18650 Cell choice: Max Constant Current 1/12th of Capacity?

[Sunder]

Lithium titiant battery 2.4V 30Ah LTO cell.
Model Number: OSN-LTO-66160
Each - 1.5-2.7 V
23 cell:
Bank - 34.5-62.1

These might be a good option.
How do we recalculate capacity based on 42.5v Minimum?

Capacity is estimated at nominal voltage, which is 2.4v

23 cells x 2.4v x 30Ah = 1656kwh for a single string.

Most houses with 2 adults and 2 kids with fully electric everything (hot water + cooking + heating) including air conditioning uses between 12kwh and 16kwh a day. Obviously, your mileage may vary. I've read of some couple with no kids energy efficient homes using less than 2kwh a day - solar boosted hot water, designed well for heating and cooling etc.

Go 23S10P would give you over 16kwh.

That's not the whole story though - some of your power is used during the day when you can pull it straight off the panels. On the flip side, you might need more than one day's worth if you expect a week worth of storms. My personal thought is that unless you have very high drain devices, just get a cheap 2kw Edit:generator, and run it when you have a string of cloudy days.

Don't hold your breath though. Shipping a kilo from China to Aust is about $20-25USD. So even if they hold to the $20 quote, which they won't, you can easily double the price quoted there.

 

[Gazza]

Don't believe everything on the web.

I don't believe anything on a good day, so its there to pick at.
They are just copy and pasted numbers and the seller has many products 1 to 50.
Translating all the figure manipulation, like pulling teeth.
It wastes a lot of time to extract answers, for maybe a good seller in 10.
Hope the effort pays off?

Let us know what you think if you see any specs that are known to be wrong.

 

[Gazza]

Capacity is estimated at nominal voltage, which is 2.4v

Yes, My question was regarding the voltage envelope (Min-Max) that the rated Ah Capacity is extracted.

With my application the charge voltage should not exceed 60v.
This means the Capacity must be drawn from below this voltage.

I know this might not be the same for another user!
The voltage below 45v becomes inefficient in energy transfer and load.
So there are compromises that I need to calculate in.
The specs could be very similar, but the shortage of specifics has me questioning.

Lithium titiant battery 2.4V 30Ah LTO cell.
Model Number: OSN-LTO-66160
Nominal 2.4v
End of charge Voltage 2.7 V
23 cells = 62.1v
90Ah Bank- 3x23cells = 69cells?

Edit: I have looked more closely at specifications for LTO.
The Voltage envelope is wider then a good Lifepo4.
You need to use down to 2v end of discharge for Capacity, with is not possible with bank requirement of 42.5v to 60v discharge/Peak Charge voltage.
The main feature of LTO is cycle life, but if a Lifepo4 lasts 8-10 years it is difficult to assess. Perhaps either could just die of old age.
This Capacity loss could be off-set to degree with lower Max discharge requirement, although I have not seen this in any specs yet?

 

[Sunder]

Yes, My question was regarding the voltage envelope (Min-Max) that the rated Ah Capacity is extracted.

With my application the charge voltage should not exceed 60v.
This means the Capacity must be drawn from below this voltage.

60v / 23 = 2.6v per cell. There is less than 3% between 2.6v and 2.7v. Almost all the power is between 2.5v and 2.0v You can see it in the sudden drop off of voltage in this discharge graph:

 

[Gazza]

I see,

I was editing above as you posted, so I need to do some math with this info!
24 Cells
How would the cell performance be with 2.5V End of charge Voltage?

Cheers

 

[Sunder]

60v / 23 = 2.6v per cell. There is less than 3% between 2.6v and 2.7v. Almost all the power is between 2.5v and 2.0v You can see it in the sudden drop off of voltage in this discharge graph:

I see,

I was editing above as you posted, so I need to do some math with this info!
24 Cells
How would the cell performance be with 2.5V End of charge Voltage?

Cheers

Well, if you read that graph, in theory, you should get about 95% of capacity. In reality, you'd get less, due to the resistance between what is read at the battery, and what is read at the charger. At the charger, you might be on 2.50v per cell. On the battery, you might be 2.45v per cell - and some of that might be temporarily elevated from charging, so you might find if you charged to 2.50v, after an hour of resting, they might settle to 2.40v or lower.

It might be only another 5%, but you're being nickled and dimed to death, to borrow a US aphorism.

 

[Gazza]

Yes, 60v / 23 cells @2.6v seems like a better starting point.

 

[Sunder]

This is the cheapest I could find on Alibaba "Wholesaler" (Like an eBay Buy It Now price - you can place an order and it's not up for negotiation)

http://wholesaler.alibaba.com/product-detail/Long-Life-batteries-2-4v-26Ah_60469829477.html

It's $64USD per cell + $41.7 shipping (Also per cell, no volume discounts).

