SOC vs RAhC

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GregH

Well-known member
Joined
Jul 1, 2011
Messages
860
Location
Irvine, CA
I think Sparky nailed it on the 0x5bc "SOC" number...
From what I've observed the last few days on my car and gleaned from the service manuals, this number is the Remaining Capacity (in Ahs)
It seems to be scaled to 1/4 Ah and has a very heavy hysteresis (tons of regen needed to bump the number up!)
During charge I observed the number increasing at about 90 second intervals when the pack is low and 100 second intervals when closer to full (approximately).
This would be true as the pack voltage increases under a constant power charge... the current would lower and the Ahs would accumulate slightly slower.
I know many of you with slightly more mature packs are getting 281, but mine is still 278.. actually 277 today (!)
The service manuals talk about a Remaining Capacity CAN message from the battery system, but no mention of "SOC" (which we typically think of as a number from 0 to 100%)
The service manuals talk of a nominal pack voltage of 345 volts (3.6V/cell) and a max 393V (4.1V/cell) and absolute max 403V (4.2V/cell) which is all reasonable for a Lithium magnate chemistry.
24000kWh/345V = 69.5Ah... x4 = 278.
I'm guessing each cell in the 4-cell modules is rated at roughly 35Ah and 3.6V nominal.

Now of course Remaining Ah Capacity and State of Charge are pretty much the same thing other than the scale..
I think on my display I'm going to stick with the raw number which I'm calling Remaining Ah Capacity (RAhC) rather than scaling to 100% and calling it SOC...

Thoughts?
 
if you could find the actual pack voltage, then you could calculate a true wh capacity scale.. and that would give you the true SoC
 
If it is measuring Ah from/to the battery, it has no good way of knowing where "zero" is without discharging the battery "completely".

Also, the top 8% of the Amp-hours should give us more energy (more miles actually driven) rather than less energy and less miles driven.

Many experience getting only 50 to 60% of the miles from the Top Bar, compared to the middle bars, even when starting near 278 (to 280) raw.

But, anything is possible.

No matter how this is derived (probably in the BMS), it performs the function of a fuel gauge, with an uncertain (variable) low end.

It appears that another mechanism, like lowest (or highest) cell voltage, is actually telling the car to STOP driving (or charging).

We think we have found battery voltage and current elsewhere.
 
I'd love to find pack voltage! And pack current... Cell voltages and indication of balancing would be nice as well, but I'd bet we'd need to "borrow" a Nissan scan-tool and snoop on the bus during a transaction in order to see how to get that.. It is probably something that needs to be asked for via a CAN message and we need to figure out how to do that. But pack voltage and current? C'mon... it's gotta be in there!
 
GregH said:
The service manuals talk of a nominal pack voltage of 345 volts (3.6V/cell) and a max 393V (4.1V/cell) and absolute max 403V (4.2V/cell) which is all reasonable for a Lithium magnate chemistry.
24000kWh/345V = 69.5Ah... x4 = 278.
I'm guessing each cell in the 4-cell modules is rated at roughly 35Ah and 3.6V nominal.


Excellent observations !!!

Thanks!
 
GregH said:
Excellent observations !!!

Great observation, thanks for mentioning it. The max pack voltage would be consistent with the 393V charging voltage seen on the sole QC station in Portland by PDXLeafer.

http://www.mynissanleaf.com/viewtopic.php?f=26&t=1401&p=109596&hilit=393V#p109596
 
Hey guys! I've been meaning to update this post but been a way for a week or so.. GaryG's CAN-do software shows the locations of the pack voltage and pack current in 0.5V and 0.5A steps and I've since been able to confirm both numbers. A little more precision would have been nice, but I'm not complaining! I've since brought up both numbers as well as a power/regen graphic representation of current on the top of my display alongside the RAhC.. It all fits nicely and behaves just as expected. The highest I've seen on my pack during charge (as high as 280 RAhC, still haven't seen 281) is 394V which implies that even at full charge, the cells are averaging 4.1V. After an overnight "80%" charge, the RAhC was about 230, or about 82% and the pack voltage was resting around 389V or 4.05V per cell.

