Battery Systems

Batteries power everything from phones to EVs and grid storage. Learn cell chemistries, series/parallel packs, capacity, C-rate, SOC and SOH, battery management systems, charging, ageing and safety.

A battery stores energy chemically and releases it as DC electricity. Cells are combined in series (for voltage) and parallel (for capacity) into packs, supervised by a battery management system (BMS) that keeps every cell within safe limits.

Lithium-ion dominates today: NMC chemistries for energy-dense EVs and electronics, LFP for long life and safety in storage and many EVs. Understanding pack design, charging and ageing is now core electrical-engineering knowledge.

An analogy

A battery pack is like a team of rowers: in series they row the same boat harder (more voltage); in parallel they add stamina (more amp-hours). The BMS is the coach keeping any single rower from collapsing.

Chemistries

Each chemistry trades energy density, life, safety and cost.

• Lead-acid: 2 V/cell, cheap, heavy, limited cycles — car starting, UPS.
• NMC/NCA Li-ion: 3.6–3.7 V, high energy density — EVs, laptops.
• LFP: 3.2 V, very long life and safer — storage, many EVs.
• Sodium-ion: emerging, cheap materials, lower density.

Series and parallel

Series raises voltage (14S of 3.6 V ≈ 50 V), parallel raises capacity and current capability. Pack notation 96S2P means 96 groups in series, each of 2 parallel cells.

Charging

Li-ion charges with constant current (CC) until the cell reaches its maximum voltage (4.2 V NMC, 3.65 V LFP), then constant voltage (CV) while current tapers. Fast charging is limited by lithium plating at low temperature and high SOC, so chargers derate current accordingly.

The BMS

Monitors every cell voltage, pack current and temperatures; protects against over/under-voltage, over-current and over-temperature; balances cells (passive bleed or active transfer) so the weakest cell doesn't limit the pack; estimates SOC and SOH; communicates with chargers and vehicles (CAN).

Ageing and safety

Capacity fades with cycles (deeper DoD and higher C-rate age faster) and calendar time (high temperature and high SOC age faster). Thermal runaway — an exothermic chain reaction triggered by internal shorts, overcharge or heat — is the main hazard; cell spacing, fire barriers, venting and fusing prevent propagation across a pack.

Formulas

• Pack voltage: V_pack = S × V_cell
• Pack capacity: Ah_pack = P × Ah_cell
• Pack energy: E = V_pack × Ah_pack
• Current at C-rate: I = C-rate × Ah

Worked example: EV pack specification

Given: 96S2P; Cell: 3.6 V, 50 Ah

Find: Pack voltage, capacity and energy.

• V = 96 × 3.6 = 345.6 V
• Ah = 2 × 50 = 100 Ah
• E = 345.6 × 100 = 34.6 kWh

Answer: 345.6 V, 100 Ah, 34.6 kWh

Worked example: Charge time at C/2

Given: 100 Ah pack; Charge current 50 A (0.5C)

Find: Approximate charge time from empty.

• CC phase ≈ 80–90 % in ~1.7–1.8 h
• CV taper adds ~0.5 h
• Total ≈ 2.2–2.5 h

Answer: ≈ 2–2.5 hours

Common mistakes

• Wrong: Connecting cells in parallel at different voltages. — Right: Equalise voltage first; large currents flow between mismatched cells.
• Wrong: Charging Li-ion below 0 °C at normal current. — Right: Low-temperature charging plates lithium; pre-heat or reduce current as specified.

Applications

• Electric vehicles: 400/800 V packs of 50–100+ kWh with liquid cooling and sophisticated BMS.
• Home & grid storage: LFP systems shift solar energy to the evening and provide backup and grid services.
• Portable electronics: Single-cell and 2–4S packs with fuel-gauge ICs.

Key takeaways

• Series adds voltage, parallel adds capacity; E = V × Ah.
• C-rate relates current to capacity.
• CC–CV charging; the BMS protects, balances and estimates SOC/SOH.
• Heat, high SOC, deep cycles and fast charging accelerate ageing.

Going further

• Equivalent-circuit models: A cell is modelled as OCV(SOC) plus series resistance and RC pairs for polarisation; Kalman filters on this model estimate SOC robustly.
• Cell-to-pack and structural packs: Removing modules (cell-to-pack) or making cells part of the vehicle structure raises pack-level energy density.
• Second life and recycling: EV batteries at 70–80 % SOH can serve years in stationary storage; hydrometallurgical recycling recovers lithium, nickel and cobalt.