Every traction battery loses capacity through two separate mechanisms. Calendar aging is the slow chemical decay that happens with time and temperature, even when the car is parked. Cyclic aging is the damage caused by moving charge in and out of the cells. V2G adds a third workload on top of driving: frequent, shallow, low-power discharges, often started from a high State of Charge (SoC). At high SoC the anode sits at a low potential, which speeds up growth of the solid electrolyte interphase (SEI) layer and permanently traps lithium or sodium ions that can no longer carry charge.
The legal picture is just as uneven. OEMs and charge point operators can record bidirectional energy flows through vehicle telematics and BMS logs, so stationary discharge is visible to them. Many battery warranties also contain a lesser-known clause: an energy throughput limit in megawatt-hours (MWh). Once that limit is reached, cover can end even if the car is inside its 8-year/100,000-mile window, and every kWh exported to the grid counts toward it.
Factor | OEM-Approved V2G Programme | Silent or Unapproved OEM |
|---|---|---|
Warranty Position | Bidirectional use explicitly covered when paired with certified hardware and tariff | Stationary discharge may be classed as misuse; cover at manufacturer discretion |
Telemetry Oversight | Export flows logged and expected; data shared with tariff partner | Discharge events logged by BMS and can surface during a warranty claim |
Throughput Limit | Energy throughput cap (MWh) often stated and sized for V2G duty | Unstated or buried cap; V2G energy counts against it |
Primary Degradation Driver | Managed cycling within a capped SoC window | Uncontrolled shallow cycling at high SoC accelerating SEI growth |
Export Economics | Viable if spread exceeds roughly 9p/kWh throughput cost | Effectively negative once warranty risk is priced in |
The arbitrage maths is thin. On dynamic UK tariffs such as Octopus Flux or Agile, the spread between off-peak import and peak export is typically 10p to 15p/kWh before round-trip losses of 10% to 15%. A £9,000 replacement pack rated for roughly 100 MWh of lifetime throughput carries an amortised wear cost of about 9p for every kWh cycled. Unless the export margin clearly beats that figure, V2G is moving value from the battery to the grid rather than creating it.
For fleet operators the rule is simple: only cycle hardware the OEM has approved in writing, cap the SoC window, and model throughput cost per kWh before signing any export contract.