Estimating state of charge
Coulomb counting, open-circuit voltage and why LFP is hard.
- Explain estimating state of charge at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real pack
Overview
There is no fuel float in a battery. State of charge is an estimate, fused from current integration and voltage models, and it drifts.
In depth
Two imperfect methods
Coulomb counting integrates current over time — accurate in the short term but accumulating sensor error. Open-circuit voltage lookup is absolutely referenced but needs a rested cell and a steep voltage curve to be precise.
A slow charge to 100% followed by a rest period lets most BMS implementations reset their reference points. Do it monthly on LFP, occasionally on NMC.
Battery Capacity Calculator
Convert pack voltage and amp-hours into usable energy.
E (kWh) = V × Ah ÷ 1000 ; usable = E × usableFractionCell capacity is measured in amp-hours. Multiplying by nominal pack voltage gives watt-hours. Manufacturers reserve a buffer at both ends of the pack, so usable energy is always less than gross energy.
Example: 400 V × 200 Ah ÷ 1000 = 80 kWh gross. With a 94% usable window that is 75.2 kWh.
- Nominal voltage ≈ cell nominal voltage × cells in series (3.65 V for LFP, 3.7 V for NMC).
- LFP packs are often released to 100% because the buffer strategy differs.
- Gross capacity is what marketing quotes; usable is what you drive on.
Key takeaways
- SOC is an estimate fused from current and voltage.
- Coulomb counting drifts; OCV needs rest and voltage slope.
- Flat-curve chemistries need periodic full charges.
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