Battery SoC stall or recalibration

Hi everyone

While in the city, I noticed that driving more than half a mile at low speed with the ICE stationary with Batt currents of 20A to 50A, later with the car standing, the SoC stalls and goes down 5% to 10% in a minute or two, reaching less than 40% and starting the ICE to recharge the battery.

Last Thursday in my usual trip (described here) it happen the same situation, so I decided to study the circumstances of SoC stalling.

Just to show you what I mean, let’s look at some charts of the last mile of my usual trip in four consecutive days. Yellow curve is the coolant temperature; bright red is the speed; light blue is the inlet air temperature; green is the friction brake actuator; violet inlet air pressure; pink is ICE RPM; bright blue at the middle of the graphic is the hybrid battery SoC, take a look at the shape of the four bright blue curves:



As you can see, the third bright blue curve as a different shape than the others. It seems that SoC stalls and goes down with no relation with the battery electric consumption, which is the same in all four days.

Taking a deep look at that trip, nothing special happen, so I decided to take the look for more time, 10000 readings which are almost the same seconds, about 2h45m to reach another situation of SoC stalling.

The first chart is the evolution of the SoC and the Battery Net* Energy computed as I explained in this post = (89% of the charging energy) – (100% of the discharging energy).



When I saw the curves, I said EUREKA! At first try I matched the SoC evolution and the equation of the Net Electric Energy that converts to SoC. The vertical scales tell us the relation of 10%SoC = 350KiloJoules.

However, this is not a perfect match of the two curves for all the time, because at some points there is a difference of up to 13 percentage points.

So, I thought to make a chart of the actual SoC, the energy predicted SoC and the difference between both.



Surprise again! There are three SoC stalls in the graphic. I never noticed the second one while driving.

Just before the SoC stall, it is possible to see a small peak in the graphic, which I suppose triggers the SoC stall.

I made a chart of some other hybrid battery parameters like temperature, voltage and internal resistance (reported by 7E2 ECU).



Bingo! The small peaks we saw on the previous chart are points where the battery voltage goes below 200Volts, while battery temperature and internal resistance seem not to be related with the triggering of the SoC stall.

There are other moments in which the battery voltage goes below 200V but do not trigger the SoC stall, so there should be another parameter that combined with the voltage triggers the SoC stall. The only other parameter missing is the SoC itself, so I made the chart of SoC Vs Voltage and marked the trigger points:



Solve it!

All three SoC stall trigger points characterized by been below 200Volts and below 50% SoC.

I also draw with a blue arrow the path that the SoC follows while been recovered.

I noticed the strange behavior of the battery internal resistance reported by the hybrid system ECU which is constant all the time at 338miliOhms. In fact it has been constant for the whole week, after last week has been at 310miliOhms.

I wanted to measure the hybrid battery internal resistance using the recorded data, so I made a chart of Voltage Vs Current while discharging near the trigger points and draw the lines to compute the slope.



It seems that the best measure for the hybrid internal resistance is 450miliOhms, far away of the 338miliOhms reported by the ECU (Anyone noticed that and knows the reason?). About a year ago I made the same computation for thousands of points and I got a similar value, so it seems that the internal resistance variation is not a triggering event.

Concluding:

1.The relation between hybrid battery SoC and Net* Electric Energy that the hybrid battery takes is 10%SoC = 350kiloJoules

2.When the battery voltage goes below 200Volts and the SoC is below 50% the SoC reported by the ECU stalls to below 40% so the battery is charged by the ICE and the SoC matches the energy predicted SoC (calibration process).

3.The SoC stalls trigger points are not related with temperature and internal resistance

4.The ECU reported battery internal resistance does not match with the actual resistance computed from observed data.

* Net electric energy = 89% of charging energy – 100% of discharging energy

I hope you like the post

Big hugs from Frank