Charging a battery cell or pack involves passing an electric current through it in the opposite direction to discharge. Converting electrical energy back into stored electrochemical energy. That description makes it sound simple. It is not.
The charging process must move lithium ions from cathode back into anode at a rate that the electrochemistry can accommodate safely. Push too hard and lithium plates on the anode surface rather than intercalating into it, accelerating degradation and at the extreme, creating dendrites that can short-circuit the cell. Charge too slowly and you waste time and reduce asset utilisation. Charge at the wrong temperature and the cell either rejects the current or ages at an accelerated rate. Overcharge by even a small margin and you drive irreversible structural changes in the cathode material.
The BMS sits between the charger and the cell, enforcing the limits that the electrochemistry demands. But the charger, the cable, the connector, the infrastructure, the communication protocol, and the thermal management system all have to work together to get energy into the pack safely, quickly, and without shortening the pack’s life.
This section covers the full charging stack: from cell-level electrochemistry through to EV charging infrastructure and standards.
Section Map
The Electrochemistry of Charging
During discharge, lithium ions move from the graphite anode through the electrolyte and intercalate into the cathode. During charge, an external voltage reverses this flow — forcing lithium ions out of the cathode and back into the anode, and forcing electrons through the external circuit from the charger to the cell.
The cell terminal voltage during charging is always higher than during discharge at the same SoC. The difference is the overpotential, driven by the internal resistance, ion transport limitations and the activation energy of the electrode reactions. This is why the voltage you charge to (e.g. 4.2V) is higher than the OCV at full charge (e.g. ~4.0–4.1V for NMC), and why charging efficiency is always less than 100%.
Three things constrain how fast a cell can be charged:
1. Lithium plating risk. At high charge rates, especially at low temperatures or high SoC, the rate of lithium-ion arrival at the anode exceeds the rate at which graphite can accept it. Excess lithium deposits as metallic lithium on the anode surface, a process called lithium plating. Plated lithium is electrochemically inactive (lost capacity) and, worse, can form dendrites that grow through the separator and cause an internal short circuit. The maximum safe charge rate is the boundary below which plating does not occur, and it is strongly temperature-dependent.
2. Electrolyte and interphase stability. At high voltages and elevated temperatures, the electrolyte oxidises at the cathode surface and the CEI (cathode electrolyte interphase) grows. Overcharging, taking the cell above its maximum voltage, drives irreversible phase changes in the cathode lattice, oxygen release (in high-nickel cathodes), and accelerated capacity fade.
3. Thermal management. Charging generates heat via I²R losses and, at certain SoC points, endothermic or exothermic entropy changes. At high charge rates this heat generation can exceed the cooling system’s capacity, causing cell temperature to rise into the range that accelerates SEI growth and electrolyte decomposition.
→ Cold temperature charge/discharge → Overcharge → Cell lifetime and degradation
Charging Protocols
A charging protocol defines how the charger varies current and voltage over time to transfer energy into the cell as quickly as possible without exceeding the cell’s electrochemical or thermal limits.
CC-CV — Constant Current, Constant Voltage
CC-CV is the standard charging protocol for lithium-ion cells and is used, in some form, in virtually every lithium-ion charger.
Phase 1 — Constant Current (CC): The charger delivers a fixed current (e.g. 1C) until the cell terminal voltage reaches the maximum charge voltage (e.g. 4.2V for NMC, 3.65V for LFP). During this phase, SoC rises approximately linearly with time. This phase typically charges the cell to around 70–80% SoC.
Phase 2 — Constant Voltage (CV): Once the maximum voltage is reached, the charger holds voltage constant and allows current to taper naturally as the cell approaches full charge. Current falls exponentially as lithium-ion concentration in the electrolyte equilibrates. Charging is considered complete when current falls below a defined threshold, typically C/20 or C/50. This phase fills the remaining 20–30% of capacity but takes a disproportionate fraction of the total charge time.
The trade-off: increasing the CC current speeds up Phase 1 but does not reduce Phase 2 time. Phase 2 duration is set by the cell chemistry and is largely independent of the charge rate used in Phase 1. This is why fast charging focuses on shortening the CC phase. Phase 2 cannot be hurried without risk.
Efficiency: A CC-CV charge cycle is typically 97–99% coulombically efficient for a healthy lithium-ion cell. The voltage efficiency (average discharge voltage / average charge voltage) reduces this to a round-trip efficiency of approximately 92–96% depending on C-rate and temperature.
Multistage Constant Current — Constant Voltage (MCC-CV)
MCC-CV uses multiple decreasing constant-current steps before transitioning to the CV phase. The first step charges at a high rate; subsequent steps reduce current as voltage limits are approached, avoiding the sudden transition of standard CC-CV. This can reduce total charge time and reduce stress on the cell compared to a single high-rate CC phase, particularly at high SoC.
