LTO 18650 batteries with a capacity of 1500 mAh

Lithium-titanate batteries have been on sale for quite a while, but somehow I never got around to buying any. So I decided to fill that gap and order a couple to test and satisfy my curiosity.
An inspection, some tests, and a little bonus.

Lithium-titanate batteries are a type of lithium-ion battery, but they use lithium titanate (Li4Ti5O12) as the anode. They offer greater reliability and a longer service life, and can operate normally at low temperatures, which is what sets them apart from other cells.

They also have another distinctive feature: their voltage is 2.4 V, so they can replace two nickel-based batteries (1.2+1.2=2.4 V), though there are a few caveats when it comes to charging.
This is useful because a “regular” lithium-ion battery replaces three nickel-based batteries, so it can be used in devices that take 3, 6, 9, and so on cells. But it won’t work in devices that use 2 or 4 cells, and LiFePO4 batteries don’t help much either. For example, I have a good cordless drill that uses a 4.8 V battery: one 3.7 V cell isn’t enough, while two are too much.
Unfortunately, it’s not that simple. For example, I haven’t been able to find a protection board for 1–2 cells, and it also needs to handle more than 20 A.

Anyway, back to the batteries.
I ordered them from a Ukrainian store. They cost 68 UAH each ($2.4), and the pair came to $7.3 with shipping. The good news is that delivery was very fast: they shipped them in the evening, and they were at my house by the afternoon. But processing the online order took a long time, and I even had to call them.

They came in a bag, with the pair of batteries tightly wrapped in film inside. Overall, the packaging was fine.


They look like ordinary 18650 cells. The only thing that sets them apart is the LTO lettering in the marking. There’s also a PSR marking at the end, but since I’ve seen similar batteries online marked PSQ, I have a strong feeling this is a date code, with the last letter indicating the year.


Dimensions and weight: everything looks fine, except that one battery is lighter than the other.


A word about the specs. I couldn’t find a datasheet. The seller’s page has a specs table, but even a quick look raises some doubts, since both the nominal and maximum charging currents are listed as 5 A.


An online search turned up several similar batteries. The first one looks almost identical, except for the PSQ marking (which is why I thought it was a date code), but it’s not available for sale. The second ones are available, and their specs are similar, but they look different and cost noticeably more.

But in my opinion, the specs here are a little more modest, especially the discharge current: up to 7 A for the first one and up to 9 A for the second. The batteries I’m reviewing are rated for up to 15 A.


Before testing, I ran a “conditioning” cycle on both batteries: charge, discharge, then charge at 1C. Even at this stage, the first battery delivered just 1215 mAh, while the second did noticeably better, at 1413 mAh.


Since I wasn’t sure about the specs, I tested charging in two modes: 0.5C and 1C, where C = 1.5 A.
First, the results for the second battery. These look pretty good: 2 hours 5 minutes at 0.5C.


At 1C, charging took a little longer, but the result was still very good. I charged it after fully discharging it at 0.2C.


The first battery, however, did much worse. I only tested it at a charging current of 1C (1.5 A). It took only a little longer than the second one, but given its lower capacity, that means it charged more slowly. The CV phase dragged on, as you can see from the graph.


Measurements. I charged at 1.5 A up to 2.8 V, with charging stopped when the current dropped to 50 mA, and discharged down to 1.6 V.

First, the first sample, discharged at 0.2C, 0.5C, 1C, 2C, 5 A, and 7.5 A (5C). What can I say? The results are very poor. At 5C, the battery delivered just 950 mAh, and even at 0.2C it managed only a little over 1400 mAh, not to mention the severe voltage sag.


Temperature at discharge currents of 3, 5, and 7.5 A.


The second battery turned out to be much better than the first, so I expanded the test and checked it at 10 A and 15 A. I can say that at 10 A it works reasonably well, but at 15 A it is clearly overloaded. So it doesn't meet the 15 A rating stated in the description and is closer to the ones I found on AliExpress, which were rated at 7.5 and 9 A.


At 3, 5 and 7.5 A, the temperature was about 10 degrees lower than with the first unit.


At 10 and 15 A, of course, it was higher, but at least at 10 A it was perfectly reasonable.


A comparison graph of the first and second units at 0.2C, 1C and 7.5 A. You can clearly see how much the first unit falls short of the second.


Internal resistance as received, after "normalization" and after all the tests.
Here, too, you can see that the first unit has higher impedance. I also noticed that it depends on the voltage; this effect is much less pronounced in "regular" lithium-ion and LiFePO4 batteries.
The resistance also doesn't match the stated 16 mΩ and is closer to 20 mΩ, as found in batteries rated for a maximum current of 7.5 A.


And now for a little bonus.
Here's what Wikipedia says about lithium-titanate batteries:


For the test, I put the second battery in the freezer for a few hours, cooling it to about -15 degrees. The battery was fully charged beforehand, just as in the previous tests. It stayed in the freezer during the test and was additionally cooled.
The battery's impedance also jumped from 18 to almost 24 mΩ.


As an experiment, I first loaded the battery at 1C. I figured this would take less time than testing at lower values, but wouldn't be so high that the battery would start warming itself up.
The second test was at 7.5 A, and the battery voltage sagged so much that I started worrying the load might shut off at its lower limit. Fortunately, it couldn't maintain the set 7.5 A below 1.8 V and dropped the current to 6.6 A. The long wires made a difference here.
The voltage drop is clearly visible, followed by a noticeable rise as the battery warms up. If the load had maintained 7.5 A the whole time, the swings would have been even greater.

In the end, I got about 1150 mAh in both modes. In the first test, that was because the current was low; in the second, the battery warmed itself up. But in both cases, the difference is quite significant.


What's more, LTO batteries can not only be discharged at sub-zero temperatures, but charged as well. This can be critical in some cases, since "regular" batteries don't allow it.
For example, two datasheets I found for higher-capacity models specify operating temperatures down to -40 and even -50 degrees.


I was also curious to see how the battery would behave if I tried charging it at -15 degrees.
After discharging the battery at 1.5 A, I started charging it at the same current. It did charge, but took in 1050 mAh, even though 1150 mAh had been "drawn" from it. The graph was also extremely sluggish; toward the end, I thought it might not make it at all.

Once charging was complete, I took it out, warmed it up, put it in the charger I'd used earlier, and it accepted another 141 mAh. I then cooled it again and ran a test at 7.5 A.


Conclusions.
In terms of capacity, the first unit meets its stated 1500 mAh rating, but that may be the only spec it meets: its resistance was a little higher, and at 15 A it was overloaded, with a severe voltage sag. Based on the tests, I can say that its actual specs are somewhat more modest and closer to the batteries on AliExpress rated at 1500 mAh, 20 mΩ and up to 7.5 A.

The second unit really surprised me. It feels like these are two completely different batteries. I've mentioned before that a battery's discharge curve is like a person's fingerprint: each type is unique in its own way. As you can clearly see above, not only do their specs differ, but so do their curves, which points to different cells.

I also expected more from its performance at low temperatures. From now on, I'll try to include this kind of test in my reviews whenever possible. I also plan to run a cycle-life test for at least a thousand cycles and see how it holds up, but first I need to settle on the test parameters.

That's all for today. I hope you found it useful.