# Charge cutoff current for 100Ah TS cells



## brainzel (Jun 15, 2009)

To get the cell charged, you have to put a higher voltage to he terminals then the full charged battery would show after charging and measuring with your multimeter.
LiFePo4 would be full at about 3.4V, Jack says 3.38V I think.
So to charge them to "full", you can go two ways:
1st) 3.38V to the terminals and wait until the current gets to stop - this would take years 
2nd) raise the voltage to 4.0V and cutt of at 0.05CA - 5Amps @ 4.0V for your cell.
If you further push current into your battery upper this point, you will overcharge it.

This works fine for a single battery, but if you have 30 or 45 in a row, you will never get to this point at the same time.
So lower the Voltage to recommended 3.6V for TS and cut of @ 5Amps and you will slightly undercharge them and everyone is happy


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## Ziggythewiz (May 16, 2010)

Do you even need every drop of range? If not you can lower the cutoff voltage further and suffer from extended cell life, say 3.5 or 3.4 vpc.


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## Jesse67 (May 12, 2009)

Thanks for the replies. I don't need every drop of range to that extent, I had lowered the voltage to 3.7V originally with the intent of not charging to 100% to extend cell life, but I've realized since then that the ending current is also part of the equation. I will likely go with a lower charge voltage as well but I want to establish a fixed value for the cutoff current. 

So.. Michael, where did you get the .05C or C/20 cutoff current to go with 4V/cell charging voltage to get 100%? This is the number I'm trying to figure out and I'd like to know what it would be for different charging voltages, like 3.8V, 3.6V etc. Has anyone done testing to determine what particular combinations actually give 100% full charge? If I want to set up a charge profile that always goes to 95% I need to initially know where 100% is. 

Thanks!

Jesse


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## Ziggythewiz (May 16, 2010)

Short of a scanning electron microscope there's no way to tell when a cell is 100% charged.

'Charged' is more of a definition that varies by manufacturer and changes every year based on further testing and warranty claims.

Probably the most common definition at the moment is to charge a cell to 3.65V and hold it there till current drops to C/20, then stop charging. Even though that's CALB's current definition of fully charged they recommend only operating the cells from 10-90% SOC.


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## Jesse67 (May 12, 2009)

I suppose that's true, I'm just wondering what most people are using and it would be great to figure out the "recipe" for a full charge at different voltages.

So the C/20 at 3.65V is from CALB for 100%, do they give a voltage and cutoff current for a 90% charge?


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## swoozle (Nov 13, 2011)

Jesse67 said:


> So the C/20 at 3.65V is from CALB for 100%, do they give a voltage and cutoff current for a 90% charge?


The percentage is based on Ah put in, so by definition it would be 90% of however many amp-hours it took you to reach "100%"


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## Ziggythewiz (May 16, 2010)

No voltage provided for 90% SOC. Based on a TS curve I saw I shoot for ~3.33 resting as 90%, which seems pretty close for my CALBs.


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## Jesse67 (May 12, 2009)

Ok, so what voltage do you charge to and what current do you cut off at to get that ~90% or 3.33V/cell resting?


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## Ziggythewiz (May 16, 2010)

I used to charge to 3.34 vpc @~5A (c/8) giving 3.3v resting. Now I have to go a little higher ~95%, I'll have to check the exact numbers.


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## ruckus (Apr 15, 2009)

Ziggythewiz said:


> I used to charge to 3.34 vpc @~5A (c/8) giving 3.3v resting. Now I have to go a little higher ~95%, I'll have to check the exact numbers.


This is good to know. The TS260ah cells in the Jaguar charge to 3.65 and settle to 3.333v, so I am seeing little need to charge so high.

My new pack will be 120 cells of CALB 60ah. Since TCCH (Elcon) chargers only go to 408v, that means the max cell charging voltage is 3.4v each. 

It sounds like I should have a fairly aggressive amp input (5A?) when charging to such a low voltage (per cell).

Thoughts?


