Molicel P45B ECM: Open-Source Equivalent Circuit Model
- About:Energy
- Aug 9
- 2 min read
Updated: Aug 13
About:Energy believes battery modelling is held back by how much of it stays hidden. Most vendors treat their methodology as proprietary, leaving engineers to choose models on trust rather than evidence, and leaving the field to re-derive the same testing and parameterisation problems in isolation. We do the opposite: we publish our testing methods, model structure, assumptions and limitations, and the raw validation data behind them, so anyone can check our working. This paper is the first in an ongoing series doing exactly that, and it's the same approach we bring to joint development with customers, moving faster because the methodology is visible to both sides from the start.
Molicel P45B Equivalent Circuit Model
About:Energy has published the first in a series of open-source battery model papers, starting with a full industry-standard equivalent circuit model (ECM) parameterisation of the Molicel P45B, a 4.5 Ah high-power 21700 cell used across eVTOL, motorsport and drone platforms. Where most academic ECM papers optimise for accuracy alone, this paper treats accuracy, build cost and run cost as equally important, since a model that only wins on accuracy rarely gets used downstream in industry. Read the preprint.
Molicel P45B Open-source Validation
The testing methodology is built around plausible real-world operation rather than discrete pulse loads from quasi-equilibrium conditions, and uses conductive surface-controlled thermal testing to hold the cell close to isothermal. That separates electrical and thermal effects cleanly, so voltage error can be attributed to the electrical model rather than a moving thermal target. The model itself uses two RC pairs, reversible heat generation and OCV hysteresis, and is validated across the full SOC, current and temperature envelope, including representative eVTOL missions and drive cycles with regenerative charging under aggressive surface cooling.
Around 30 validation conditions were run across four temperatures and the cell's full current envelope, with RMSE voltage error binned in 10% SOC increments rather than reported as a single average, so performance across the range is visible rather than hidden. The full parameter sets, validation data, validation metrics workbook and documentation are released on the Zenodo data repository under CC-BY 4.0, continuing About:Energy's approach of publishing methodology in the open rather than treating it as a black box. The same testing and validation approach is used across the rest of the Voltt cell library.
