The electrical design of a battery system spans everything from the electrochemical behaviour of a single cell through to a 800V pack architecture switching hundreds of amps. It is the thread that runs through every other discipline — thermal management is driven by I²R heating, mechanical design must accommodate busbar routing, the BMS exists entirely to measure and control the electrical state of the pack.
The voltage window is tighter than you expect. Most lithium cells have a total voltage range of about 1.5–2V, and the usable window the BMS permits is often narrower still. Exceeding the upper voltage limit by even a few hundred millivolts accelerates ageing significantly. Dropping below the lower limit risks copper dissolution and dendrite formation. The BMS is not just a safety device — it is actively managing lifetime.
This section covers the electrical fundamentals and the HV system in depth. Use the map below to navigate by topic.
Where to Start
New to the electrical fundamentals? This is a suggested reading order through the core material. For the complete list of every page in this section, see the Full Index below.
- Ohm’s Law – the whole picture: the single relationship that underlies everything electrical.
- Series and Parallel – how cells combine to build voltage and capacity.
- Voltage Architecture – how that combining is structured across the pack.
- Resistance – why real cells and packs don’t behave like ideal ones.
- C-rate – the standard way of expressing charge and discharge rate.
- Open Circuit Voltage – how voltage measurements relate back to state of charge.
- High Voltage System – the safety-critical system these fundamentals feed into.
Full Index
A complete, always up to date index of every page in the Electrical section, organised by hierarchy. Click any heading to expand it.
Electrical Fundamentals
The Cell Electrical Model
The single battery cell is represented in circuit diagrams by a standard cell symbol — the longer line is the positive terminal, the shorter line negative. The chemistry does not change the symbol.
A more complete electrical model adds the internal resistance of the cell in series:
V_terminal = OCV − (I × R_int) during discharge V_terminal = OCV + (I × R_int) during charge
This simple model — open circuit voltage minus the resistive voltage drop — is the foundation for every battery electrical calculation. The OCV gives you the voltage at rest; the internal resistance tells you how that voltage changes under load.
→ Open Circuit Voltage (OCV) → Internal Resistance — DCIR and ACIR
OCV and DCIR
Open Circuit Voltage (OCV) is the cell terminal voltage when no current flows and the cell has been allowed to rest. It is a direct function of the cell’s state of charge and chemistry — an LFP cell at 50% SoC has an OCV of ~3.27V; an NMC cell at 50% SoC has an OCV of ~3.7V.
DC Internal Resistance (DCIR) is the cell’s resistance to a direct current demand. It is typically quoted at 25°C, 50% SoC, for a 10-second pulse. The DCIR is not a fixed value — it varies significantly with state of charge, temperature, and age. At −10°C, the DCIR of a typical NMC cell can be 3–5× its value at 25°C, dramatically reducing available peak power in cold conditions.
The product of DCIR and current squared (I²R) is the rate of heat generation in the cell during charge and discharge. This directly links the electrical and thermal design — a cell with a lower DCIR runs cooler at the same current, ages more slowly, and can deliver more power before hitting the minimum voltage limit.
→ DCIR explained → Electrical Formulas
Series and Parallel — Building a Pack from Cells
Individual cells are connected in series to increase voltage and in parallel to increase capacity. The combined configuration is written as S×P — for example, 96S46P means 46 cells in parallel (forming one group with 46× the single-cell capacity) and 96 of those groups in series.
96S46P with 3.7V nominal, 5Ah cells:
- Pack voltage: 96 × 3.7V = 355.2V
- Pack capacity: 46 × 5Ah = 230Ah
- Pack energy: 355.2V × 230Ah = 81.7 kWh
The critical constraint is that the weakest cell in any series string limits the performance of the entire string. A single cell with 5% lower capacity than its neighbours reduces the usable energy of the entire string by 5% — not just of that one cell. This is why cell matching during pack assembly and cell balancing during operation both matter.
→ Series and parallel in depth → Single-sided 21700 busbars → Battery power demand solution
Watt, Watt-Hour, and the Units of Electrical Energy
Power (W) = Voltage (V) × Current (A)
Energy (Wh) = Power (W) × Time (h)
1 Wh = 3,600 joules. The watt-hour is the standard unit for comparing cell and pack energy — a 100Wh cell stores 360,000 joules of electrochemical energy.
In pack design, both power and energy matter but they pull in different directions. A high-power cell (low DCIR, optimised for fast charge/discharge rates) typically has lower energy density than a high-energy cell. Understanding the application duty cycle — whether peak power or total stored energy is the binding constraint — determines which cell metrics to prioritise.
Continuous Current Rating
The continuous current rating is the maximum current the pack can sustain indefinitely without any component exceeding its thermal limit. It is one of the first electrical design parameters to establish, because it sizes every downstream component in the HV system.
Plotting continuous power against nominal pack voltage gives the current design points for a given application. From this single calculation, you can derive:
- Cell-to-cell busbar cross-sections
- Module busbar cross-sections
- Contactor current rating
- Fuse blow current and I²t rating
- Connector contact rating
- Current sensor range
A useful starting point for copper busbar sizing is 5A/mm² of cross-sectional area — this gives a reasonable initial estimate before detailed electrothermal analysis.