Stepped Constant Current — Constant Voltage (SCC-CV)
A variant of MCC-CV used specifically for fast charging. The current profile steps down at defined voltage thresholds based on characterisation testing of the specific cell, keeping the charge rate as high as the cell can safely accept at each SoC point. The step-down thresholds are cell-specific and must be derived from testing, they are not transferable between cell types.
Boost Charging (CC-CV-CC-CV)
A two-stage CC-CV protocol where a second CC-CV sequence is applied after the first, with a rest period between stages. Used in some fast charging applications where a brief rest between stages allows heat to dissipate and lithium concentration gradients in the electrode to relax, permitting a higher second-stage charge rate than would be safe without the rest.
Pulse Charging
Charging current is applied as pulses rather than continuously, alternating between a charge current and a brief rest or discharge pulse. The discharge pulse is claimed to promote more uniform lithium insertion into the anode by relaxing concentration gradients between pulses. Commercial applications are limited; the evidence base for significant lifetime improvement over optimised CC-CV is mixed.
Variable Current Profile (VCP)
An optimisation approach where the entire current profile over the charge duration is computed to minimise charge time subject to cell temperature, voltage, and ageing constraints. Requires an accurate cell model and real-time temperature measurement. Increasingly used in research and in high-performance automotive charging strategies as cell models improve.
Non-Linear Voltammetry (NLV)
A novel charging protocol that uses a non-linear voltage waveform rather than a simple constant voltage or constant current. Published results show reduced charge time and improved capacity retention compared to CC-CV. Early-stage adoption in commercial products.
→ Non-Linear Voltammetry → All charging protocols overview
Charge Rate, C-Rate, and Temperature
The maximum safe charge rate is not a fixed number. It is a function of temperature, SoC, and cell age. The same cell that safely accepts 3C at 25°C may only safely accept 0.5C at 0°C and should not be charged at all below −10°C (for most NMC and LFP cells) due to the severe lithium plating risk at low temperatures.
Temperature effect on maximum charge rate:
| Temperature | Typical NMC max charge rate | Typical LFP max charge rate |
|---|---|---|
| −20°C | Not recommended | Not recommended |
| −10°C | 0.1–0.2C | 0.1C |
| 0°C | 0.3–0.5C | 0.2–0.3C |
| 10°C | 0.7–1.0C | 0.5–0.7C |
| 25°C | 1.0–3.0C | 0.5–1.5C |
| 45°C | 1.0–2.0C (heat limit) | 0.5–1.0C (heat limit) |
These are indicative — actual limits are cell-specific and must be derived from characterisation testing. The consequence of exceeding them is lithium plating at low temperature and accelerated degradation at high rate and temperature.
Cell preconditioning — some advanced BMS and charging systems preheat the cells to a minimum temperature before beginning fast charging. This is standard on the Tesla Model 3/Y (which will display a “preconditioning battery” message when navigating to a Supercharger) and increasingly common on other platforms. Preconditioning adds time to the total charging session but reduces plating risk and enables higher initial charge rates.
→ Cold temperature charge/discharge
Charge Limits and Safety
The BMS enforces three independent limits during charging. All three must be satisfied simultaneously — the most restrictive applies:
Maximum cell voltage — the upper voltage limit set for the specific cell chemistry. Exceeding this by even 100–200mV drives irreversible cathode degradation and, in high-nickel cells, oxygen release. The BMS measures every individual cell voltage (not pack voltage) and terminates the CC phase when any cell reaches V_max. Cell-level voltage measurement is mandatory, pack voltage averaged across the string cannot detect a single high cell.
Maximum cell temperature — typically 45°C for fast charging, with derating beginning at 35–40°C. At elevated temperature, SEI growth rate increases exponentially and electrolyte decomposition accelerates.
Maximum charge current — the cell manufacturer’s rated maximum C-rate for charging, which decreases with SoC in most CC-CV implementations. The BMS enforces this as a hard limit; the charger communicates its current capability and the BMS responds with an allowed charge current via the communication protocol.
These limits are not independent — they interact. A cell at 40°C can safely accept a lower charge rate than the same cell at 25°C. An aged cell at high internal resistance generates more heat at the same charge rate. The BMS must manage all three limits simultaneously and adjust in real time as conditions change.
Overcharge is the critical failure mode. Charging beyond V_max:
- Drives structural transformation of the cathode (layered → disordered spinel → rock salt)
- Causes oxygen release from high-nickel cathodes
- Drives electrolyte oxidation and gas generation
- Increases internal pressure, potentially venting the cell
- At the extreme: triggers thermal runaway
Overcharge protection is an ASIL-rated BMS function in automotive applications. Failure to protect against overcharge must be assigned an appropriate safety integrity level under ISO 26262.
