Simple battery capacity calculation

I searched the forum and didn't find this. If it is redundant my apologies and we can let this thread die. If not, we can compare notes and see what is what easily.

OK, so by battery capacity I mean what the car thinks it is (useable capacity). Perhaps not completely definitive but it does give a basic metric for comparison between cars and a single car over time.

Canion is required of course, and one starts with a fully charged pack showing 100% SOC.

after a completely logged longish trip, plug these values into the formula below.

(Wh out - Wh regen)/(100% SOC - current SOC%) = Wh that the car thinks the pack has available when fully charged.

assuming that the system keeps the same amount in reserve over the cars life we can watch for degradation in capacity in a more quantitative way.

If this calculation were done soon after Mitsu's recalibration procedure that we are to do, in place of dealer "battery checks" it should be quite accurate.

Thoughts?

Aerowhatt

We've discussed this before and decided that doing a capacity check by running the battery down by driving isn't as accurate as a steady load. For example, a discrepancy of the Wh Reg value has been discovered which will skew the results. I can't remember the topic title, though, if somebody wants to look.

I just got the 36,000mi battery test done at the Mitsu-dealer (see separate thread).
CaniOn shows me know 4.06V at most of my cells and 4.055 at 4 cells with a 100% SoC. I have seen, that a new battery should have 4.10V at 100% SoC.
How many percent have I lost compared to a new cell?
When Mitsu did the battery test, did they open up the discharge window towards the bottom to make up for the lost maximum Voltage?
Regards,

Carsten said:

CaniOn shows me know 4.06V at most of my cells and 4.055 at 4 cells with a 100% SoC. I have seen, that a new battery should have 4.10V at 100% SoC...

Click to expand...

The cell voltage at 100% SoC at room ambient should be very close to 4.095v-4.100v (=360vdc for the pack), but this has little to do with the battery capacity. What was the battery temperature when this measurement was made, as I suspect the BMU will cut back on the voltage at higher temperatures? (Sorry, a bit off-topic)

Between 28 and 33 degree Celsius. It looks like starting at cell 2 with 33degrees it is every 6th cell with that high temperature.
Regards,
P.S.: I would put photos in, if I would know how to.

PV1 said:

We've discussed this before and decided that doing a capacity check by running the battery down by driving isn't as accurate as a steady load. For example, a discrepancy of the Wh Reg value has been discovered which will skew the results. I can't remember the topic title, though, if somebody wants to look.

Click to expand...

I would like to read that discussion if someone can direct me towards it?

I've done this several times and am getting very consistent results. :?

Aerowhatt

Aerowhatt said:

PV1 said:

We've discussed this before and decided that doing a capacity check by running the battery down by driving isn't as accurate as a steady load. For example, a discrepancy of the Wh Reg value has been discovered which will skew the results. I can't remember the topic title, though, if somebody wants to look.

Click to expand...

I would like to read that discussion if someone can direct me towards it?

I've done this several times and am getting very consistent results. :?

Aerowhatt

Click to expand...

A extense thread... http://myimiev.com/forum/viewtopic.php?f=23&t=1789&start=110

Carsten said:

I just got the 36,000mi battery test done at the Mitsu-dealer (see separate thread).
CaniOn shows me know 4.06V at most of my cells and 4.055 at 4 cells with a 100% SoC. I have seen, that a new battery should have 4.10V at 100% SoC.
How many percent have I lost compared to a new cell?
When Mitsu did the battery test, did they open up the discharge window towards the bottom to make up for the lost maximum Voltage?
Regards,

Click to expand...

I take my car many times to 100% SoC. I usually look to the cell voltages, when it is at 100%. I surely have look at them at this high SoC many, many times. And what I see is that at 100% SoC, sometimes, cell voltages are 4,105V and 4,11V (total voltage 361V), but other times 4,075V and 4,08V. I believe it depends on the accuracy how the car had calculated his SoC in the last trip. If it gets it a little wrong, giving a value below what it should, then it charges only the amount of energy that it thinks is sufficient to full the battery, and then the value at 100% will be with less voltage in the cells. And sometimes it makes SoC corrections when charging (around 90% SoC, going immediately to 95% or 85%), trying to match the total voltage with the SoC.

So, I think if you take a look another time, your 100% SoC will give you different voltages in the cells. The most important thing is that they are very close, a difference of 0,005 V will be normal.

My I-MiEV, with 4 years and 6 months, has now only 35 Ah of battery current capacity (a new one should have about 48 Ah), so shows a significant loss. Not the worst case, showed in this forum, but I know it shows significant higher degradation then most of the cars with the same age. I blame the long trips I made with it in the hot summers of 2012 and 2013, when the car was almost of the time charging or in trip, day by day. But I can see it charging, many times, to 4,105V and to 4,11V, so I think charging to 4,06V/4,055V could not be related to loss of capacity.



You can try to make a new charge, after it is at 100% with that voltages (4,06V) after some hours of the end of charge, and see what it does. Mine would immediately correct the SoC to a lower value and began charging to go to 100%

Thanks Malm,
I will try the double charge tonight.
Regards,