# Determine which LiFePO4 cells are "bad" in pack



## Ziggythewiz (May 16, 2010)

Do the logs give you individual cell readings or just full pack info?


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## mora (Nov 11, 2009)

Ones with highest internal resistance are to suspect. You could remove them from pack and measure internal resistance and capacity at 1C at least. Maybe it is good idea to do this for every cell in your pack if you have time and equipment to do so.

In my experience TS cells are not happy when discharged at rates over 3C. Newest cells even operate at 3.0V at 1C load.


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## tomofreno (Mar 3, 2009)

kickngas said:


> My question is, how would I determine which cells are the "bad" ones in case I want to replace them to make for a healthier pack?


 The ones that are sagging badly during acceleration. Doesn't the Orion read out individual cell voltages? If not, get some Celllog8's, hook them up and data log cell voltages while aggressively accelerating. Graph voltage versus time to pick out the bad ones.

Also check your connections if you haven't for a while.


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## GizmoEV (Nov 28, 2009)

tomofreno said:


> Also check your connections if you haven't for a while.


I would definitely start with this suggestion. I would remove all the connections, put NO-OX-ID on each one and reassemble. Test again.

Another thing I have heard from 1 or 2 places is that the TS cells like you and I have "like to be charged to 4.00V once in a while." I haven't found the documented evidence to support that statement, however. I don't recall if you have a top balanced pack or not but if you do it might be worth considering to see whether any difference is made.

I haven't noticed any drop off with my pack but I have a 200Ah pack so it is really gently used compared to yours.


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## kickngas (Feb 3, 2011)

We are going to remove all connectors next week and have the NO-OX-ID ready! That was going to be the first step. I have a student that likes programming and will be working on a way to data log the Orion so we can graph out how each cell is performing. 
Gizmo, thanks for your helpful advice over the past couple of years. We can set the charge points higher and hit the 4.0 volts. Ironically we had talked about that yesterday, however on the same spec sheet, it shows only discharging to 2.5 volts. 
I feel that anyone using Lithium cells are still considered early adopters since the lack of solid evidence on charging, discharging and monitoring is just not there yet. Nice to have this message board where we are all trying to figure it all out....even with the healthy arguments! Learning is fun.


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## kennybobby (Aug 10, 2012)

*Overcharging?*



kickngas said:


> We are going to remove all connectors next week and have the NO-OX-ID ready! That was going to be the first step. I have a student that likes programming and will be working on a way to data log the Orion so we can graph out how each cell is performing.
> Gizmo, thanks for your helpful advice over the past couple of years. We can set the charge points higher and hit the 4.0 volts. Ironically we had talked about that yesterday, however on the same spec sheet, it shows only discharging to 2.5 volts.
> I feel that anyone using Lithium cells are still considered early adopters since the lack of solid evidence on charging, discharging and monitoring is just not there yet. Nice to have this message board where we are all trying to figure it all out....even with the healthy arguments! Learning is fun.


Did that same spec sheet not indicate an upper limit for charging voltage on the order of 3.65 volts? It is very easy to overcharge at that voltage, much less at 4.0! Over charging produces heat and causes the internal cell pressure to increase and this pressure causes the case to puff out (or the safety vent to open)--once they get puffy or vent, they sag and lose capacity.

"Learning is fun"--i totally agree, but an education is expen$ive. You can pay tuition to learn at college (be a college puke), or you can learn it the hard way and pay for your mistakes (at the ******* university)...


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## dladd (Jun 1, 2011)

regardless of how you got to where you are, the way to find the bad cells is to log the voltage sag while driving. Sounds like you have the hardware to do that, and a student who is willing to figure out how. You're set.


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## kickngas (Feb 3, 2011)

Not that I ever would, but this is directly from the manual:
At 20C +/- 5C, the cell is discharged at a current of 1/3C3 till voltage of
the cell reach 3.0V, and then start to perform constant current charge at a
current of 1/3C3 under 20C +/- 5C till voltage of the cell reach 4.2V and
simultaneously switch to constant voltage charge. When charging current
value decreases to 5% of initial value, charging completes.

