OVMS User Manual page 18

OVMS: Renault Twizy User Guide
each. Cell chemistry is supposed to be LiMn based, so each cell has a theoretical voltage range of
about 2,5 - 4,3 V, nominal voltage around 3,6-3,7 V. Renault restricts the usable SOC window to
maximize battery cycle life. Charging top voltage is 57,8 V for the pack = ~4,13 V per cell.
Important for good performance of a battery pack is that all cells in a serial configuration are at
about the same voltage level and temperature. If a single cell drifts out of line in either direction,
that's a sign of some possible defect.
Now there are 14 cell voltages and 7 module temperatures on the CAN bus, plus two pack voltages.
So, what the OVMS can do to detect potential problems, is to monitor all cell voltages and
temperatures, and to produce an alert if it detects a critical deviation. It also collects the maximum
deviations from the mean values over a usage cycle and sends these statistics on SMS request as
well as to the OVMS server for further analysis and evaluation.
The limits are, the 14 voltages are collective for 3 parallel cells each, and the 7 temperatures each
cover a module of 6 cells. So we're not quite on the single cell level yet, but it's sufficient to detect
critical situations.
Battery max power levels
The Twizy BMS shapes the available maximum power levels for charging and discharging the
battery pack depending on parameters like SOC and temperature. This info is sent on the CAN bus
every 100 ms and read by the SEVCON (while driving), which in turn uses the limits to control
maximum motor drive and recuperation power levels.
Typical Twizy power limit curves look like this:
100 90
This graph additionally shows the default SEVCON linear drive power cutback below 15% SOC.
As you can see above, the battery will do 19000 W if fully charged and 18500 W above 70% SOC,
so this may be a tuning option.
80 70 60
Discharge
50 40 30
SOC [%]
Charge
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2019/05/02 RT3.9.1
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