Structure and composition of electric bicycle power battery

1、 Battery type and chemical composition

Electric bicycle power batteries are mainly divided into three categories, each with its own chemical characteristics and application scenarios:

1. Lead acid battery
   • Electrode material: Lead and its oxides as positive and negative electrodes, sulfuric acid solution as electrolyte.
   • Features: Low cost, mature technology, but low energy density (short battery life), heavy weight, gradually replaced by lithium batteries.
2. Lithium ion battery (mainstream choice)
   • Positive electrode material:
     • Lithium iron phosphate (LiFePO ₄): High safety, ignition point up to 800 ℃, long cycle life, low cost (0.6 yuan/Wh).
     • Ternary materials (such as lithium nickel cobalt manganese oxide): high energy density, longer range, but higher cost (0.8 yuan/Wh).
   • Negative electrode material: graphite or graphite like carbon material, providing lithium ion insertion/extraction channels.
   • Electrolyte: Organic solvents containing lithium hexafluorophosphate (such as carbonates), used as lithium ion transport media.
   • Diaphragm: A polymer microporous membrane that allows lithium ions to pass through but blocks electrons, preventing short circuits.
3. Graphene lead-acid battery
   • Upgrade point: Adding graphene to lead-acid batteries to improve energy density, charging speed, and cycle life.
   • Positioning: A transitional product between lead-acid batteries and lithium batteries.

2、 Battery structure hierarchy

The power battery adopts modular design, with clear hierarchy from battery cells to battery packs:

1. Cell
   • Core unit: composed of positive electrode, negative electrode, separator, electrolyte, and shell.
   • Working principle: During charging and discharging, lithium ions migrate between the positive and negative electrodes through the electrolyte, completing energy conversion.
   • Packaging form: cylindrical (such as 18650 battery), square or soft package (aluminum-plastic film packaging).
2. Module
   • Composition: Multiple battery cells connected in series/parallel, fixed by brackets and busbars to increase voltage and capacity.
   • Example: 13 string modules (nominal voltage 48V) are commonly used in electric bicycles.
3. Battery Pack
   • Integrated system: includes multiple modules, battery management system (BMS), temperature control system, housing, and connectors.
   • Shell material: steel, aluminum or plastic, meeting the requirements of protection (dust and water resistance), heat dissipation and lightweight.
   • Layout: Usually installed at the bottom of the frame, lowering the center of gravity to enhance stability.

3、 Battery Management System (BMS)

BMS is the 'brain' of the battery, ensuring its safe and efficient operation. Its core functions include:

1. Condition monitoring
   • Real time collection of voltage, current, temperature and other parameters to estimate remaining charge (SOC) and state of health (SOH).
2. Balanced management
   • Balance the voltage difference between battery cells, avoid overcharging/overdischarging of single battery cells, and extend the overall lifespan.
3. Safety protection
   • Overcharge/overdischarge protection: prevents battery damage or thermal runaway.
   • Overcurrent/short circuit protection: Cut off abnormal currents to avoid fire risks.
   • Temperature management: Control the temperature through heat sinks, fans, or liquid cooling systems (the optimal operating temperature for lithium batteries is 20-45 ℃).
4. communication interface
   • Interact with vehicle dashboard, charger and other devices to display battery status and fault information.

4、 Safety and protection measures

From design to use, the entire chain ensures battery safety:

1. Structure Design
   • Explosion proof valve: Release internal pressure to prevent explosion.
   • Sealed shell: prevents electrolyte leakage, avoids corrosion or short circuit.
2. thermal management
   • Passive heat dissipation: The aluminum casing naturally dissipates heat.
   • Active cooling: fan or liquid cooling system (high-performance models).
3. electrical protection
   • Fuse: When overcurrent occurs, it melts and cuts off the circuit.
   • BMS intervention: Actively limit the charging and discharging power when abnormalities are detected.
4. using standards
   • Avoid modification: Changing non original batteries or adding battery cells without authorization may cause short circuits.
   • Original charger: matches the battery charging curve to prevent overcharging.
   • Regular inspection: Check if the battery casing and wiring are intact to avoid water ingress or physical damage.

↑ Electric bicycle power battery pack (lithium-ion battery cells)

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