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LiFePO4 battery

LiFePO4 battery – definition

A LiFePO4 battery is a rechargeable lithium-ion battery that uses lithium iron phosphate (LiFePO4, also called LFP) as the cathode material. In mobile robotics, it is a DC energy source for the traction system, onboard computer, sensors, communication hardware, and payload electronics of an unmanned ground vehicle (UGV).

LiFePO4 chemistry is commonly selected for UGV power systems because it provides a relatively flat discharge-voltage curve, good thermal stability, and a high cycle-life potential when operated within the limits specified by the cell manufacturer. It is not equivalent to a generic “lithium battery.” Battery voltage, allowable current, charging profile, protection thresholds, connector type, and battery-management-system configuration must be treated as system-specific parameters.

For Leo Rover and Raph Rover projects, a LiFePO4 battery should be considered part of the complete electrical architecture. The battery must be compatible with the rover’s input-voltage range, motor-controller requirements, onboard DC/DC converters, payload power budget, and safety protection design. Neither Leo Rover nor Raph Rover becomes autonomous solely by using a higher-capacity battery. Energy storage affects mission duration and payload capability, while autonomy requires a navigation, perception, and control stack.

Cell voltage and pack architecture

A LiFePO4 pack is built from one or more electrochemical cells connected in series and, where higher capacity is required, in parallel. Series connections increase nominal voltage. Parallel connections increase available capacity in ampere-hours (Ah).

For example, a four-cell series pack is commonly described as a 4S LiFePO4 battery. Using a nominal cell voltage of 3.2 V, its nominal pack voltage is approximately 12.8 V. The actual voltage changes with state of charge, load current, temperature, cell age, and internal resistance.

Parameter Typical LiFePO4 value Engineering meaning Source
Nominal cell voltage 3.2 V Reference value used for pack-level nominal-voltage calculations. EVE Energy LiFePO4 cell specifications
Charge upper-limit voltage 3.65 V per cell Maximum charge voltage specified for many LiFePO4 cells. It is not a nominal operating voltage. EVE Energy LiFePO4 cell specifications
Lower discharge limit 2.5 V per cell A common manufacturer-specified cell cutoff limit. A BMS may use a higher practical cutoff to preserve reserve capacity. EVE Energy LiFePO4 cell specifications
Nominal voltage of a 4S pack 12.8 V Calculated as 4 × 3.2 V. Derived from the nominal cell voltage above.

The energy stored in a battery is often estimated in watt-hours. The nominal estimate is useful for mission planning, but it does not account for converter losses, motor-current peaks, terrain resistance, temperature, or BMS cutoff behavior.

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

Example:
12.8 V × 20 Ah = 256 Wh

Battery management system and UGV protection

A LiFePO4 pack for a mobile robot should normally include a battery management system (BMS). The BMS monitors cell voltages and may provide cell balancing, overcharge protection, over-discharge protection, over-current protection, short-circuit protection, and temperature-related charge or discharge inhibition.

The BMS is a protection subsystem, not a replacement for electrical-system design. A UGV integration should still use correctly rated wiring, connectors, fuses, emergency power isolation, and DC/DC converters. Motor startup, wheel blockage, climbing, loose terrain, and high-payload operation can produce current peaks that are much higher than the average current measured during idle operation.

Before connecting a battery to Leo Rover, Raph Rover, or an external payload power rail, verify the following against the relevant rover documentation and battery data sheet:

  • Nominal and maximum battery-pack voltage.
  • Continuous and peak discharge-current ratings.
  • Allowed charging voltage and charging-current limit.
  • BMS low-voltage cutoff and recovery behavior.
  • Polarity, connector pinout, fuse rating, and cable gauge.
  • Whether payload power is supplied through a regulated DC/DC output or directly from the battery bus.

Leo Rover uses an onboard Raspberry Pi compute unit and supports ROS 2 Humble. A battery integration should therefore account for both traction power and stable supply voltage for the Raspberry Pi, motor controller, cameras, LiDAR units, IMUs, and network hardware. Brownouts can interrupt the compute unit even when the pack retains usable energy. The rover documentation should be the authority for supported power interfaces and permitted electrical configurations.

Raph Rover is intended for applications requiring greater payload capacity. A larger payload does not only increase static mass. It can also increase traction demand, acceleration current, and the energy required for terrain traversal. Battery sizing for Raph Rover should therefore be based on measured mission current rather than capacity alone.

LiFePO4 battery monitoring in ROS 2

ROS 2 does not define a mandatory battery topic name, publishing frequency, or hardware driver. However, the standard sensor_msgs/msg/BatteryState message provides fields for voltage, current, charge, capacity, design capacity, percentage, power-supply status, health, technology, and cell-voltage data.

A battery-monitor node can read values from a smart BMS through CAN, UART, USB, Modbus, or another hardware interface and publish them for diagnostics, logging, or mission logic. The message field power_supply_technology includes POWER_SUPPLY_TECHNOLOGY_LIFE for lithium iron phosphate batteries.

ros2 topic echo /battery_state
ros2 interface show sensor_msgs/msg/BatteryState

The following topic inspection command is useful after integrating a battery-monitor driver. It confirms the message type and Quality of Service configuration used by the node.

ros2 topic info /battery_state --verbose

Battery percentage should not be calculated from voltage alone unless the BMS manufacturer explicitly provides a valid voltage-to-state-of-charge model for the installed cells and operating conditions. LiFePO4 has a comparatively flat discharge curve, so voltage-only estimation can be inaccurate during much of the usable discharge range. Coulomb counting combined with cell-voltage and temperature data is generally more informative when supported by the BMS.

Standards, transport, and documentation

Rechargeable lithium cells and battery packs must be selected and handled according to applicable product, transport, and laboratory safety requirements. IEC 62133-2 covers safety requirements for portable sealed secondary lithium cells and batteries. IEC 62619 addresses safety requirements for secondary lithium cells and batteries used in industrial applications. Transport testing for lithium cells and batteries is specified in the UN Manual of Tests and Criteria, Part III, subsection 38.3.

For a research UGV, retain the battery data sheet, BMS documentation, charger specification, wiring diagram, and test records. These documents are necessary for repeatable integration, fault analysis, and safe replacement of battery packs.

See also