BMS-05 — Current Measurement

1. One Sensor, the Whole Pack

Current data matters in four places: SOC calculation (Coulomb counting), SOP (power limits), overcurrent/short-circuit protection, and internal resistance estimation. Because current is the same at every point in a series circuit, a single sensor (usually inside the BJB, on the negative or positive main line) is enough to measure current for the whole pack — which also makes that one sensor’s accuracy critical.

2. Three Methods, Three Trade-offs

Shunt resistor: a precision resistor of known, tiny value (µΩ–mΩ) placed in series with the current path. Current is calculated from the voltage drop across it:

I = V_shunt / R_shunt

With R_shunt = 100 µΩ and V_shunt = 20 mV, I = 200 A. A shunt offers high accuracy and low cost, but it dissipates power in the current path (P = I² × R — for a 100 µΩ shunt at 200 A, that’s ~4 W) and needs separate isolation.

Hall-effect sensor: measures the magnetic field generated by the current, providing galvanic isolation inherently and creating no power loss. The open-loop version is simple and cheap but has limited linearity; the closed-loop (compensated) version zeroes out the measured field with an opposing current, improving accuracy — at higher cost.

Fluxgate: works on the core-saturation principle, runs closed-loop, and offers very low offset — the most accurate option, but also the most expensive and largest.

Criterion	Shunt	Hall (open-loop)	Hall (closed-loop)	Fluxgate
Accuracy	High	Medium-low	Medium-high	Very high
Offset/thermal drift	Low	High	Medium	Very low
Isolation	Needs a separate stage	Built-in	Built-in	Built-in
Power loss	Yes (I²R)	None	None	None
Cost	Low	Low-medium	Medium	High

ASSUMPTION — The R_shunt, offset, and thermal drift values used in this lesson are representative examples; real values depend on the specific sensor model’s datasheet.

The choice comes down to balancing accuracy requirements (especially low offset for Coulomb counting), cost, and isolation needs.

3. Why Even a Few Amps of Offset Is a Problem

Coulomb counting integrates current over time; even a constant offset accumulates into a large SOC error:

ΔAh_error = I_offset × t

A constant +20 A offset produces a 20 Ah error in charge accounting over 1 hour. In our hypothetical 225 Ah pack, that alone is a ~8.9% SOC error — unacceptably large. This is exactly why real systems try to keep offset in the milliamp range and periodically zero it out via OCV feedback (BMS-08).

4. When Things Go Wrong

Sensor open/short circuits, excessive offset drift, and saturation (current exceeding the measurement range) are typical faults. The BMS can catch inconsistencies by cross-checking the current reading against voltage changes (plausibility, BMS-13). Once a fault is detected, power is typically restricted to stay on the safe side, or SOC is flagged “low confidence”; HV may be disconnected in critical cases.

5. How It’s Verified in Production

Current sensors go through zero-point and gain calibration against known reference current sources in production; EOL testing confirms offset stays within budget. HIL testing injects sensor-loss/saturation scenarios to verify the BMS’s response (BMS-19, BMS-20).

6. How It Connects to Other Systems

Current data is the core input to SOC estimation (BMS-08), the current-limit component of SOP (BMS-10), an input to SOH estimation (BMS-09), and the direct source for overcurrent protection/derating logic (BMS-13).

Summary

• Shunt is accurate and cheap but dissipates power and needs isolation; Hall is isolated and lossless but carries offset/thermal drift; fluxgate is the most accurate but the most expensive.
• Low offset is critical for SOC accuracy — even a few amps of drift turns into a large error within hours.
• Calibration and EOL testing guarantee offset/gain error stays within budget in production.

Sources

• Gregory L. Plett, Battery Management Systems, Volume I — current measurement and Coulomb counting.
• Current sensor manufacturer datasheets/application notes — general concept.