A stand test rests a battery pack, fully assembled with its BMS connected and powered, at a fixed state of charge and temperature for an extended period, then measures the voltage and state-of-charge loss over that interval. It is the pack-level counterpart to a cell-level calendar ageing test, but it deliberately leaves the system electronics live rather than isolating the cell, because at pack level the BMS, contactors, current sensors and any always-on communication interface draw current of their own throughout the rest period.
Why it needs to be a separate test
A cell-level calendar test isolates the cell from any external circuit, so the voltage and capacity loss it records comes only from the cell’s own self-discharge chemistry. A pack sitting on a shelf or in a stationary vehicle is never that clean: the BMS keeps monitoring cell voltages and temperatures even when the vehicle is “asleep”, and that monitoring draws a small standing current from the pack. ISO 12405-1 already builds this distinction into its storage test clauses, specifying a “no-load SOC loss” test with the BMS taken out of circuit alongside a separate “SOC loss at storage” test with the BMS left active – the two are run as a pair precisely because they attribute the loss differently: one to the cell alone, the other to the cell plus everything the live system draws around it.
The gap between the two can be substantial. Individual BMS monitoring ICs are commonly specified with standby or deep-sleep currents in the single-digit microamp range – for example, STMicroelectronics’ L9961 battery monitoring IC is datasheet-rated at around 2 µA in deep-sleep and 3 µA in standby – but a pack contains many such channels alongside contactor coils, isolation monitoring and wiring losses, and the combined draw is what actually determines how long a stationary pack can sit before its state of charge drops to a level that risks over-discharge or triggers a low-voltage fault.
Practical implications
- A stand test result is only meaningful if the BMS operating mode during the test is recorded – deep sleep, standby and fully awake can differ by orders of magnitude in draw, so a headline “loss per month” figure needs its BMS state stated alongside it
- Because the loss during a stand test combines cell self-discharge with system quiescent draw, a rising stand-loss trend over successive test intervals does not on its own distinguish cell ageing from a BMS or wiring fault – the no-load, BMS-isolated result for the same pack is needed to separate the two
- Vehicles and stationary storage systems that may sit unused for long periods (dealer stock, seasonal storage, spare packs) are the main practical reason to run this test, since the system-level loss rate sets the maximum safe unpowered stand time before a maintenance charge is needed
References
- ISO 12405-1:2011, Electrically propelled road vehicles — Test specification for lithium-ion traction battery packs and systems — Part 1: High-power applications, Clauses 7.4 (No-load SOC loss) and 7.5 (SOC loss at storage), International Organization for Standardization
- STMicroelectronics, L9961 battery monitoring, balancing and protecting IC datasheet
See also: Recovery Capacity, Remaining Capacity, Storage, Self-Discharge Test Procedure