Lithium Titanate Battery

[serialk11r]

If you have heard of these, they're the most durable lithium chemistry, and they're starting to become commercially available at a decent price. The benefit is that they work down to -40C just like lead acid, and have a cycle life of 20000 cycles.

So I took a bit of a gamble and ordered one of these off a seller on Aliexpress known for sketchy products (grade B, used, defective cells). Kind of like this, but 5S (12V nominal), and I requested that they fit a 200A BMS to it. I measured the weight at 15.6lbs, right at their advertised 7kg.

https://www.aliexpress.com/item/4000906602346.html

It looks exactly like that case, and the case is honestly quite nice. It's sturdy and smooth, seems like a quality plastic molding.

Inside, well, maybe not so much, but I wasn't expecting much given how inexpensive it was. The battery slid around a little bit inside the case. The lid was only glued on at 2 of the 4 edges, so it was conspicuously sticking up in one corner. The voltage meter at the top is completely inaccurate. The charger puts out 14V, but the BMS seems to cut off charging at 13.3V, which is kind of weird. The customer support was pretty useless, they told me to not worry about it sliding around inside...[facepalm].

Okay to the disassembly:
Lid removed, whole assembly taken out of the case


Those are doubled up 8AWG wires going to some meaty copper lugs, which should do the job just fine. The resistance in any car's existing wiring loom is probably higher. The rest of the pack is shrink wrapped together with the BMS, which is dumb but not a problem for an automotive starter battery application, where the battery is only working hard for a few seconds at a time.

Shrink wrap cut open


A bunch of pouch cells taped together with fiberglass reinforced tape, then two layers of shrink wrap. I didn't get a look at the terminals but they're probably soldered to the wires, and then some foam was packed around. Not the sturdiest construction, but...I guess it will do. We can see the heat sink for the BMS is separated from the pouches by a PCB, but there is no airflow going to it, so this is definitely not going to work as an EV battery under high duty cycle.

Crappy closeup of the BMS

It's marked "car starter BMS" on the PCB, and has a ridiculous number of MOSFETs. I guess that is pretty reassuring, I'm not going to question the 200A continuous rating.

Crude capacity test to come tomorrow. My only hall effect ammeter is a high current one, so the resolution is very poor (only within 1A ish), but I will try to attach it to a car and turn on the headlights and ignition for a ~30A load. Right now it's sitting a little above 13V, aka full. I'll measure the load current and check the voltage at 30 minutes to get an approximate capacity by referencing the standard voltage vs. SOC chart.

[Stubby79]

Interesting; appreciate the tear-down and feedback on things like this!

[19bonestock88]

Does the LTO chemistry offer any further benefits than operating temperature range? Energy density? Charge/discharge rate?

[Stubby79]

Voltage: 2.4v nominal.
Charge/discharge cycles: up to 10,000.
Charge efficiency: >95%
Discharge efficiency: 85%
Specific energy: 0–80Wh/kg
Temperature range: -30 to 55°C

Benefits & drawbacks:
Low energy density.
Inefficient discharge.
High charge/discharge rate.
One of the safest lithium chemistries.
Wide temperature range.
Ridiculously long cycle life.

YMMV.

[serialk11r]

One correction to the above: The discharge efficiency of 85% that's widely cited is wrong, it's the same as other lithium chemistries. However, because it has high cycle endurance and low internal resistance, you can cycle it at extremely high current (10C continuous charging! That's a 6 minute full charge) where the efficiency is below 90%, while other chemistries are measured at low current.

The only true drawback it has is that the energy density is poor. You have only 70% of the voltage of LFP per cell, so it's going to be heavier.

Anyways, I just hooked it up to the car and turned on all the electrical loads, which I measured to be 54A with my crappy ammeter. Ran it for just about 20 minutes and...the battery is almost dead?

Here's the weird thing...when I disconnected the battery from the car, it sat at 11.20V, and now it's up to 11.30V and slowly climbing. This may be balance current. However, these are supposed to be doubled up 20Ah cells in parallel, so 18Ah should not be draining any cell anywhere near empty. Maybe the cells themselves are "recovering" from being loaded up, but AFAIK they're not supposed to do that.

I suppose I shouldn't have expected much given GTK sells 10 20Ah cells for 220 bucks, while Yinlong 40Ah cylindrical cells are 2x that price lol. I guess we'll see if the voltage miraculously recovers to 12V. If it really is only 20Ah capacity, then this battery is kind of junk because it weighs almost as much as a 20Ah lead acid -_-

[serialk11r]

Alright, it's not budging at 11.3, so these only have 20Ah capacity, unless my current measurement is completely wrong (I doubt it, 54A sounds completely reasonable for ignition + headlights + fans). I'm gonna ask for a partial refund, see if I can swap in some better cells or figure out which ones are bad.

I guess they're technically still more useful than a small lead acid since the internal resistance is lower, but this isn't very encouraging. I did hook it up to a dead Group 24 lead acid battery with jumper cables, and it managed to crank a 3.5L V6 engine without much trouble, so in a way the performance is quite impressive. I recall doing a similar exercise once with a dead 10Ah AGM battery hooked up to my fully charged 30Ah lead acid, and it really struggled to crank a 1.8L 4 cylinder.

EDIT:
Okay I just read this paper and decided I need to reserve judgment until I do another test. Apparently, some cells have a steeper voltage curve, and it's possible that if I run this battery down to 10.0V I can get nearly double the energy out. Still not the advertised 40Ah, but over 30Ah would not be bad for what is effectively a 300 dollar discount.

[freebeard]

'm gonna ask for a partial refund...

According to the pictures you cut it open.

[serialk11r]

Originally Posted by freebeard

According to the pictures you cut it open.

Best case, the battery works below 10.5V and nothing happens. The store doesn't know I took it apart (though I didn't touch any electrical connections, just the shrink wrap), and they might still be willing to refund me some money in exchange for not leaving a bad review if the battery poops out at 11V.

Worst case, I just lost 200 bucks and reuse the case with some less sketchy cells. I used contact cement to glue the lid back on so I should be able to pry it off and pull the adhesive out with no damage.

[Stubby79]

If you have any kind of AH or watt-meter, charge it, run it until the BMS kicks in and see what your total really is. 10v might not even be the floor for this chemistry for all I know.

[redpoint5]

Capacity at high C-rates is drastically lower than at lower (whatever the test criteria was) C-rates.

To determine if capacity matches advertised, we'd first need to find what the parameters of the advertised ratings were (temperature, C-rate) and then replicate those.