I'm sure if you got in contact with the seller, they might be able to arrange a cheaper economy shipping. For example, using EMS instead of UPS is about 1/3rd of the cost, but EMS is not listed there.

 

[Gazza]

Morning,

Large 3.2v prismatic lifepo4 cells are the best value I have found so far, for my application of 48v 100Ah.

Made price inquiry for 96x pieces, Lithium titiant battery 2.4V 30Ah LTO cell.
Seems very unlikely this cell is available at the price shown, a contender if it is.

Here is a smaller lifepo4 cell: 26650 3.2v 3ah 3.2ah lifepo4 cell
https://ewtbattery.en.alibaba.com/p...729633/26650_3_2v_3ah_3_2ah_lifepo4_cell.html
-

I will be using BMS regardless of the Battery Cell type, possibly building it myself if required. (Not Zenners) LOL.
I have found very little saving in cell costs, in options using more cells.
The additional component cost and complexity is a problem dealing with hundreds of cells.

There are people that believe it is cheaper to use small cells.
I am not interested in the arguments around balancing, the problem starts with the actual cell price.

After hundreds of quotes for small cells in numbers to make a 48v 100Ah Bank, the large prismatic is the same price or a lot cheaper.

I could possibly assemble from smaller cells cheaper, but it could end up expensive false economy.
Most of the cells in comparative price range of large prismatic, have low or very confusing Life Cycle.


Shipping Notes

Those from the US, just note I am in Australia. I closed my Fedx account, even mail out of US is over the top.
I buy specialised PCB boards from America through standard mail, which is good given a unique product,
but you only have to shop from Australia on Ebay to know the problem for general products out of US, most sellers know it is not viable.

In regards to china, I have purchased 40KG of "essentially metal pieces".
The company made up packages the maximum allowable through standard mail, and it was very viable.
I did have a small heart attack after finding it was all insured or costed at about $1.
Scary, but it all worked out. (Things could go wrong very badly dealing in china, the risk is up to individuals)

Open to offers or ideas.

Cheers.

 

[Sunder]

Random thought... At 1m, deep you roughly halve the temperature variation. So if your climate is like 15*C at night to 40*C in the day, your median temperature is 27.5*C, with 12.5*C either way. So at 1m deep, you should have a 6.25*C variation either way - never colder than 21.25*C and never hotter than 33.75*C (Hope that makes sense)

At 2m deep, it's about 1/4 of the variation, and at 4m deep, it usually doesn't vary much at all.

Have you considered using LiPo, but once the battery is stable, and doesn't need tinkering, put it in a water proof box, and bury it 1-2m below ground? I'm sure on a farm, you'd have the right tools to dig a hole quickly.

Just a random thought I had. Haven't looked into this too deeply. I'm sure there are things I haven't thought of with this.

 

[Gazza]

LiPo is still an option at the right price.
The first requirement is to do calculations on existing environment.

I have not read a lot of Lipo Data sheets to understand the characteristics,
lets consider the test environment where things are averaged to the real world.

If you extract Capacity during the day, all ion transfer is happening in the hottest part of 24 hour cycle.
In this case the charge capacity exceeds the discharge x5, so the day time charging is main factor.

The Battery Storage area is insulated with passive venting, temperature is 20 to 37C. (40C Max for safe tolerances).
So this is the existing environment.
--

Battery Cooling

I have been thinking about Cooling the environment with any spare money from the hypothetical saving.
The numbers would have to stack up extremely well, and using the Battery to power its own life support system is far from perfect.

I have also considered putting the Bank in a hole, raising and lowering is doable, but ventilating a hole will equalise the temperature.
Sealing a small deep hole to stablise temperature may end bad.
This needs to be turn key, but I would seriously consider small scale Lipo in a hole for a Backyard project.

Bit of common sense, it could cost a few dollars to try, not much to lose.
You would want a secondary container to ensure no soil contamination.

Small fridges are cheap and can be very efficient.
Solar Panels have been mounted on water, 30L can be cooled with about 100w.

None of which I am suggesting or discouraging, just science

 

[Sunder]

Did you ever get an answer about pricing? I have not.

 

[Gazza]

Did you ever get an answer about pricing? I have not.

-
I have offers of Prismatic Lifepo4 3.2V 100AH for US $90, negotiating for lower minimum order Numbers.
-
Still negotiating for price on Titanate Cells.
-
I have doubts Titanate will work for this application.
The Battery Connects Directly to DC Load.
Max Load Voltage: 60v (Above 60v life span is low)
Min Load Voltage: 40v (Bellow 40v Load is under powered)
-
27S x 2.25v each cell = 60.75v Charge Cut-off Voltage.
27S x 1.5v each cell = 40.8v Discharge Cut-off Voltage.
-
Here: I have modified a LTO discharge graph with required voltage envelope.