At -18C +/-5C, the cell is discharged at a current of 1/3C3 till voltage of
the cell reach 2.2V, and then start to perform constant current charge at
1/3C3 under -18C +/-5C till voltage of the cell reach 4.2V and
simultaneously switch to constant voltage charge and duration is 2 hours.
After that, Trickle charge will begin. Charging completes when charging
current value decreases to 5% of initial value.


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## Ziggythewiz (May 16, 2010)

I think the early manuals said 4.2V, then 4, 3.8, and now at 3.65. Next year they'll say 3.5.

They don't really know what's best for these cells, and we're their beta testers.


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## kennybobby (Aug 10, 2012)

*Which manual?*



kickngas said:


> Not that I ever would, but this is directly from the manual:
> At 20C +/- 5C, the cell is discharged at a current of 1/3C3 till voltage of
> the cell reach 3.0V, and then start to perform constant current charge at a
> current of 1/3C3 under 20C +/- 5C till voltage of the cell reach 4.2V and
> ...


The charger or the cells?

Sounds like the charging procedure for a different chemistry cell--definitely not the currently advertised procedure for LiFePO cells.


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## GizmoEV (Nov 28, 2009)

*Re: Which manual?*



kennybobby said:


> The charger or the cells?
> 
> Sounds like the charging procedure for a different chemistry cell--definitely not the currently advertised procedure for LiFePO cells.


The cells. That is what the TS manual and paperwork said back in 2009.

4.0V does not noticeably heat my November 2009 TS cells if that is the max voltage and the current is low. Having said that, since my Zivan charger won't stop based on a target voltage AND ending current and in stead just tapers back the current to close to zero I am now only charging to 3.455vpc. The charger's timer shuts it off and after several hours of sitting the average cell voltage of my pack is just under 3.4V so I figure I have it set really close to charging to 100%.


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## GizmoEV (Nov 28, 2009)

kickngas said:


> We are going to remove all connectors next week and have the NO-OX-ID ready! That was going to be the first step.


I ended up getting a soft bristle stainless steel brush to clean the tops of the terminals. It seemed to shine both the Al and Cu up nice without leaving streaks in them. I then coated them with NoAlOX and haven't had any issues since I installed them nearly 3 years ago. If I take anything apart I will be cleaning the posts and using NO-OX-ID. I don't think it it gets as gummy as NoAlOX does.



kickngas said:


> Gizmo, thanks for your helpful advice over the past couple of years. We can set the charge points higher and hit the 4.0 volts. Ironically we had talked about that yesterday, however on the same spec sheet, it shows only discharging to 2.5 volts.
> I feel that anyone using Lithium cells are still considered early adopters since the lack of solid evidence on charging, discharging and monitoring is just not there yet. Nice to have this message board where we are all trying to figure it all out....even with the healthy arguments! Learning is fun.


You are welcome. We definitely are early adopters. It is critical that we share our successes and failures as much as possible so all of us can learn. These packs aren't cheap. I need mine to last another 3 years and I'll definitely be past the break even point compared to if I had stayed with lead acid. In any case, I have definitely enjoyed the greater usability of my rig going with a LiFePO4 pack!

I look forward to what you find out.


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## bga (May 25, 2009)

Whoooah!
Before getting too carried away disassembling the pack, it may be telling to measure some temperatures after a short, spirited, drive. An inexpensive infra-red remote temperature sensor can be used to quickly spot all the cell and terminal temperatures. This may save a lot of guessing.


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## GizmoEV (Nov 28, 2009)

bga said:


> Whoooah!
> Before getting too carried away disassembling the pack, it may be telling to measure some temperatures after a short, spirited, drive. An inexpensive infra-red remote temperature sensor can be used to quickly spot all the cell and terminal temperatures. This may save a lot of guessing.


