LTO battery management

[Scrappyjoe]

Hello,

I am considering using Toshiba LTO cells for my VW T3 Syncro hybrid project.

I am going to model my hybrid implementation on the GoWesty hybrid bus (see here - the relevant images are in the first 5 min ) , but I will be using a Zombieverter along with an outlander rear drive unit and inverter.

I want to use 152 20ah cells in series, to give me a small 6kwh, 350v pack that can deliver up to 60kw continuously, and to drive the electric motor via signals from the rear petrol engines engine load signals.

Anyhow, Lithium Titanium Oxide batteries are apparently very stable, and I was thinking of grouping the packs into 7 x 22s modules to give me 50v nominal, balancing them, and then running a BMS across the 7 modules.

Does anyone have any opinions or guidance on LTO cells? I’m expecting they won’t need serious cooling.

And, yes, I know, they are expensive and big and I could get a bigger lithium ion pack for the same price - but I am not on a country where this is an option.

[arber333]

Hm... what if you order a complete used Volt pack as a spare part? Would that qualify for customs in your country?

[Scrappyjoe]

Used vehicle parts are far harder to import than new ones, to protect local manufacturing. Used EV packs are extremely hard and expensive to import, because you are dealing with used car parts plus hazardous goods. I have only ever found one shipper willing to ship me used EV batteries, and the cost for 48kwh worth of modules was $2500, excluding import duties. If I was set on a lithium ion pack, it would be cheaper and easier to buy new modules on Alibaba.

Customs are 20% VAT + 15% tariff as far as I am aware.

[bigmotherwhale]

The energy density of LTO is quite low, and I imagine the quality will vary quite a bit, would it not be just as easy to get LFP? You could probably find whole modules designed for EV conversions on there.

[m.art.y]

A while ago I was very interested in LTO and was seriously researching them. It is a very stable chemistry that is able to deliver a lot of amps and they supposedly almost don't lose capacity in the freezing cold. Also they are advertised as able to handle some 20000 cycles although I am not sure if that is true. If you get them in good condition (not run down) they will definitely work but your range will be lower for the same weight as compared to Li-ion. I have heard of others using them with success. I myself eventually decided against them due to their high price. If your best place to buy batteries is China you may also consider LFP (which may work well in your climate as well) and Li-ion.

[Scrappyjoe]

I really like the stability of LFP and LTO - I take my bus to some pretty harsh environments; last month I was in Namibia and my water-cooled ICE engine had a hard time not overheating in 40 degree celsius heat day after day. What I am really looking for in a battery is something small, virtually unbreakable through human error (over discharge, over charge, temperature extremes, and with a high C rate.

An 8kwh pack that can deliver 80kw continuously would be spot on; I'll settle for 6kwh/60kw.

If I could find an LFP cell that can deliver that, I would seriously consider it. But, with no LFP that I can find able to deliver 8-10C, I'm left with Lithium Ion, and LTO. Sure, I can get high C-rate Lithium Ion, but then I have to acquire the skill to build a cooling system for it, and it'll have to be really well engineered, because most of us rely on big packs that stay well away from their peak discharge rates, rather than super fancy cooling, to avoid thermal runaway.

[bigmotherwhale]

with no LFP that I can find able to deliver 8-10C, I'm left with Lithium Ion, and LTO. Sure,

I am very surprised by that, LFP chemistries are well known for their high C rating, and you should be fine with the higher temps as long as your design is ok, they really struggle in low temperatures. Most LFP batteries have operating temps above 60c that I have seen.
The good thing about LFP is that its a tried and tested method, many conversions have used them successfully.

I know a good Chinese supplier of LFP batteries try Jenny Wu, jennywu896@gmail.com

[bexander]

LFP cell with high C rating.
https://www.evlithium.com/Headway_Battery/898.html

[Scrappyjoe]

bexander, the energy density of those headway cells is 77Wh/kg, which is less than the energy density of the LTO cells at 89Wh/kg. Granted, the headway cells are cheaper but they also have less rated cycles. The headway cells are also cylindrical, and therefore quite a bit harder to pack efficiently.

bigmotherwhale, thanks for the contact - I've seen her recommended on the diysolarforum as well. While I have heard it mentioned occasionally that LFP have high C ratings, most data sheets I have seen (for, say, EVE LF230, or CALB L135F72, but also many others that are typically sold here for solar applications) have either 1C or (rarely) 2C ratings. The CALB L135F72 is in fact the only cell I have been able to find from a reputable manufacturer that has a 2C or greater rating. Without wanting to put you on the spot, could you please explain to me how I could validate the claim that LFP have high C rates? It's not a snarky question, I accept that there may be some subtlety around the data sheets that I do not understand.

[bigmotherwhale]

You explained it pretty well yourself, the LFP batteries that are sold with low C are designed for solar, generally LFP have high C rating compared with Li-Ion especially for charging, I may be wrong but i thought this was common knowledge?

What LTO cells are you comparing the C rate with? the Toshiba SCIB cells are only 5C rated for their 25ah high power version.

I have to say it would be amazing how long these LTO cells would last potentially decades, the thing that puts me off using them more than anything is the sheer number you would need due to their low voltage, to get 300v you would need 125 of them compared to only 90 LFP cells or 81 NMC

Due to the cost and safety aspect as well as recent developments in capacity there will be alot more EV packs containing LFP cells in the near future im sure some of these will trickle down to AliExpress and become available.

