The essential answer
Self-discharge is a loss of stored charge or usable energy while a device is not intentionally delivering its main load. External standby electronics can create an additional drain that should be measured separately. Terminal voltage change alone is not a universal measure of lost energy because voltage can relax after charging or vary nonlinearly with state. A practical retention test compares recoverable energy after controlled storage intervals using the same discharge protocol, temperature, and endpoint criteria.
How to reason about it
Begin with reproducible conditioning and a defined initial state. Separate devices or repeated matched cycles can provide immediate-discharge references and delayed-discharge measurements. Keep the storage environment controlled and document any instrumentation that remains connected, since even a meter draws some current. After storage, discharge through the same profile to the same cutoff and compare recovered energy. Distinguish intrinsic storage loss from energy used by protection circuitry, communications, or balancing. The fractional retention may depend on temperature, initial state, and elapsed time, so a single percentage per day should not be extrapolated indefinitely without a model. A constant absolute leakage and a constant fractional decay imply different long-term behavior. Voltage recovery or relaxation can complicate early observations; an energy-based comparison is often more directly relevant to the application than the shape of the open-circuit voltage trace alone.
ILLUSTRATIVE · NOT A REPORTED RESULT
An example to work through
Assume a hypothetical device yields 10.0 Wh when discharged immediately after conditioning and 9.5 Wh after seven days under identical conditions. Retained usable energy is 95%, and the difference is 0.5 Wh. If a separately measured always-on circuit consumes 2 mW throughout those seven days, its energy demand is 0.002 × 168 = 0.336 Wh. The remaining 0.164 Wh cannot automatically be assigned to one internal mechanism; it includes other losses and experimental uncertainty. This constructed calculation assumes both discharge tests are comparable and that the auxiliary measurement is representative.
Where interpretation fails
A falling open-circuit voltage may reflect equilibration rather than proportional energy loss. Conversely, a flat voltage plateau can conceal substantial loss of capacity. Comparing voltage alone across different storage chemistries or states can therefore reverse the apparent ranking of retention performance.
The next decision
Measure recovered usable energy after the intended hold time and report standby demand separately. State temperature, initial condition, and discharge cutoff. Use a decay model only after testing it across multiple intervals; otherwise present observed retention for the tested duration without extrapolating a lifetime claim.
Sources & context
These sources support the concepts noted below. Our synthetic examples and research framing are educational interpretation; they are not findings from these organizations or validation of our ventures.
- Battery Energy Storage System Evaluation Method — DOE ↗
Background on energy-in/energy-out assessment and importance of state of charge.
Source check: Official PDF search excerpt reviewed; apparent parenthetical ratio typo in snippet not adopted.
Educational research context, not a validated device specification or clinical protocol. Read our editorial and correction policy.