# Li Sag



## green caveman (Oct 2, 2009)

The SOC meter, left over from a Lead Acid 102V pack in the Electravan started showing low readings. No one seems to know what that indicates, but it used to always read 100+% so we started looking for issues.

When the ammeter (which I don't really trust much more than the SOC meter) read ~100A, the DVM reads 84V. That's accelerating, uphill. However, when you're coasting, it very quickly goes back to the 108-109V range, 109V is about it's resting voltage. Nothing seems to be overheating.

Should I be concerned? The Thundersky discharge curves seem to suggest that I should. If so, what should I be looking for? When you put the van on jack-stands and run the wheels, you see a volt or so of sag.

The van has an 80Ah 33 parallel/series cell Thundersky pack. No BMS, EV1 controller. The pack's a weird mixed pack of 60's and 20's, been there since the beginning of the year - ~1000km driving, never more then 12 miles between charges. The wall to the wheels is about 500W/mile, so 12 miles should mean that it's really never been close to 80%DOD. It's hilly, so we tend to be a little conservative with the measurements and distances.

The first time the SOC when crazy, I charged it and measured all the cells by hand - they were within 0.01V.


----------



## Ziggythewiz (May 16, 2010)

Is your SOC meter an AH counter or just a dummified DVM? My analog ammeter is off by ~40A, so while accelerating it's only off by 80%, but when cruising it's off by 100%, and just gets worse from there. A 25% sag at a little over 1C does sound pretty bad, but maybe you're pulling more than you think.

I recommend getting a JLD404 so you can see what's really going on and know your SOC.


----------



## green caveman (Oct 2, 2009)

Ziggythewiz said:


> Is your SOC meter an AH counter or just a dummified DVM?


It's not clear what it is. I don't think it's just a DVM because it doesn't track with the pack voltage. However, I don't think the old EV1 is smart enough to count coulombs, so I doubt it's an AH counter.

We pulled out the panel to see if we could figure out, primarily the ammeter, which may have been the start of the SOC behavior. With the LA pack it did seem to provide some passing indication of the pack status.



Ziggythewiz said:


> My analog ammeter is off by ~40A, so while accelerating it's only off by 80%, but when cruising it's off by 100%, and just gets worse from there. A 25% sag at a little over 1C does sound pretty bad, but maybe you're pulling more than you think.


Should probably check the zero on the meter but to get that sag it would have to be about 5C = 400A. The fuse is 350A, so it's possible that it's pulling more than 100A, but I doubt that much. 

The van, with the little pack, probably weighs in at under 2000lbs. 40kW would probably strip the gears in the box.



Ziggythewiz said:


> I recommend getting a JLD404 so you can see what's really going on and know your SOC.


We have a little monitor board that we can hook up to the Arduino/Android tablet. The reason we haven't done this yet is we can't figure out the value of the current shunt. We were going to go for a quick and dirty calibration against the analog meter, but now I'm not so sure. Could add a shunt, but I hate to have two. Could use one to calibrate the other, but that's just a project.


----------



## Karter2 (Nov 17, 2011)

green caveman said:


> The van has an 80Ah 33 parallel cell Thundersky pack. No BMS, EV1 controller. The pack's a weird mixed pack of 60's and 20's,.


 Can you detail how those mixed cells are arranged/connected ?
What do you mean by.. " *33 parallel* cell Thundersky pack". ?
I assume you ment a 33 cell series conected pack for the >100v pack voltage, ..but i dont understand how those mixed capacity cells are arranged


----------



## green caveman (Oct 2, 2009)

Karter2 said:


> Can you detail how those mixed cells are arranged/connected ?
> What do you mean by.. " *33 parallel* cell Thundersky pack". ?
> I assume you ment a 33 cell series conected pack for the >100v pack voltage, ..but i dont understand how those mixed capacity cells are arranged


Yes, you're right, I should go back and edit that. The pack is pretty strange, which, now you mention it, may account for the voltage sag.

There are 20 60AH in a chain in parallel with 20 20Ah in a chain. Then there are 13 sets of 4x20Ah in series with that. So the pack is a nominal 105.6V pack.

So now, we get to consider the internal resistance of the 20Ah chain vs. the 60Ah chain...


----------



## Karter2 (Nov 17, 2011)

> ...There are 20 60AH in a chain in parallel with 20 20Ah in a chain


 Really !  are you sure ?
That doesnt sound like an ideal config .


----------



## Ziggythewiz (May 16, 2010)

Haha...that's the part you take issue with? I don't think there's much issue with the 20x60 paralleling, but throwing the 4x20s in series is bound to cause some unbalancing.


----------



## tomofreno (Mar 3, 2009)

green caveman said:


> Yes, you're right, I should go back and edit that. The pack is pretty strange, which, now you mention it, may account for the voltage sag.
> 
> There are 20 60AH in a chain in parallel with 20 20Ah in a chain. Then there are 13 sets of 4x20Ah in series with that. So the pack is a nominal 105.6V pack.
> 
> So now, we get to consider the internal resistance of the 20Ah chain vs. the 60Ah chain...


 So at 100A you are pulling maybe 2.5C through the 20Ah cells, if half the current is going through them. Seems like too much sag in that case. But then you don't trust the ammeter, so maybe it is 3C...100A at 84V is only 8.4kW power, around 6.7kW shaft power, not much to accelerate a van up a hill. Maybe you are pulling much more current than you think?


----------



## green caveman (Oct 2, 2009)

Karter2 said:


> Really !  are you sure ?
> That doesnt sound like an ideal config .





Ziggythewiz said:


> Haha...that's the part you take issue with? I don't think there's much issue with the 20x60 paralleling, but throwing the 4x20s in series is bound to cause some unbalancing.


