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davewill
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Re: Linearity of SOC

Tue Nov 22, 2011 4:15 pm

Herm wrote:
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.
Where do you get the time element? If you're using time of reception, the noise could be delays in the messages.
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klapauzius
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Re: Linearity of SOC

Tue Nov 22, 2011 4:37 pm

I was wondering about the noise too...Naturally taking products of noisy quantities results in a an even noisier quantity, but the amps and voltage during charging should not be that noisy? Especially if you integrate them over some time to get energy? TickTock, how many samples i.e. volts*amps are you integrating over each time step?
It seems the range of the energy is all over the place from 0-200 Wh? What is the Amp/voltage resolution that you can get?

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evnow
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Re: Linearity of SOC

Tue Nov 22, 2011 6:12 pm

TickTock wrote:I integrated battery_volts*battery_amps*delta_time; then plotted the change in total energy each time the gid count changed.
I'm ofcourse looking at discharge - with any regen in between.
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evnow
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Re: Linearity of SOC

Tue Nov 22, 2011 6:13 pm

Herm wrote: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.
I don't capture Voltage now - I'll have to retrofit Gary's changes to my version ...
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surfingslovak
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Re: Linearity of SOC

Tue Nov 22, 2011 7:07 pm

Herm wrote:the 281 scale is probably not the actual capacity, but a voltage related status of the battery..
Yes, that's quite likely. I'm also under the impression that turtle mode is triggered when pack voltage drops below a certain threshold. In regard to SOC linearity, unless you haven't seen it already, you might find this chart published by AESC helpful:

Image

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TickTock
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Re: Linearity of SOC

Tue Nov 22, 2011 7:08 pm

Herm wrote:
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.
I have that posted here
http://www.mynissanleaf.com/viewtopic.p ... &start=257

I don't have the irdrop removed, but that is very small during charging (.07 ohms * 10 amps = 0.7v). Unfortunately, the kids have the computers tied up so I can't count the number of samples right now.

Herm
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Re: Linearity of SOC

Tue Nov 22, 2011 8:06 pm

TickTock wrote: I don't have the irdrop removed, but that is very small during charging (.07 ohms * 10 amps = 0.7v). Unfortunately, the kids have the computers tied up so I can't count the number of samples right now.
Thanks that does look fairly linear regarding kWh per GID for a charging event and probably very similar for a discharging event. Lithium-ion cells have a high coulombic efficiency, but it does drop a bit at the extremes of the SOC.. perhaps that is the non-linearity that EVNOW is seeing. Your plot may even display the coulombic efficiency curve, and that means the software is accounting for it, not just a simple guess by using the voltage.

I think GID does represent energy storage, within those limitations. Next question: will the GID display change as the battery ages? :o

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TickTock
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Re: Linearity of SOC

Tue Nov 22, 2011 8:18 pm

I am getting between 3 and 40 samples on the EVcan per gid and I think that is the primary source of the "noise" since the current is pretty much constant and the voltage moves very slowly. I did, during my inspection, discover a mistake in my original graph so I created a new one. This one shows the result of two separate charging logs. I could see no correlation in the noise between the two. Even the bi-modal distribution in the upper counts is not correlated.

Image

klapauzius
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Re: Linearity of SOC

Tue Nov 22, 2011 8:24 pm

Does the CAN bus provide a sample time along with the data for current and voltage?
Otherwise, being at the mercy of the message time, these measurements are pretty useless due to the noise.
One would think that the leaf internally is doing some time-sensitive computations, so there must be a sample time stamp somewhere on the bus??

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TickTock
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Re: Linearity of SOC

Tue Nov 22, 2011 9:01 pm

Each message is timestamped. The battery voltage and current is sent very frequently (I got >1.5M updates during the ch100a log) but the message containing the gid count is much less frequent and less regular (only 3000 messaged during the log and 3-40 per count). I got gid updates on average every 6.8s but sometimes as quick as 0.5s and sometimes as long as 50 seconds. However, these max update interval values do not correlate to the max energy step values (and should still be fast enough) so I no longer think that is the source of the error. Since the voltage and current is very stable, I think this just means that the car is using something else to generate the gid count that has a fair amount of error.

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