OVMS User Manual page 41

OVMS: Renault Twizy User Guide
Background
Battery power levels are electrical, while motor power configurations are mechanical. To translate
between these, we need the combined power efficiency of the motor + controller + BMS. The
power efficiency is nonlinear dependant on speed, load, voltage, temperature and more parameters,
so we need to select a single value that fits most situations.
The observed minimum power efficiency for driving is ~88.4%. So if your power profile has a
mechanical maximum of e.g. 16 kW, the maximum electrical power needed will be...
16 kW / 88.4% = 18.1 kW
Taking that +/- 0.1 kW as your reference drive power usually works quite well. Now if the BMS
limits power to e.g. 17.5 kW, that translates to an automatic DRIVE level of...
17.5 kW / 18.1 kW = 96.7 %
Recuperation is more complex. Efficiency is ~88% for low power as well (recup level < 40%), but
can be close to 100% for higher recuperation levels. An asynchronous AC generator has a low speed
turning point below which it needs more power input to build the magnetical field than can be
generated from recuperation. The SEVCON does brake feathering below ~5 kph to accomodate this
and to provide a smooth handling in low speeds.
The following chart is based on a test drive using a CAN logger to capture all drive data. It shows
the theoretical mechanical power available (yellow, zero at 1.1 kph) and the actual electrical
recuperation power feeded into the battery (blue points, y offset -300W).
Recuperation profile was the default torque map with level raised to 65% to produce the limiter
effect. BMS limit was 7500 W, but the chart shows the limitation will actually start at ~6000 W and
allow a maximum battery input of ~6500 W:
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2019/05/02 RT3.9.1