→ Continuous current rating → Fuse sizing → Contactor sizing
Voltage Architecture
Low Voltage, Extra-Low Voltage, and High Voltage
Battery systems span a wide voltage range, and the safety and design requirements change significantly at defined thresholds:
| Voltage range | Classification | Key implication |
|---|---|---|
| ≤50V AC / ≤120V DC | Extra-Low Voltage (ELV) | Reduced shock risk; SELV/PELV/FELV categories |
| ≤60V DC | Low Voltage (LV) | Safe working voltage threshold in automotive |
| >60V DC | High Voltage (HV) | Mandatory isolation, HVIL, PPE requirements |
| >1000V DC | Very High Voltage | Specialist HV standards apply |
The 60V DC threshold is the critical boundary in automotive battery design. Below it, component sizing and safety requirements are manageable with standard approaches. Above it, the design must address creepage and clearance distances, mandatory isolation monitoring, interlock systems, and a complete set of HV safety procedures.
→ Extra Low Voltage (ELV) → Higher voltage packs
400V vs 800V
The choice of pack voltage architecture is one of the most consequential early design decisions in EV and BESS development.
At a given power level, current = power ÷ voltage. Doubling the voltage halves the current, which halves I²R losses in conductors, halves the cross-sectional area needed in busbars, and reduces the physical size of contactors and connectors. It also enables faster charging — a 350kW charger at 800V delivers ~437A, versus ~875A at 400V.
The tradeoff: 800V requires more cells in series, more demanding isolation requirements, and components rated for higher voltage. Charger infrastructure at 800V is less widely deployed than at 400V, though this is changing rapidly.
Split-pack charging bridges the gap — an 800V drive pack is split into two 400V sections connected in parallel during charging, allowing compatibility with 400V infrastructure. Used in production in the Porsche Taycan, Audi e-tron GT, and several other platforms. → Bank Charging
HV System Safety
Creepage and Clearance
At voltages above 60V DC, air gaps between conductors can spark, and surface paths between conductors can track. The minimum distances required to prevent this are defined by IEC 60664 and are a function of the working voltage, pollution degree, and material group.
Creepage is the minimum distance between two conductors measured along the surface of the insulating material. Clearance is the minimum distance through air.
These values constrain PCB layout, connector design, busbar routing, and the design of any housing containing HV conductors. They cannot be compromised by cost reduction or packaging pressure — they are safety-critical dimensions with legal compliance implications.
→ Creepage and clearance in detail
Isolation Resistance
The HV bus must be electrically isolated from the vehicle chassis (and any accessible metal parts) at all times. ISO 6469-3 specifies a minimum isolation resistance of 500Ω/V — so a 400V pack must maintain at least 200kΩ isolation resistance, and an 800V pack at least 400kΩ.
The BMS continuously monitors isolation resistance and opens the main contactors if it falls below the minimum threshold. Isolation can degrade over the pack’s lifetime through dirt ingress creating conductive pathways, coolant leakage across HV surfaces, ageing of insulating materials, cell venting or thermal runaway events, physical damage to the pack, or — less commonly but not rarely — tools left in the pack during service.
→ Isolation resistance of a pack
Short Circuit Protection
A short circuit between the positive and negative HV bus — whether internal (cell-level) or external (connector damage, wiring fault) — produces a current limited only by the total system resistance. In a 400V pack with 200µΩ total circuit resistance, the theoretical short-circuit current exceeds 2,000,000A in the first microseconds, limited in practice by inductance and the fuse/contactor response time.
The HV system is designed to interrupt this current before it destroys components or starts a fire. Passive fuses handle overcurrent events faster than any contactor can respond; pyro fuses handle commanded disconnection; contactors handle normal make/break cycling and fault isolation commanded by the BMS.
→ Short circuit → Fuses
High Voltage System
The HV system is the complete electrical architecture between the cell stack and the external HV connection — the components that switch, protect, measure, and monitor the high-voltage circuit.
→ High Voltage System — full section
The HV system comprises:
Battery Disconnect Unit (BDU) — the assembly housing the main contactors, fuses, pre-charge circuit, and current sensor. All HV switching and protection is routed through the BDU.
Busbars — the primary conductors connecting cells, modules, and the pack output. Sized from the continuous current rating; designed for thermal management, mechanical durability, and low joint resistance.
Contactors — controllable HV switches operated by the BMS. Always specified as normally open in EV applications — failure results in disconnection, not a live circuit.
Fuses — passive and pyrotechnic protection against overcurrent. Passive fuses respond to I²t; pyro fuses can be commanded by the BMS independently of current level.
Current Sensor — measures pack current for BMS coulomb counting and SoC estimation. Accuracy directly affects SoC accuracy over the lifetime of the pack.
Connectors — the HV interface to charger, motor, and auxiliary loads. Must meet IP67/IP69K and include an HVIL contact.
HVIL — the High Voltage Interlock Loop that detects disconnection of any HV connector before the HV terminals separate.
Power Electronics — the inverter, onboard charger, DC-DC converter, and their interfaces to the battery pack.
Related Sections
The electrical design of a battery does not exist in isolation. The topics below are directly connected:
- BMS Hardware and Algorithms — the control system that measures the electrical state of the pack and commands the HV system
- Thermal — I²R heating in all electrical components drives thermal management requirements
- Pack Design — busbar routing, BDU packaging, and HV connector position are integrated into the pack enclosure design
- Mechanical — busbar mechanical mounting, vibration loading, and connector retention are mechanical design problems
- Safety — HV system safety interlocks, thermal runaway response, and functional safety requirements
- Legislation, Rules and Regulations — ISO 6469, IEC 60664, UN ECE R100, and the standards that define HV system requirements
- Battery Calculators — series/parallel configuration, pack energy, current rating, and Wh/kg calculators