-
My question for anybody with experience using Titinate or perhaps even Lifepo4.
-
Q/ What Maximum Charge Voltage, required to maintain >2v per cell for discharge.
Conditions: The cell is first charged @ specified C rating, before waiting 24hrs.
-
The specified Capacity is not going to be realised, the question is if charging such battery to 2.25v would even give 25Ah.
For all the experts, what Minimum Charge Voltage would give around 25Ah?
Is it even possible with such Voltage envelope limitation?
-
Gazza

 

[Gazza]

I'm using A123 20ah. Love the large format. 983 cycles . Heavy for a bike great for solar. They are very expensive now. But love large ah cell and lifepo4 being long life. Just find one double the c-rate you need.

Hi Zip,
The seller loves to extract some obscure c-rate requirement from you to fiddle the specs.
My approach is to ask for Tested c-rate for capacity of the cell, before claiming it to be also my c-rate.
-
I double the applications required c-rate for my own calculations for good tolerance.
-
Questions:
What has been your experience?
Do you refer just to discharge c-rate or charging as well?
Do you double the c-rate just for good tolerance or do you have more detailed reasons to offer?
-
Thanks,
Gazza

 

[Sunder]

The seller just responded to me with $22USD for a 30Ah battery + sea freight. So I have tentatively accepted the order pending the sea freight quote.

When I get them, I intend to do:

1 Capacity Test
2 ACIR Test
3 DC IR Test - estimated by applying known load across multiple load points
4 Controlled over-charge/over temp destructive tests.

If they all turn out good, I'm going to order about 200 units for an off grid solution. If not, it was always a $220USD + Shipping gamble.

 

[Sunder]

I have doubts Titanate will work for this application.
The Battery Connects Directly to DC Load.
Max Load Voltage: 60v (Above 60v life span is low)
Min Load Voltage: 40v (Bellow 40v Load is under powered)
-
27S x 2.25v each cell = 60.75v Charge Cut-off Voltage.
27S x 1.5v each cell = 40.8v Discharge Cut-off Voltage.

Not sure what you're trying to calculate with those numbers above.

60v/2.75 = 21.8, round up to 22S as the base of your battery.

Reversing that, 60v / 22 cells = top charge of 2.72v per cell - pretty much perfectly the recommended voltage 0.03v is going to have virtually no difference to capacity.

40v / 22 = 1.8v per cell. well above the low voltage tolerance of LTO, but getting 98% of the capacity.

A 22 cell battery pack would be perfect for a 40-60v operating range.

 

[Gazza]

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[Gazza]

Lithium Titanate Cells (LTO)
I am interested to know if or when someone Tests Remaining Capacity for Partial State of Charge.

It has been done here with Lifepo4 http://lygte-info.dk/info/BatteryChargePercent UK.html

The End off Discharge voltage can be lower for LTO and the voltage envelope is narrower.

When you try to extrapolate findings here it does not work for LTO: Increasing the cycle life of lithium ion cells by partial state of charge cycling
https://www.researchgate.net/public..._ion_cells_by_partial_state_of_charge_cycling

I extrapolated as percentage of the Voltage Envelope Width, LTO from Lifepo4 cell data and where Lifepo4 would have 60% Capacity, the same percentage of voltage envelope applied to LTO and the capacity is zero.

Lithium iron phosphate the LiFePO4 LiB cell (abbreviated as C/LFP)

Li4Ti5O12 LiB cell which uses lithium titanate (Li4Ti5O12) as its negative electrode and Li[NiCoMn]O2 as its positive electrode (abbreviated as LTO/NCM).

There is also the same, approximately 1v drop between End of Charge Voltage and Voltage at Rated Capacity, although the LTO has narrower voltage envelope.

The other factor is the Lifepo4 Discharge Graph has a knee at start and end which is not seen in the commercial LTO used in study.

You can see the OSN LTO has different properties again below

 

[Sunder]

You've lost me on this one.

You can extend the life of lithum cobalt based cells (basically anything with a 4.1 or 4.2v per cell full voltage) by under charging or leaving a reserve, which is why many cell manufacturers list their cycle life as 80% depth of discharge.

A much smaller effect also occurs with LiFePo4 - so much smaller that nobody I know of undercharges it. For LiFePo4, life extension is mostly about keeping the C rate low. This isn't even to preserve capacity, this is to preserve power density.

LTO is a much lower voltage and lower volatility chemistry, and suffers from neither of thes problems, and therefore, does not benefit from trying to fix a problem that doesn't exist. LTO can usually be completely discharged for tens of thousands of cycles, and can pull multi-C rate loads without damage. Some can pull over 280C safely.

The trade of of course is more cells in series, more weight and more volume, which makes them unsuitable for mobile devices and motor transport. This means all their applications are relatively low volume, and expensive.