You guys have no sense of fun! Actually, the cells have stayed in balance really well.


----------



## green caveman (Oct 2, 2009)

tomofreno said:


> So at 100A you are pulling maybe 2.5C through the 20Ah cells, if half the current is going through them. Seems like too much sag in that case. But then you don't trust the ammeter, so maybe it is 3C...100A at 84V is only 8.4kW power, around 6.7kW shaft power, not much to accelerate a van up a hill. Maybe you are pulling much more current than you think?


It's an internal resistance question (I think). So 1/4 of the current should go through the 20's and 3/4 through the 60's.

BUT you're right, if the internal resistance of the 20's and 60's is not 3:1 then the 20's could see more current. However, the voltage (~67V) at the end of the chain of 60's vs the chain of 20's has to be the same (they're tied together).

The way to check would be to check the balance at the bottom, which I haven't done. I suspect that they will be balanced.

How would you feel about the pack if, instead of a chain of 20's and a chain of 60's in parallel, they were paralleled as pairs of 60/20? Electrically/resistance wise, I think that's exactly the same, but I haven't met anyone who's sure of whether they are the same or different.


----------



## Karter2 (Nov 17, 2011)

> Actually, the cells have stayed in balance really well.


 Static ballance maybe,.. but have you checked under load ?


----------



## Sunking (Aug 10, 2009)

State of Charge meters are pretty much useless on an operating system that is under load or charge. SOC is only partially usefull when the battery has been rested for a few hours, and then it is only a ball park meter.

But to answer your question all batteries have voltage sag under load. It is the function of the battery internal resistance and load current. The more current you draw, the greater the sag will be.


----------



## Ziggythewiz (May 16, 2010)

green caveman said:


> How would you feel about the pack if, instead of a chain of 20's and a chain of 60's in parallel, they were paralleled as pairs of 60/20? Electrically/resistance wise, I think that's exactly the same, but I haven't met anyone who's sure of whether they are the same or different.


Buddy pairs would act similar to a single string, with the cells charging and draining together. With the separate strings they will charge pretty close but drain independently under current, then even out under light/no load.

Each method has strengths and weaknesses, in this case the primary concern would be damaging the 20AH string by pulling more amps through than you think.


----------



## Karter2 (Nov 17, 2011)

Are these all "Thundersky" cells ?
I cannot seem find any reference to a 20Ahr TS cell ?


----------



## green caveman (Oct 2, 2009)

Karter2 said:


> Are these all "Thundersky" cells ?
> I cannot seem find any reference to a 20Ahr TS cell ?


Me either. The cells were a couple of years old, but supposedly unused. Maybe the problem.


----------



## green caveman (Oct 2, 2009)

OK, some very preliminary data. So the pack is 20 20Ah in parallel with 20 60Ah and then there are 13 (4x20Ah) to complete the 80Ah pack.

Added an Allegro Hall Sensor in the 20Ah and the 60Ah strings. Sensor goes to an Arduino, Bluetooth to Android. So far only to BlueTerm so I'm hand converting a strictly non-random sample (I'll try to get the data into SQLite and graph it in the next few days). I mis-connected the voltage, so I don't have those numbers.

The most interesting part is that the current readings in the 20Ah string are about the same as the current reading in the 60Ah string. I wouldn't expect a perfect 3:1 split, but I'd expect better than 1:1.

So, for example, accelerating I see 164A from the 20's and 184A from the 60s (the analog ammeter in the van reads, probably 120A, so which do you believe?). 

Crusing, you see 24A from the 20's and 40A from the 60s.

Thoughts? Ideas? I'll fill in details as I can get them (it's supposed to snow here which may slow things down). 

Any more ideas for instrumentation? (I'm working on each cell, but it's a way's away). New pack configuration (new pack?)?


----------



## Ziggythewiz (May 16, 2010)

Interesting info.

Smaller cells usually have better C rates than larger ones, so that could account for some of the difference. What are the temps like there?

I run lithium:lead and usually see 2:1 sharing but in the cold the lead has done most of the work during acceleration. Totally unexpected.

I measure with separate JLD404s and shunts and have DVMs on each string. Eventually I'll have half or 1/3rd pack DVMs on the lithium to better see what it is doing (I have CA40s alongside SE40s).


----------



## green caveman (Oct 2, 2009)

Ziggythewiz said:


> Smaller cells usually have better C rates than larger ones, so that could account for some of the difference.


Could be, but I doubt the 20Ah are good for close to 10C - even for a short time.



Ziggythewiz said:


> What are the temps like there?


40's F



Ziggythewiz said:


> I run lithium:lead and usually see 2:1 sharing but in the cold the lead has done most of the work during acceleration. Totally unexpected.


Unexpected. I would expect the lead to have the lower resistance at all times and so be pulling larger currents.



Ziggythewiz said:


> I measure with separate JLD404s and shunts and have DVMs on each string. Eventually I'll have half or 1/3rd pack DVMs on the lithium to better see what it is doing (I have CA40s alongside SE40s).


I can add more sensors, they're ~$6 each and they're easy to get into the Arduino. I'm just not sure what more would tell me except to validate the existing data (not a bad thing). I'll get the software working so that I have history and trending which will give a better idea of what is going on.


----------



## Ziggythewiz (May 16, 2010)

green caveman said:


> Could be, but I doubt the 20Ah are good for close to 10C - even for a short time.


The older 40AH CALB cells were specced at 10C max, while those above 100AH were 6-8C. I assume other big brands are similar.

If I had your pack the main things I'd want to see are voltage and amps on the 60AH string, the 40AH string, and voltage on the 20AHx4 chunk. I'd record stuff and compare load sharing and sagging at various temps and SOCs.


----------