[Scrappyjoe]

My original reply:

This listing states 8C for the 20ah Toshiba SCIB cell. - https://batteryhookup.com/products/2-3v ... matic-cell
The quote I got from Alibaba said 10C for the same cell, which I'd take with a pinch of salt.

I agree there could be a bit of sample bias in the LFP cells I am looking at. But when I search for instance EVLithium or googling around, the most common LFP cells sold on EV conversion shop sites are the EVE LF230 or CALBs with 1C or 2C ratings, so I figured if _EV shops_ can't find higher discharge rate cells they use not exist.

After a bit more googling

That said, while I was googling around to give 'evidence' to my reply, I discovered that EVWest have very recently added an LFP cell to their lineup, and it looks to be exactly what you suggested - an ~8C LFP cell!

So what I am taking from this is that your basic theory is correct, i.e LFP high discharge cells have been very hard to find, but as they are being more frequently used in EV applications, they are starting to trickle through to the open market slowly.

[bigmotherwhale]

as a rule of thumb as the cells get smaller the C rating gets higher and vice versa, with the slight exception of the new tesla cells which have different cell termination, this is why I think you see such low C rating for LFP chemistry because they are designed for energy storage, if you had a 280ah cell with a 10c rating thats 2800 amps! obviously not practical, If you look at the LFP hobby cells which i have been using as a ICE starter battery for the past 10 years they are rated at 30C, There are no chemistry differences just the amount of conductor used inside. What im trying to say is its not the chemistry its the design.
Those EVwest lifepo4 cells are very cool, i wish i had a pack of them for home storage.
Don't let me put you off using the LTO they certainly have their benefit especially in low temperatures, i would be surprised to see if they are economical for you as they are not used all that much.

[m.art.y]

Also why is the C rating that important here? If you have a smaller 20ah cell that can deliver 5C or if you have 100ah cell that can deliver 1C the end result is the same. I would recommend using at least 2P configuration in your battery design. That way you will get higher C ratings, higher charging power and won't stress your cells as much.

[Scrappyjoe]

M.art.y, the intent is to place the motor on the front diff of a VW T3 Syncro, and retain the ICE in the rear. Because of this, I don’t need a huge amount of energy storage. The motor will assist the ICE when torque demands are high. I want to keep weight low - let’s say 100kg for the pack, because. But I need to be able to produce 60kw+. So this naturally leads me to look at high discharge rate, smallish cells.

I’d also like very stable chemistry, because

[Scrappyjoe]

Thanks for the insight bigmotherwhale, it clears up the confusion for me somewhat.

To bring this back on topic (pity the poor person who reads this thread in the future hoping for an answer to the original question) are there any specific considerations one has to make when using LTO cells in an EV application?

It seems to me that they can be fully discharged without damage. What happens to them when over charged? Do they need cooling if run hard? Should they be packed in the same way as, say, LFP, whereby one wants to pack them tight to prevent swelling? Id be more inclined to skip a BMS with LFP than lithium ion. Is LTO also likely to be more ‘forgiving’ of designs that don’t use an BMS? Etc.

[m.art.y]

LTO is supposedly the most forgiving chemistry out there. And it can offer huge discharge rates. But it depends on what condition you can get them. As they also can be run down from what I have seen. If you would need cooling it would depend on how you'll use them. Since you are looking to make a hybrid and only have a small pack it suggests you will only use the electric motor occassionally when the terrain demands that - so several times during a said trip. If so then I think you can get away without actively cooling the battery. From what I have seen battery cells only produce heat when discharged at higher rates or when continually charged at higher rates. In my car during normal driving the amp draw goes up and down and I can't observe any noticeable heat generation.
When it comes to BMS you can get away without it (I have driven my car for about a year without one) but you have to have a way to check the cells manually from time to time. Also it is a bit more difficult to find a BMS for LTO - although I have seen options on aliexpress.
You can also get cylindrical Yinlong LTO cells. But only in used condition as they are not generally produced to be sold to general public (made for Chinese buses). I don't think you would need compression with either prismatic or cylindrical LTOs but cylindrical cools down better/don't heat up as much. When I was searching I found an alternative Chinese manufacturer that sells cylindrical LTO cells new.

[bigmotherwhale]

It makes sense now, you are building a hybrid so you want the highest C rate for both charging and discharging
You will always need a BMS, you cannot skip this part no matter what anyone says, its asking for problems.
There are a lot of hybrid packs out there, they are really cheap because no one knows what to do with them, they don't suit energy storage because of their low capacity and they don't fail enough in vehicles to be sought after for that reason. I have a pack from a Kia Niro for this purpose (mild hybrid) it only cost me a few hundred £ and is capable of 30kw ish even though its only 1.6kwh 240v, my original plan was to use a 48v system with A123 nanophosphate cells. I also have a pack from a later model Prius which is also high discharge, have a look round your breakers, you might chance upon something you can use.

[Scrappyjoe]

Salvage prius is definitely doable here. I was researching this, but got the distinct impression that Prius cells are very hard to repurpose into DIY applications. Something about quite high self discharge rates, which get worse as they get older, so you end up playing whack a mole with cells going bad

https://www.diyelectriccar.com/threads/ ... -ev.45808/

[bigmotherwhale]

Thats the wrong type of Prius battery, you want 2015 onwards Li-ion, Using the nickel metal hydride cells is a pita as you rightly said.