High-accuracy current measurement for electric vehicle battery packs

21st February 2013

Transducer specialist LEM's CAB high-accuracy current transducer enables makers of battery packs for Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs) to achieve a new level of precision when measuring current flow into and out of the battery.

Car makers need this data to maximise the range of the vehicle, to keep the battery in good condition, and to enable use of the optimum size of battery pack.**
LEM's CAB transducer offers current measurement capability of up to ±400A, with unlimited overload capacity, and with excellent accuracy: Coulomb-counting error over the driving cycle is reduced to under 0.1 per cent. It is galvanically-isolated and non-intrusive; no electrical connection to the power circuit is needed. This ensures safe operation and, in contrast to techniques that use a sensing resistor in the power feed, generates no waste heat or losses.
The CAB takes the form of a small panel-mounted module measuring just 71 x 52 x 21 mm, excluding mountings, that has an aperture through which the primary (battery feed to the vehicle motors) conductor passes. It operates from the vehicle's 12V power supply, and is rated for operation from -40 to 105ºC. The CAB transducer transmits its data to the EV/HEV's control circuitry using the automotive-industry standard CAN bus; LEM can supply variants that deliver data via other bus and interface standards, on request.
In the CAB transducer, LEM chose to use its proven implementation of the fluxgate principle. Fluxgate transducers use an advanced magnetic current-measurement principle that inherently cancels or nulls many of the offset and drift errors that appear in simpler current-sensing techniques.
To ensure long battery life, EV and HEV batteries are typically never fully discharged, nor charged to the maximum, and are therefore over-sized compared to their nominal rating. Car makers must further increase this safety margin if they need to make allowance for inaccuracies in energy measurements. With the advent of the CAB transducer, vehicle designers can have complete confidence in state-of-charge (SOC) estimation, and can reduce the size and weight of the battery pack accordingly.
** State-of-charge (SOC) is a critical measurement function in EVs and HEVs. During the driving cycle the battery is either driving the vehicle, or current flow is reversed in regenerative braking or charging, returning energy to the battery. In these transient conditions battery voltage does not give a good indication of SOC – each unit of charge (Coulombs) delivered by, and returned to, the battery has to be accurately tracked, which demands precise current measurement.

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