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## Jesse67 (May 12, 2009)

Do you mean a fairly high cut off current? 5A would be a very low charge current for even a 60Ah cell. From my knowledge you would need a very low cut off current to get a reasonably full charge at only 3.4V/cell charging voltage. Ziggy, do you know how close you were to full when you were only charging to 3.34V/cell and cutting off at 5A? That's the lowest cell voltage I've ever heard anyone charging to. 

Jesse


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## Ziggythewiz (May 16, 2010)

Jesse67 said:


> From my knowledge you would need a very low cut off current to get a reasonably full charge at only 3.4V/cell charging voltage. Ziggy, do you know how close you were to full when you were only charging to 3.34V/cell and cutting off at 5A? That's the lowest cell voltage I've ever heard anyone charging to.


Depends what you consider "reasonably full". CALB's recommended usage window is 10-90% SOC. To follow that recommendation cutoff shouldn't even be determined by current, but by voltage. I expect more users and vendors will move away from the whole CC-CV thing in the next few years.

I haven't saved enough pennies for a PL6 or 8 yet, so I can't verify it, but I think charging to 3.4 and resting at 3.3 put me ~85-90% SOC. My lithium pack is just a booster so I'm much more interested in cell life than peak range or performance. 

My current naturally tapers from C/5 to C/8 because charge resistance increases with SOC. While my termination voltage may be lower than most people's, my current is also lower so it's similar to higher powered chargers cutting off at 3.45 or 3.5 vpc.


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## Jesse67 (May 12, 2009)

Ah, charging with no CV phase essentially, in which case the SOC will be determined by the voltage and the charge current. The lower the current, the higher the ending SOC. I believe that is a good reason stick with the CC CV profile though, regardless of your initial charge current you'll end up with the same state of charge. So if you have a high powered charger but have access to a variety of power supplies, eg 110V at work, 220V 50A at home, you'll still end up with the same state of charge if the CV stage voltage AND cutoff current is the same. Although I suppose it wouldn't be more than a few Ah difference.


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## brainzel (Jun 15, 2009)

I don't have a TS100Ah to measure, but I made a chart with another LiFePo4 cell, in this case GBS20AH to show up the difference in SOC at different voltages.
@ 3.34V = 12.25Ah/20Ah = 61.25%
@ 3.40V = 14.00Ah/20Ah = 70%
@ 3.50V = 17.25Ah/20Ah = 86.25%
@ 3.55V = 19.00Ah/20Ah = 95%

Data based on:
20Ah GBS LiFePo4 cell
charging @ 10A
3,59V upper end voltage
cut off @ 3,59V / 1A

So you have to know, that a early cut off would decrease your capacity and range. If you could deal with it, then go for it 
But thats not economically good, while you spend a lot of money for a big pack, but use only 60% of it. You could save 40% money and weight ;-)


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## Ziggythewiz (May 16, 2010)

Are those resting, or charge voltages?



brainzel said:


> So you have to know, that a early cut off would decrease your capacity and range. If you could deal with it, then go for it
> But thats not economically good, while you spend a lot of money for a big pack, but use only 60% of it. You could save 40% money and weight ;-)


Economically I'd rather use 60% and have the cells last 5x as long as if I used them 100%. For 50 miles or less range weight is irrelevant.


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## brainzel (Jun 15, 2009)

Of course charging voltage.

I would never recommend 100% discharge but also not 40% oversizing.

Charging to 90%/95% and discharging 80% to 20% of the nominal capacity should be fine for a long, long, endurance.

So instead of 7.9kWh pack (24x100Ah cells) charge to 60% and discharge to 10% to get to your daily destinations, I would take 70AH cells (5,5kWh) and save about $1.000 Dollars

<mho>
If they "only" least for 10 years, then I would buy the new chemistry of cells with mostly higher density, lower weight and less expensive and put the "old" batteries (they still work pretty, but only have about 80% capacity left!) into my home solar storage cluster ;-)
</mho>

But this is only my humble opinion


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## Ziggythewiz (May 16, 2010)

Use 60% (30-90% SOC) is not charge to 60%. 



brainzel said:


> I would never recommend 100% discharge but also not 40% oversizing.
> 
> Charging to 90%/95% and discharging 80% to 20% of the nominal capacity should be fine for a long, long, endurance.