I've also seen evidence of pack balancing although not directly.. I'd be curious to hear if anyone else has seen this... After a full charge and the blue lights were off, I plugged in my display and of course got no data (car off, not charging but still plugged in). About an hour later I came down to the garage and there was data on the display! Ie the car had woken up.. Did it charge? Did it balance? I don't know.. Maybe one of the folks with time recording on their EVSE unit can say if it comes on periodically for small charges every 15-60min(?) while balancing the cells... This is what I'd expect as the balancing electronics can only slowly discharge the "full" cells compared to the 3.3+kW charge..
 
A period of about 45 minutes "doing nothing" followed by about
5 minutes of somewhat-reduced-amperage charging has been observed
to occur at least 2 times after the "main" charging portion is completed.

I assume the "doing nothing" was a time of discharging the highest cells.
So, for now, I call these "equalization" cycles.

Open Questions:
1. Do they occur after every 100% charge?
2. Do they occur after any 80% charge?
3. How many cycles can occur (we have seen 2)?
4. Is this really the cell-equalization mechanism?
5. If they never follow an 80% charge, must a car be charged
to 100% occasionally to maintain cell-equalization?
6. What situation stops or prevents the equalization?
 
garygid said:
Open Questions:
5. If they never follow an 80% charge, must a car be charged
to 100% occasionally to maintain cell-equalization?

There is not need to equalize the cells if you never charge to 100%, but its guaranteed that when you do charge it to 100% after a couple of months of 80% it will go into an equalization cycle at that time.. it all depends on how well matched the cells are and how even the heat is distributed across the pack.. hotter cells will have a higher voltage.
 
Herm said:
garygid said:
Open Questions:
5. If they never follow an 80% charge, must a car be charged
to 100% occasionally to maintain cell-equalization?

There is not need to equalize the cells if you never charge to 100%, but its guaranteed that when you do charge it to 100% after a couple of months of 80% it will go into an equalization cycle at that time.. it all depends on how well matched the cells are and how even the heat is distributed across the pack.. hotter cells will have a higher voltage.

What exactlly happens during an equalization cycle?
 
One of the reasons why I do a 100% charge every month or two regardless of whether or not I need to...

garygid said:
There is not need to equalize the cells if you never charge to 100%, but its guaranteed that when you do charge it to 100% after a couple of months of 80% it will go into an equalization cycle at that time.. it all depends on how well matched the cells are and how even the heat is distributed across the pack.. hotter cells will have a higher voltage.
 
Herm said:
There is not need to equalize the cells if you never charge to 100%
Sure there is - unless the charging algorithm stops when the lowest cell is 80% full, you still want the cells balanced to maximize your 80% range.
 
drees said:
Herm said:
There is not need to equalize the cells if you never charge to 100%
Sure there is - unless the charging algorithm stops when the lowest cell is 80% full, you still want the cells balanced to maximize your 80% range.

There is no need, as long as the BMS never lets you discharge a cell below its safe point, it would take a really severe imbalance before your 80% range was affected.. probably enough to trigger a warranty event. We dont know but the Leaf may do an occasional balancing at 80% also
 
No, it only takes a small imbalance to lose range, even if charging to 80% instead of 100%.

And even a "comparatively severe" loss of range is not likely to trigger a warranty event, if it APPEARS to be "capacity" related.
 
I don't know for sure, but I'd guess balancing only happens at 100%
In the mid range (5-95%) it's very hard to detect cell imbalance. Because of the charge curve of the chemistry it's much easier near zero or full.. The pack would have to be pretty unbalanced to see anything at 80%. I wonder if the car would ever go to 100% even if told to do 80% for a long time...

5 minutes low current on and 45min off sounds about right.. I would assume when any cell hits 4.15 or 4.2V during charge, then charge stops and resistive balancing drains the high cells down to 4.10v.. At probably 1/10th the charge speed.. Otherwise the resistive elements would generate too much heat. If after this cycle some cells are still 4.10v then the cycle repeats.. More charge till some guys get too full then discharge them down to 4.1v

Someday we'll be able to see the individual cell voltages then we'll know for sure.

So 2 cycles (45+5min) is the most anyone has seen?
How many people have these data loggers on their EVSE units?
 
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