→ Overcharge in depth → BMS functional safety → Thermal runaway
Fast Charging
Fast charging has become one of the primary competitive metrics in EV development. The industry benchmark is the time to charge from 10% to 80% SoC, the range over which fast charging rates are achievable before the CV taper begins.
Progress has been rapid. In 2011, the Nissan Leaf charged at 50kW (0.5C on a 24kWh pack), taking approximately 30 minutes from 0–80%. In 2024, the BYD Seagull charges at 40kW and the Hyundai Ioniq 6 charges at 233kW, taking approximately 18 minutes for 10–80% on an 800V architecture.
What enables faster charging:
Higher voltage architecture (800V). At 800V, the same power level requires half the current of a 400V system. Lower current means lower I²R heat generation in the pack and cabling, allowing higher charge rates to be sustained without the thermal management system being overwhelmed. 400V fast chargers are limited to approximately 150–175kW by thermal constraints; 800V systems have delivered 350kW in production (Porsche Taycan, Lucid Air) and roadmaps exist for 500kW+.
Better cell design. Thinner electrode coatings, larger graphite particle surface area, and silicon anode additions all reduce the ionic transport resistance that limits fast charge rate. Tabless cell designs (Tesla 4680, LG 4695) reduce internal electrical resistance and improve current distribution, enabling more uniform lithium insertion during fast charging.
Thermal management improvement. Immersion cooling and direct refrigerant cooling provide significantly higher heat removal rates than water-glycol cooling plates, enabling sustained high charge currents without cell temperature rising into the degradation zone.
Active preconditioning. Heating cells to their optimal charge temperature before and during fast charging, using the heat pump or dedicated battery heater, ensures the full fast charge rate can be applied from the start of the session rather than being thermally limited.
→ Fast charging in depth → Charging at the speed of a petrol pump
EV Charging Infrastructure
Charging Levels
EV charging infrastructure is categorised by power level:
| Level | Power | Typical connector | Use case |
|---|---|---|---|
| AC Level 1 | 1.4–2.4 kW | Type 1 (J1772), domestic socket | Overnight home charging |
| AC Level 2 | 7–22 kW | Type 2 (IEC 62196), CCS AC | Home wallbox, workplace, public |
| DC Fast Charging | 50–150 kW | CCS2, CHAdeMO | Public rapid charging |
| DC High Power | 150–350 kW | CCS2, NACS | Motorway/highway charging |
| DC Ultra-High Power | 350 kW+ | MCS (trucks), NACS, CCS2 | Heavy transport, premium EVs |
AC charging is handled by the vehicle’s on-board charger (OBC) — the AC/DC converter is inside the car, sized to match the vehicle’s charging power target. DC charging bypasses the OBC entirely: the DC charger converts AC to DC externally and delivers DC directly to the battery pack via the BMS-controlled charge circuit.
Connectors and Standards
CCS (Combined Charging System) — the dominant standard in Europe (CCS2) and North America (CCS1). Combines AC Type 2 or Type 1 socket with two DC pins in a single combined connector. Defined in IEC 62196-3. Supports up to 350kW DC.
NACS (North American Charging Standard) — Tesla’s connector, standardised as SAE J3400 in 2023. Now adopted by Ford, GM, Rivian, Polestar, and most North American OEMs as the primary connector for new EVs. Increasingly deployed at public charging infrastructure across North America.
CHAdeMO — Japanese DC fast charge standard, dominant in early-generation Japanese EVs (Nissan Leaf, Mitsubishi i-MiEV). Declining in new vehicles as CCS and NACS expand, but installed base remains significant.
GB/T — Chinese DC and AC charging standard, mandatory for all EVs sold in China. Not interoperable with CCS or NACS without adapters.
MCS (Megawatt Charging System) — IEC 62196-6, designed for heavy commercial vehicles (trucks, buses, ships). Supports up to 3.75MW charging at up to 1,250V / 3,000A. Under active deployment in Europe and North America.
ISO 15118 — the communication protocol standard that sits above the connector. Defines the bidirectional digital communication between vehicle and charger — charge power negotiation, plug-and-charge authentication, and Vehicle-to-Grid (V2G) energy exchange. ISO 15118-2 is the current production standard; ISO 15118-20 extends it for V2G and wireless charging.
→ EV charging connectors in detail → EV charging overview
Battery Swap
Battery swapping replaces a depleted pack with a fully charged one at a swap station, eliminating charge time entirely from the user’s perspective. The vehicle drives in, a robotic system removes the depleted pack and installs a charged one, and the vehicle drives out — typically in under five minutes.
Commercially deployed at scale by NIO in China (over 2,500 swap stations, swapping 30kWh, 75kWh, or 100kWh packs depending on model). CATL’s EVOGO (Choco-SEB) system standardises a swappable block format for broader OEM adoption. Aulton Energy operates the largest two-wheeler swap network globally.