That's 70-75%, so you'd save 10-15% vs 60% use, not 40%. I've seen claims of 50-67% increased life for 70%DOD vs 80% so to me that 50-67% is worth more than 10-15%.


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## dladd (Jun 1, 2011)

brainzel said:


> I don't have a TS100Ah to measure, but I made a chart with another LiFePo4 cell, in this case GBS20AH to show up the difference in SOC at different voltages.
> @ 3.34V = 12.25Ah/20Ah = 61.25%
> @ 3.40V = 14.00Ah/20Ah = 70%
> @ 3.50V = 17.25Ah/20Ah = 86.25%
> ...


That's a C/2 charge rate!  This results in more Ah put into the cells during the CV phase than if you are charging at a lower current. I personally have been charging my 130ah cells at 6-9a (depending on which charger I use) and 3.5vpc appears to be very close to full. There is only a few minutes of current taper at the end of charge before the charger cuts off when charging at a lowly c/20.


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## Jesse67 (May 12, 2009)

Yes a high charge rate but interesting data none the less, thanks for sharing! I'm assuming those lower voltage data points did not include a CV stage dropping to C/20 before ending the charge but just measured the Ah in at the point when the voltage first hit each value? 

Like dladd says with such a high initial charge rate then a larger portion of the total charge goes in during the CV phase so if my above assumption is correct a charge to 3.4V with a full CV stage may actually get significantly fuller than 70%.


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## brainzel (Jun 15, 2009)

The datasheet to this LFP020AH cell gives an optimum charge current of 10Amps, maximum 20Amps.
So this graph was made to confirm the given specs.

Like I said before, I have no TS100Ah for testing, so these data is based on a similar but different type of LiFePo4 cell.


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## dladd (Jun 1, 2011)

brainzel said:


> The datasheet to this LFP020AH cell gives an optimum charge current of 10Amps, maximum 20Amps.
> So this graph was made to confirm the given specs.
> 
> Like I said before, I have no TS100Ah for testing, so these data is based on a similar but different type of LiFePo4 cell.


I didn't realize they recommend charging at c/2, I take back my 'shock and awe' emoticon.  

It is interesting to me to see all the different variations of charging, currents, when to cut off etc. It's all intermixed, variables of current and voltage together, there doesn't seem to be any one simple way to do it. High current, long taper, low current no taper, 3.5vpc, 3.6vpc, 3.4vpc, so many choices. Here's a plot of a cell I cycled at 39a (.3c, which is CALB's recommended charge current), you can see that once the cell hit 3.6v, it tapered to .05c (where the charger shuts off) in about 10 minutes. If you do a simple slope calculation from 39a to 6.5a you'll see that about 3Ah were added to the battery after it reached 3.6v at 39a. On this 130ah cell, that is 2% of capacity, not much. In reality it's even less than that since the current falls non-linearly. You can also see, just visually, how quick the voltage shoots straight up once it goes past 3.4v. It happens quick!


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## Ziggythewiz (May 16, 2010)

Yup, and to make it more fun there's vanilla LiFePO4, and the LiFeYPO4 and LiFeMnPO4 flavors (are there more?) which would have some effect, but I haven't seen many comparisons. I'd love to see average curves of 100AH cells of each generated under the same conditions and plotted together, both for charge/discharge as is commonly done, but also using rest periods which I've only seen documented once.

Once I save up for a PLx I want to make a stepped charge curve to correlate resting voltage vs SOC every 5% and do some capacity tests to see how accurate that would be. 

It would be nice to have tons more data from the manufacturers. Especially with CALB recommending charging to 90% but only providing voltages to indicate full charge to too much discharge.


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## Jesse67 (May 12, 2009)

I found the same thing, only about 3.25Ah was going in between 3.5V and cutoff at 3.7V and in reality charging to 3.5 with a CV stage the difference would be even less. I'll be lowering my balancing voltage, no harm done, keeps them away from any chance at overcharging. I'm hard enough on the cells on discharge, hopefully with a cell friendly charge cycle I can keep them happy for a long time to come.


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