs

Li-ion batteries have high energy density relative to many rechargeable battery types, but they require controlled charging, mechanical protection, and thermal management. Battery performance is not constant across the discharge cycle. Under high current demand, internal resistance causes voltage drop. This can reduce available propulsion power or trigger BMS protection even when the estimated SoC is not zero.

For field robotics, the battery pack should be evaluated under representative conditions. Relevant tests include driving on the expected terrain, operating payload sensors continuously, logging voltage and current, measuring enclosure temperature, and checking recovery after a BMS protection event. An IP-rated rover enclosure does not remove the need to protect battery connectors and charging interfaces from contamination and moisture.

Normative references and standards

Battery safety, transport, and ROS message semantics are covered by different documents. The following references are appropriate starting points for engineering documentation and verification.

See also