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evnow
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Linearity of SOC

Tue Nov 22, 2011 11:36 am

I've been tracking the starting and ending SOC units for almost all trips in the last couple of months. What I've noticed is that ... at the top end, SOC units have less energy than in the middle esp. the 280 to 270 band has little actual energy compared to say 230 to 220. I don't have done enough data at the bottom end to draw any conclusions.

The following shows the Wh/SOC unit calculated using the average of the starting and ending trip SOC units (the denominator). The actual energy used is calculated as miles drives / mpkwh (this is the numerator). The Wh/SOC should be around 75 (21,500/281 or 21,000/281) if the SOC units were linear.

We see quite a bit of variability - between 40 & 110 Wh/SOC. But the trend shows the Wh/SOC going down as the SOC # goes up.

Image

I wonder whether we should consider this when displaying the SOC %.

ps : By eliminating outliers we get ...

Image

My guess is, when the SOC # is below 100, the Wh/ SOC again goes down.
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Herm
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Re: Linearity of SOC

Tue Nov 22, 2011 11:41 am

the 281 scale is probably not the actual capacity, but a voltage related status of the battery.. for a true linear SOC you would use a kWh counter.. apparently both voltage and current measurements are available in the CAN-bus so it should be possible to generate a kWh reading.

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evnow
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Re: Linearity of SOC

Tue Nov 22, 2011 11:44 am

Herm wrote:the 281 scale is probably not the actual capacity, but a voltage related status of the battery.. for a true linear SOC you would use a kWh counter.. apparently both voltage and current measurements are available in the CAN-bus so it should be possible to generate a kWh reading.
Instantaneous Voltage & Current only gives you Power. Not energy stored.
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TickTock
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Re: Linearity of SOC

Tue Nov 22, 2011 12:34 pm

Very interesting. I got a very linear response when I plotted it. Very noisy, but linear. If anything the curve has slightly positive slope, not negative like yours.

Image

This was plotted from a log taken during a single charge. I did confirm on a second log that the noise is, in fact, noise and not repeatable non-linearity.

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Re: Linearity of SOC

Tue Nov 22, 2011 12:54 pm

evnow wrote:Instantaneous Voltage & Current only gives you Power. Not energy stored.
Obviously you'd have to take rapid samples (many a second) of V&C to accurately calculate energy...

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evnow
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Re: Linearity of SOC

Tue Nov 22, 2011 1:28 pm

drees wrote:Obviously you'd have to take rapid samples (many a second) of V&C to accurately calculate energy...
Still won't give you energy stored. You need to integrate the power over the length of the drive till the car goes to turtle and stops.
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Re: Linearity of SOC

Tue Nov 22, 2011 1:31 pm

TickTock wrote:This was plotted from a log taken during a single charge. I did confirm on a second log that the noise is, in fact, noise and not repeatable non-linearity.
How did you get the kwh used per SOC unit ?

The first 10 units (280 to 270) go so fast - I'm sure it doesn't have as much energy. It is possible between say 50 and 270, the units have a fiarly constant amount of energy.

ps : My method assumes the m/kwh shown by Leaf is accurate.
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Re: Linearity of SOC

Tue Nov 22, 2011 2:45 pm

evnow wrote: Still won't give you energy stored. You need to integrate the power over the length of the drive till the car goes to turtle and stops.
Duh!, you know what I meant.

Tick-Tock, could you do a graph of pack voltage minus IR losses vs GID?.. assume a constant value for R, I just want to see how linear it is.

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TickTock
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Re: Linearity of SOC

Tue Nov 22, 2011 3:34 pm

evnow wrote:
TickTock wrote:This was plotted from a log taken during a single charge. I did confirm on a second log that the noise is, in fact, noise and not repeatable non-linearity.
How did you get the kwh used per SOC unit ?

The first 10 units (280 to 270) go so fast - I'm sure it doesn't have as much energy. It is possible between say 50 and 270, the units have a fiarly constant amount of energy.

ps : My method assumes the m/kwh shown by Leaf is accurate.
I integrated battery_volts*battery_amps*delta_time; then plotted the change in total energy each time the gid count changed.

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Re: Linearity of SOC

Tue Nov 22, 2011 3:54 pm

TickTock wrote: I integrated battery_volts*battery_amps*delta_time; then plotted the change in total energy each time the gid count changed.
Ohh!, the graphs is a kWh vs GID.. now it makes sense.. noisy but probably the best you can get out of the data.

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