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LTO-Titanate, recent flood of cells on EBAY
#11
"I know which BMS and balance chargers work best with them"

...I'm intrigued, please continue...links?
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#12
This one for up to 24 or 32 cells.
https://www.aliexpress.com/item/Free-shi...0.0.yNbasb

I also have Chargery 6s and 10s dc balance chargers with lto fw.
 Chargerys  newest ac/dc -chargerI  is 12s balance 40a 1500w.
Available in next few weeks.
About $3**. Though

Finding more but not tested yet.
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#13
(02-24-2018, 03:21 PM)spinningmagnets Wrote: The last time I checked (a few months ago), LTO cells were rare, whether new or used. Today, there are quite a few selections. LTO chemistry (Lithium Titanate Oxide) is exceptional due to its rated number of cycles, typically in the 7,000 range. This is not a typo, common 3.7V Li-NCA/Li-NCR in our beloved 18650 formats, are sometimes rated as 1,000 cycles if cared for and you don't charge above 4.1V per cell. The 3.2V nominal LiFePO4 is sometimes rated at a couple thousand cycles.

Best info I can find says LTO is a 2.4V nominal cell, with 1.8-LVC and a full charge of 2.85V (I just googled a discharge graph, and it appears that there is a long and flat middle, with no usable range under 2.2V, or above 2.6V)

If making a 12V pack, five cells in series (5S) would be 12.0V on the nose...11.0V LVC, and a full charge of 14.25V

The full charge number intrigues me, since a typical car alternator produces 13.8V (2.76V per cell), so charging a "suitcase" pack from your cars electrical system would be a great option. I'm fairly certain if you used only five large cells (none in parallel), you could get by safely with no BMS. The thermal runaway characteristics (fire safety) is about the same as LiFePO4, much better than average...energy density is about half of NMC, so...twice the battery volume required per the same Amp-hours of range.

7,000 cycles...dayum! (some claim 14,000?)

edit: LTO seems to perform well when cold, can go to zero volts with no damage, and are known for high charge and discharge rates (30C is common in ads).

The battery pack that I am planning to build will be 11s and in 24V modules because at 12V the voltage range is just a bit too high or too low for use with anything that expects a standard lead brick input range. I am planning to actually have a 22s (48V) module.

The cut off is quoted at 1.5V but that is scraping the bottom of the barrel and more useable cut-off would be closer to 1.9V or higher at low C (this is if you are not pulling more than 1C) and depends on the actual LTO cell as the lower cut-off profile varies at 1C and higher.

2.7V-2.85V is typically the maximum beofre damaging the cells and 5x2.85 gives you 14.25V so above the standard 14.1V charge on a typical lead brick.

1.9V x 5 gives 9.5V, well below the nominal cut off for a lead brick, and if you are expecting to discharge at more than 1C the voltage will not hold up for 5 cells beyond around 85% capacity, depending on which cell you are using.

Changing to 6 cells pushes the maximum charge voltage requirement above 14.1V (2.35V) but would provide better hold up.

Charging to 2.7/2.85V may only get an extra 5%.... and your risking additional max charge heat stresses

When increasing up I found that 11s seems to work best (in thoery at this point) and will know in a couple of months when I get 300 cells delivered..

There are lots of LTO cells on eB and Ali and some of them are actually used and re-cycled, touted as new. All of them are well above the actual market price....
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#14
I wouldn't rule out 6s completely, depending on your 12V equipment. Overvoltage cutoff is often around 15.5V and undervoltage cutoff usually under 11V and above 10V. 6s LTO charged to 2.55V, discharged to 2V, 12.0V to 15.3V, is almost perfect.
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#15
I'll definitely be building a second pack with 4S LiFePO4, but...

For the 6S LTO I now have, I very much like that I can get "some" charge from a 12V cars utility socket (typically a 13.8V alternator), but...when charging with solar or a wall-socket based charger, the top voltage should be very adjustable to get max Amp-hours out of it when a long run-time is needed...
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#16
40Ah LTO cell 1C discharge rate (will be different for different cells)
Starting voltage 2.7V

Drops to 2.4V with only 0.81% of the capacity drawn - very quick drop at the top
With LTO the top few % of capaicty going to 2.7V charge is quite small.

1.9V cut off at 1C will have discharged 86% of the cell.

1.85V cut off will have then drawn 94.6% of the charge

Dropping to 1.5V is scrapoing the barrel and dropping a lot of V in series.
iomagico likes this post
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#17
Here's a charge curve for visual thinkers like myself:

[Image: LTO-DisCharge-Rates.gif]

Useful range is from 2.6V to 2.2V for this example.
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#18

 

Factory modules are 10s for 24v.
I use 6s for automotive starting battery and 11s or 12s for 24v.
32s for 72v.
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#19
(03-08-2018, 03:22 PM)rev0 Wrote: Here's a charge curve for visual thinkers like myself:

[Image: LTO-DisCharge-Rates.gif]

Useful range is from 2.6V to 2.2V for this example.

This is the 1C profile for the 40Ah cell I quoted the figures for.
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#20
What about self-discharge ?
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