That may be true but if the terminals were not cleaned and sealed with something like NO-OX-ID then the resistance will go up, maybe not enough to detect with a temp gun. I know Bill Dube reported similar results with TS cells in his Cabriolet and found that he needed to clean all the terminals and seal them to get his performance back. I don't recall how long he went from assebly to the sag that kickngas is describing. If they all heat up the same how would you know if they were bad? I'm not saying not to check, just that if it doesn't show anything don't assume that the connections are all good.

What about measuring the voltage drop across a connection? Do you have a load you can put on the pack and then use a volt meter to measure the voltage drop between the post and connecting strap? It doesn't sound easy but if the current draw is known then the resistance can be calculated. Maybe go from one battery post to the one across the strap. That would give you the voltage drop across two connections and the strap. Would it be correct to assume that the resistance is higher at the connections than in the strap?


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## GizmoEV (Nov 28, 2009)

A couple of articles I have bookmarked might be useful. I think I got these from another DIY thread.

*The Trouble with Torque in Electrical Connections: Torque and force are not the same*
http://www.maintenanceworld.com/Articles/shackmann/thetrouble.html

*Creating Reliable Electrical Connections*
http://www.maintenanceworld.com/Articles/shackmann/creatingreliable.html


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## vans_off_der_Wal (Aug 31, 2012)

Does anyone know the Torque specs on the 100aH Thundersky cells? I've been looking for awhile and have brought up nothing.


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## bga (May 25, 2009)

GizmoEV said:


> ...
> If they all heat up the same how would you know if they were bad? I'm not saying not to check, just that if it doesn't show anything don't assume that the connections are all good.
> 
> What about measuring the voltage drop across a connection?
> ...


Measuring the voltage drop across a connection under load is tricky in a car, more tricky when there are 50 of them to do.
Because of the power involved, most electrical issues will show up as heat. 1V under load will equate to several hundred watts, sufficient to measure.

I agree that thermal measurements are not infallible, but this is a quick measure and can often identify a problem. Terminal resistance issues are rarely uniform, so the differences between cells will likely be telling.
With a thermal gun type sensor it is non-contact, so sparks and burnt fingers are not likely.


Bolt torque on TS160 cells is surprisingly low, see this thread: http://www.diyelectriccar.com/forum...commendations-thundersky-batteries-52956.html
20Nm would be for the larger M8 terminals on 100AH cells etc. The small cells have M6 bolts and much lower torque. I would also suggest 'belleville' conical spring washers that provide consistent joint compression over successive thermal cycles. It may be necessary to stack two or more washers to obtain a loading that is compatible with the bolt torque.

The terminals are Alum and Copper, so this torque is important to avoid stripping the terminal. If the bolt is lubricated, the required torque will be lower. A small torque wrench is useful to obtain a consistent result.


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## kennybobby (Aug 10, 2012)

*Re: Which manual?*



GizmoEV said:


> The cells. That is what the TS manual and paperwork said back in 2009.
> 
> 4.0V does not noticeably heat my November 2009 TS cells if that is the max voltage and the current is low. Having said that, since my Zivan charger won't stop based on a target voltage AND ending current and in stead just tapers back the current to close to zero I am now only charging to 3.455vpc. The charger's timer shuts it off and after several hours of sitting the average cell voltage of my pack is just under 3.4V so I figure I have it set really close to charging to 100%.


Are these yttrium cells (LiFeYtPO) that may have different cut-off limits?


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## GizmoEV (Nov 28, 2009)

*Re: Which manual?*



kennybobby said:


> Are these yttrium cells (LiFeYtPO) that may have different cut-off limits?


No, these were before the FYP cells. These are LFP cells. AFAIK, they kept lowering the final charge as they learned more, likely from their customers! The theoretical full voltage is right at about 3.4V so charging a cell with a particular procedure and letting it sit for a day or two will give you a good indication of whether you are over charging or not. That is how I have arrived at my 3.455vpc figure for my particular situation.


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