Safoutin, M., Cherry, J., McDonald, J., and Lee, S., "Effect of Current and SOC on Round-Trip Energy Efficiency of a Lithium-Iron Phosphate (LiFePO4) Battery Pack," SAE Technical Paper 2015-01-1186, 2015, doi:10.4271/2015-01-1186.
While equivalent circuit modeling is an effective way to model the performance of automotive Li-ion batteries, in some applications it is more convenient to refer to round-trip energy efficiency. Energy efficiency of either cells or full packs is seldom documented by manufacturers in enough detail to provide an accurate impression of this metric over a range of operating conditions. The energy efficiency of a full battery pack may also be subject to more variables than would be represented by extrapolating results obtained from a single cell, and can be more demanding to measure in an accurate and consistent manner. Roundtrip energy efficiency of a 22.8-kWh A123 Li-ion (Lithium Iron Phosphate, LiFePO4) battery pack was measured by applying a fixed quantity of charge and discharge current between 0.2C and 2C rates and at SOCs between 10% and 90% at an average temperature of 23°C.