Ideal Range Configuration - OVMS User Manual

OVMS: Renault Twizy User Guide 2019/05/02 RT3.9.1
To reset the mean value and get a direct single capacity estimation, set feature #13 to 0 before the
charge process reaches 100%. Feature #13 stores the running average capacity value (percentage)
persistently, so it's not lost over power losses or reboots. Note: feature #13 will be shown as an
integer value by the app and feature queries, but the internal precision is at 1/100 %.
Background
Lithium-ion batteries lose capacity by aging (calendary time) and usage (charge cycles). There are
lots of parameters for the capacity degradation. Generally, storing with high SOC at very high
temperatures, charging with high currents at very low temperatures and deep discharging are the
primary causes for raised degradation.
The Twizy battery is normally rented, with Renault providing a new battery if the capacity drops
below 75%. We need to pay for a capacity check by Renault if the result is above 75%, so we
should be sure it's worth the check when we reach that point.
The capacity of a lithium based battery cannot be measured directly, instead it needs to be derived
from other measurements – just like the SOC. There exist some advanced instruments probing for
the voltage drop under load (elasticity), but the basic method available to us is to check the capacity
by discharging & charging and measuring the energy amount (coulomb count) that fits into the
battery, then relate that to the nominal capacity.
Ideally we should discharge to 0%, then charge to 100%. That's because these are the most reliable
SOC points the BMS can provide. Keep in mind, the SOC is also just an estimation derived by
complex algorithms from coulomb counting and many other parameters of the BMS. Even 0% is
not necessarily really "empty", it's just the BMS saying "no more discharging, please".
The charging process of a lithium battery consists of basically two phases, the "CC" (constant
current) followed by the "CV" (constant voltage) phase usually beginning around 94% SOC. The
CC phase current is not necessarily constant though, for example if it needs to be cut back due to
high temperature.
Capacity can be given in Wh or Ah. Capacity measurement based on the Wh sums induces an error
due to the energy discharge/charge efficiency and inner battery resistance related side effects. The
coulombic discharge/charge efficiency of a lithium-ion battery on the other hand is always near
100%, so is the best choice for capacity measurements.
The OVMS capacity algorithm detects the end of the CC phase by monitoring the battery voltage
during charge. It scales the coulomb count measured during the CC phase from the CC phase SOC
window to a theoretical full range CC phase beginning at 0%. It then adds the coulomb count from
the CV phase, resulting in the overall Ah capacity. The percentage is then calculated based on the
assumption of 108 Ah default usable capacity.

Ideal range configuration

The Twizy's range estimation is based on the energy use of the last 3 km driving, and it will drop
faster on high usage than it will raise on low. So if you happen to have a hill climb at the beginning
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