The fundamental constraint of battery swapping is standardisation. Unlike fast charging, where a connector standard allows any EV to use any charger, swap systems require dimensional and mechanical compatibility between vehicles and swap packs — creating a significant design constraint and limiting the format freedom of successive vehicle generations.
→ Battery swap vs fast charge → Designing swappable batteries → Swappable battery packs
Vehicle to Grid (V2G) and Bidirectional Charging
Vehicle-to-Grid (V2G) reverses the normal charging flow, allowing the EV battery to discharge energy back into the grid or into the building. This requires bidirectional power conversion, both in the vehicle (bidirectional OBC) and in the charger/EVSE.
V2G has significant potential value: a fleet of EVs collectively represents a very large distributed energy storage resource that can provide grid balancing services (frequency regulation, demand response) and allow owners to earn revenue from their stored energy. A 75kWh EV at 50% SoC holds as much energy as a typical UK home uses in three days.
The challenges:
- Accelerated degradation — additional charge/discharge cycles from V2G degrade the pack faster than charge-only use. BMS-managed V2G strategies that limit cycle depth and operate within optimal SoC and temperature windows mitigate but do not eliminate this effect
- Grid connection complexity — V2G requires grid synchronisation, anti-islanding protection, and utility agreements that vary significantly by country and network operator
- Standards — ISO 15118-20 defines the communication protocol; harmonised grid connection standards are still developing
Vehicle-to-Home (V2H) and Vehicle-to-Load (V2L) are simpler bidirectional modes that do not require grid synchronisation — the vehicle supplies AC power to the home or to external devices via a standard socket on the vehicle. V2L is already available on the Hyundai Ioniq 5 and 6, Kia EV6, Ford F-150 Lightning, Rivian R1T, and others.
→ Vehicle to Grid (V2G) → Power electronics — OBC and bidirectional charging
The On-Board Charger (OBC)
In AC charging, the on-board charger is the AC/DC converter installed in the vehicle that converts grid AC to the DC voltage required by the battery pack. The OBC is rated by its maximum AC input power — a 7.4kW OBC on a 230V/32A single-phase supply, or an 11kW or 22kW OBC on a three-phase supply.
The OBC communicates with the BMS to determine the allowable charge current and voltage at each moment, and communicates with the EVSE (charging station) via the pilot signal or ISO 15118 protocol to negotiate available power.
In DC fast charging, the OBC is bypassed — the DC charger connects directly to the HV bus via the charge inlet and the BMS-controlled charge contactors. The charger and BMS communicate directly via CCS Combined Charging Communication (DIN 70121 or ISO 15118-2) to negotiate charge power.
Effect of Charging on Pack Lifetime
How the pack is charged has a larger effect on its lifetime than almost any other operational variable. The key factors:
Charge rate — higher charge rates mean higher I²R heat generation and higher lithium plating risk at any given temperature. Limiting peak charge rate extends lifetime, particularly for the upper portion of the SoC range.
Maximum SoC — regularly charging to 100% accelerates calendar ageing significantly. Most BMS systems allow a daily charge limit to be set below 100% (e.g. 80%) for routine use, reserving 100% for long journeys. This single change can extend battery calendar life by 20–40% depending on chemistry.
Charging temperature — charging at elevated temperature (>40°C) accelerates SEI growth and electrolyte decomposition. Charging at very low temperature (<5°C) risks lithium plating. The optimal charging temperature for lifetime is 15–35°C.
Charge frequency — each charge cycle contributes to cycle ageing. Opportunity charging (frequent partial charges) typically causes less degradation per unit of energy delivered than full charge/discharge cycles, because the depth of each individual cycle is lower.
→ Cell lifetime and degradation → Usable SoC window → Calendar ageing
Related Sections
- BMS — Algorithms — the BMS enforces all charge limits and communicates charge capability to the charger
- Thermal — thermal management during fast charging is the binding constraint on charge rate
- Cell lifetime and degradation — charging strategy is the primary controllable variable for pack lifetime
- Electrical — High Voltage System — charge contactors, fuses, and connectors are part of the HV system
- Power Electronics — OBC, DC-DC converter, and bidirectional charging architecture
- Safety — overcharge is a thermal runaway trigger; charging safety is a functional safety requirement
- Pack Sizing — charge rate requirements feed directly into cell selection and thermal sizing
- Legislation, Rules and Regulations — IEC 62196, ISO 15118, SAE J3400, and EV charging standards
- Battery Energy Storage Systems — charging strategy for grid-connected BESS differs significantly from EV
- Swappable batteries — the alternative to charging for fleet and two-wheeler applications
→ Charging Definitions and Glossary
See also: GB 17761-2024, Charging Connectors