If LTO is not for you, then making compromises to extend the life of other chemistries is not only fine, it's industry practise. I'm just laying it out for you as I see it. If you want another opinion, there were a few here who experimented with LTO years ago when it was new, but as almost everyone here is an eBike or eMotorcycle enthusiast, it hasn't suited our applications due to weight and volume, so there are not many. If you search Toshiba SCIB on this site, you might find out who has played with these in the past.

 

[Sunder]

Turns out they will only hold to the price if I deal outside of Alibaba. They'll only do bank transfer or western union, no paypal. So I'm 99.99% sure it's a scam.

 

[Gazza]

60v / 22 cells = top charge of 2.72v per cell - pretty much perfectly the recommended voltage 0.03v is going to have virtually no difference to capacity.
40v / 22 = 1.8v per cell. well above the low voltage tolerance of LTO, but getting 98% of the capacity.
A 22 cell battery pack would be perfect for a 40-60v operating range.

I lost you with this, because you are approaching it as a fact check. (I aim to be constructive!)

Look at the Blue Graph above for OSN LTO, insert your suggestion of 1.8v to 2.72v and estimate your Capacity.

I have done a lot of work here.
I do not claim any Battery is better or worse until fully testing.

When learning from customers on the internet, virtually all users scream high c-rate and or capacity per size/kg as the only real benchmark of interest.
PSoC is for what ever the user of a particular cell needs it for, defining as life cycle is just a customer choosing their interest.

No one is interested in using lower capacity then that stated, other then through increased C-rate.
It is psychological nonsense.

If you can extract your voltage envelope from any cell type, and the remaining Capacity is cost effective per cell? This is my interest.
I say if, in the context of testing the cells.

You can see this can be done with Lifepo4, I do not say it will or will not work for me using LTO!
If the Seller provides me some samples I will find out.

I understand the point you try and make, but the only fact is what I find after testing for my specific application and need.

 

[Gazza]

Voltage Envelope

Looking at a hand full of different discharge graphs,
it would be simplistic to believe PSoC findings can be transferred across cells of the same specification.
Discharge graphs with knee at ends, to strait linear lines, play a bigger roll.

Even without seeing practical results, it would not seem impossible to start with a bad Cell 40% under the stated Specified Data.
Apply Papers finding for 60% PSoC voltage, and end with zero capacity. Through the Manufacture dodge specs, cell discharge curve or both.

This is for Lifepo4 cells, but the effects for Commercial LTO with linear discharge can be seen previous post.

Just a consideration that may, or may not be correct.

 

[Gazza]

by LockH » Wed Dec 28, 2016 9:19 pm

This would seem to be playing out as shown, based on many Quotes for 18650 and Prismatic Cells.
The better value being 3.2v Prismatic Lifepo4 35Ah, 50Ah and 100Ah.
Available for < US $1 if you are looking at > 100 pieces on Alibaba.
Also of similar price are some cylindrical lifepo4 with Bolt or Screw.

The many quotes for Tesla size cells worked out more expensive, this before you decide how to construct.
I cannot see any quality advantage worthy to sway from 3.2v Lifepo4 Prismatic 50Ah, unless you need greater then .2C.
Even then, there are many Prismatic rated at higher c-rate for larger load.

Not an argument, more a conclusion for my own application of .2C Discharge and .5C charge.

 

[Sunder]

I lost you with this, because you are approaching it as a fact check. (I aim to be constructive!)

Look at the Blue Graph above for OSN LTO, insert your suggestion of 1.8v to 2.72v and estimate your Capacity.

Ah, think I got you now.

But before I reply to your question, have a look at this graph and tell me what capacity you'd get if you stopped the discharge at 2.0v (for simplicities sake)

Your first question is going to be "Which C rate" right? At 50C, it could be as little as 20%. At 1C, it could be as much as 90% - from the same cell. Now look at your discharge curve - you have to also ask "Which C rate" of which none is listed. That graph, on its own, is not of much use, because you don't know how the test was conducted.

The graph that OSN provided looks like it's a battery under stress. It didn't start at the full 2.75v, and it appears to be a graph that is "shifted down". That says to me either:

1. The C rate used for the was quite high; or
2. The cells are quite poor quality, and sag too much under a reasonable C rate.

Unfortunately, I think for the price I paid, it's 2. This is especially true because if they could advertise say, 35Ah by lowering the test C rate, they probably would.

But since I am expecting to use them for very low current draws, it might not matter so much to me, as it does to others. If you could be as kind as to ask them what C rate the graph was generated in, I'd be in your debt. I'd ask myself, but the graph doesn't indicate it's their cell, and I'm dealing with sales staff only, not techs. I don't want to confuse them while they're still processing my order. Her English is not fantastic.

 

[Sunder]

Going back to a much earlier topic on this thread, see in the graphs below the rate of loss by temp and SOC and chemistry.