LiitoKala Lii-40A 21700 4000mAh batteries

I've reviewed and tested various batteries, from camera batteries to cylindrical cells in all sorts of sizes, but this is the first time I've had 21700 batteries in my hands. Technically, they're ordinary batteries, just a little larger, but I was still curious to find out what they can do.


Essentially, this form factor "grew" out of the popular 18650: a little longer and a little thicker, which lets it "pack" a bit more capacity. That's exactly what I want to check today.

The batteries are sold in lots of:
1 pc - $4.53
2 pcs - $7.58
3 pcs - $10.38
4 pcs - $13.35
5 pcs - $16.50
6 pcs - $19.12

I bought them in lots of 5, since the per-unit price was high, and lots of 5 let me place orders for 10 at a time.

A lot of batteries were ordered, but 10 of them ended up in my hands. They were packed in a somewhat unusual way, in cardboard boxes of different sizes. Two plus three were in one parcel, and one plus four in the other. At one point I even thought the second parcel was short one, as sometimes happens.


All the batteries look exactly alike. The labeling quality and overall appearance are both a 5 out of 5.


According to the markings on the casing, the batteries have a rated capacity of 4000 mAh and a voltage of 3.7 V, so they're standard lithium-ion batteries. No manufacturing date is given, which I think is a serious drawback, since in some situations it's better to use batteries from the same or similar production batches.


There is no protection circuit, but that was clear from the start, both from the product listing and from their intended use in electronic cigarettes.


The battery is slightly longer than the 70 mm stated in the name, measuring 71 mm, and its diameter is also a little larger.
Just out of curiosity, I weighed all the batteries and found that their weights vary slightly. The photo shows the lightest and heaviest of the 10.


To give you a better idea of their size, I placed one next to a 26650 and an 18650.


The batteries arrived almost completely discharged. On the first charge, about 3700 mAh "went in".
I was a little worried before testing, since these batteries are 5 mm longer than the familiar 18650s and definitely won't fit in a standard holder. But it turned out they fit lengthwise in my charger, and I can even squeeze four in at once, unlike 26650s, of which only two fit. The two in the middle do sit a little higher than the outer ones, but that doesn't stop them from charging properly.


In this case, my charger served as a battery holder; all the tests were performed using other devices. To connect to the terminals, I used small pieces of nickel strip. It's not the best solution, but unfortunately there weren't many alternatives.




Since these batteries are rated for currents above 10 A, I switched to a more powerful electronic load for the rest of the tests. Its measurement accuracy is lower, of course, but still quite sufficient for estimating capacity.


For the connection, I used two layers of nickel strip with a cross-sectional area of 1.6 mm² (0.2 × 8 mm), for a total of 3.2 mm² per contact.
The clamping force was moderate, since I used the same charger, but the contact plates fit very tightly.
The plate connected to the positive terminal had extra insulation in case the battery insulation melted through.



At this point, though, a question came up: what current should I test them at?
The battery description says 40 A continuous, but elsewhere it lists 15 A and 5C.


The product title, meanwhile, says 30 A and 5C.
So I ended up with four different maximum continuous discharge currents:
1. 15 A
2. 5C (20 A)
3. 30 A
4. 40 A.

So which one should I believe?


Looks like I'll have to find out the real current by successive approximation (scientific guesswork). I decided to increase the discharge current gradually while monitoring the battery temperature, to find out the maximum current these batteries can handle.

Since I'd already tested them at 10 A, I decided to try 20 A (5C) next.

The battery delivered 4,194 mAh, or 13.2 Wh. I don't have much confidence in the latter figure, though, because voltage drop across the contacts and wires affects it; the actual result is higher.
The battery heated up to about 90 degrees, which I considered the upper temperature limit.

After the successful 20 A test, I took the second battery and loaded it at 30 A.
The results were worse here. I had to run the test in two stages because after 5 minutes the battery had heated up to the same 90 degrees, so I stopped the test. I cooled the battery down and continued, discharging it completely.
In the first stage, I drew 2,474 mAh, or 7.49 Wh, from it.
The total capacity delivered was 4,041 mAh, or 11.66 Wh, which is quite good.


Temperature measurement results:
1. Full discharge of the first battery at 20 A.
2. First part of the second battery's discharge at 30 A; discharge time: 5 minutes.
3. Second part of the second battery's discharge at 30 A; discharge time: 3 minutes 9 seconds. Total time: 8 minutes 9 seconds.



The less-than-ideal current connection certainly had some effect, so I'd say the actual discharge ratings are more like this:
20 A continuous
30 A for short bursts.
It's hard to say about 40 A—perhaps only for very brief bursts.


I can't say how well these batteries hold up over time. A year and a half later, I discovered an unpleasant issue with the 26650 models: a faulty safety vent. The defect showed up even while they were just sitting on a shelf.
But these batteries have a different form factor, a different positive contact design, and most likely come from a different cell manufacturer.
The batteries meet their stated capacity, but LiitoKala was a bit misleading about the continuous discharge current. The description lists four (!) different discharge currents—good luck figuring that out.

That's all from me. I hope you found this review useful.