Testing small LTO batteries

[Dacian Todea]

The clue is that those are not supercapacitors even tho I use the board I designed for supercapacitors.

  

You can enlarge the image using the right mouse click and selecting open image in a new tab.

I added the wire stripper and ferrule crippler in the photo as some of you asked what I'm using. 

[Jim A]

32650 cells ??

[Kohala Jim]

It looks like standard radial aluminum electrolytic caps as the scoring on top indicates, however, caps just don't make sense
The IDC connection terminal is consistent with a 6S monitor/balance connection and there are 6 cells (or caps?)
Looks like some kind of small UPS or battery backup unit.
Pure, speculative conjecture...

[Bernd]

Hello Dacian,

thanks for sharing this competition.

Here are my thougths. 

-These blue tanks are energy storages devices. I do not know their name, but this is the function. And we see, that the energy tank is huge.

-i love to see, that you found 2 new connector options for your boards, so the journey of "how-do-we-connect-all-this" will continue :-)

-the right side connects DC  cables.

-between the DC cables a switch could be in place, let´s call it a relay

-it is designed for 8s only (or simulates a 8s config)

-the 8 grey wires are cell connectors, as they cannot (directly) connecto to sbms0. I think these will go to a battery bank.

-the 5 black screw connectors remind me on your EXT16 boards... I asume the SBMS0 can control via IO ports something here.

-i first read "ON" and "+)" and the connectors, but i think it is "GROUND" and "BAT (+)"....

At least one of my thougths is wrong... But which one...

After thinking on this maybe for one hour (which i really enjoyed) here is my guess:

It is a device, that turns on or off another device with high energy needs. It is a ERBPS550 - Electrodacus Relay Battery Protect Switch.as it can handle up to 550A.

[Dacian Todea]

Thanks for playing :)  as you seen maybe from edited title they are LTO (Lithium-titanate, Li4Ti5O12) cells. 
They are installed on the 8s PCB's I made some long time ago for Supercapacitors and so in the extra two places I just installed jumper wires 16AWG solid wires as this is just a 6s setup thus it looks like all cell sense wires are used but they are shorted on the board.

This is the Aliexpress add I purchased this from https://www.aliexpress.com/item/4000372981333.html  so 59.7CAD for 6pcs and you can see the spec there 23mm diameter and 68mm long and they have about 63g (real weighted by me).

The internal impedance is for all cells around 11.3mOhm (at 1kHz) while DC internal resistance is about 2x that around 22mOhm.

I wanted to get this for some years now to test so I can have better default parameters for LTO (battery type 3) in my SBMS as the ones there are firly conservative with cut-out at 2.6V but I will likely change that to 2.7V

Still for 12V systems 5s will have a bit to low of a voltage and 6s is a bit to high but you need to chose 6s will work better and if you need something like this for a 18V power tool then 8s seems to be a good choice.

In any case I charged them to 2.8V (as per spec) but it was constant current charging only so I stopped there did not also do a constant voltage until current dropped as I did not wanted to stress this cells.

In any case here are the results. The data was collected manually as I just did not had the time to setup any automated log but resolution is good enough to get an idea of how this cells work.

On the charge side there is no clear cut off place like there is with LiFePO4 where it is clear going above 3.5V makes no sense as battery can not accept any more charge here it seems it will continue to accept charge even after the already high limit of 2.8V.   On the discharge end it is as clear as with LiFePO4 as you can see below 2.2V the voltage drops very fast showing that there is nothing else available so a 2V cut out make sense for this same as 2.8V for LiFePO4.

 

Below a few photos of the SBMS0 screen during charge and discharge.

Just before starting to charge

Just after starting to charge at 1A constant current that is about 0.4C for this battery.

End of charging (the default limits where lower so I forced changing manually to 2.8V)

Just after I started discharging.

Around middle of discharge. Ignore the SOC as I needed to restart the SBMS0 due to force charging to 2.8V and exceeding the overcharge lock limit.

The end of discharge and since discharge was done at constant power (DC-DC converter powering an LED) and the voltage dropped significantly the current increased.

Just a few minutes after discharge ended the voltage recovered above 2.2V but it will drop back below 2V in just a few seconds if I connect the load.

Battery capacity (sorry for the blur image). 2364mAh (close enough to spec 2500mAh) and 33.5Wh     33500mWh/2364mAh = 14.17V / 6s = 2.36V nominal per cell again close enough to claimed 2.4V 

Discharge current curve notice the fast increase of current at the end as voltage dropped rapidly.

The power curve fairly constant at around 17.2W  so around 0.5C discharge rate.

Some final conclusions:

LTO is great but just to expensive in therms of cost amortization when compared to LiFePO4 as a typical 40Ah 2.3V LTO is around 40CAD  while a 200Ah 3.2V CATL can be had for around 75CAD  (none of this prices include shipping or import taxes) the 310Ah CATL is around 115CAD all average prices from Alibaba.

So 40Ah x 2.3V = 92Wh   thus 1000/92 = 10.87 x 40CAD = 435CAD/kWh

And LiFePO4 310Ah x 3.2V = 992Wh basically 1kWh thus  115CAD/kWh

Shipping for same capacity will be lower for LiFePO4 as it will be 2x lighter than LTO

So LTO is 3 to 4x more expensive for same capacity but is unlikely will be able to last that much longer as good LiFePO4 should last at minimum 10 to 15 years thus that will require LTO to last 40 to 60 years to be even and even so paying 3 to 4x the price for same capacity in advanced is not a good strategy even if the battery where to last 40+ years.

[Oberon Robinson]

Actually LTO was my first thought when I saw your initial post, but all the other LTO cells I've seen had a terminal at each end, so I got confused!  I did briefly consider LTO for my van build, their one big advantage is low-temperature charging, which for a van that could easily see -30°C during its travels, would be a big benefit.  But ultimately for me it wasn't worth the price - it will be interesting to see if their pricing comes down significantly over the next few years.