Vapcell T39 21700 3900mAh: just high-current batteries

Today's review will be relatively short. That's not to say the batteries aren't interesting—in fact, quite the opposite—but there were no surprises or mysteries with them.

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These batteries don't really have any complicated backstory. You can find them on Google and on the manufacturer's website, and sometimes even on AliExpress, although they're temporarily unavailable there right now.
Still, there's something that sets them apart from similar cells, but I'll get to that toward the end of the review.


Specifications from the Vapcell website
Model: T39
Size: 21.1±0.2 * 70.5±0.3mm
Typical Capacity: 3900mAh (0.2C discharge)
Min capacity: 3800mAh (0.2C discharge)
Nominal voltage: 3.60V
Max Continuous Discharge Current: 35A
Max Discharge Current: 50A
End-of-charge voltage: 4.20V
End-of-discharge Voltage: 2.5V
Weight: 68g ( Appr)
Internal resistance: 6mΩ ( Appr) （AC 1kHz）
Standard Charge: 3000mA,CCCV 100mA cut-off
Quick Charge Current: 6000mA

They look like other high-current models from Vapcell, which are usually red and sometimes black. That's what you might call color coding.


No surprises with the dimensions or weight; everything is quite standard.


They arrived charged to about 25%. The first cycle showed a capacity of almost 4000mAh, which is already nice.


Two charging modes are specified: standard charging at 3A and fast charging at 6A.
At 3A, charging takes about an hour and a half, with a moderate CV phase.


As with the batteries in the previous review, I tested fast charging at 5A rather than 6A, but I don't think that makes a significant difference.
In my opinion, everything looks fine here too; the CV phase has barely changed.


Since these batteries are listed on the Vapcell website, I also found their test graph there. By the way, Vapcell is one of the few companies whose test results are quite realistic.


I tested the cells in 10 modes: 0.2, 0.5, 1, 2, 3, and 4C, and 20, 25, 30, and 35A.
The batteries had no trouble delivering not just their rated capacity, but a little more. They also performed well at 35A, and I think they could handle even more without any problems.


Temperature rise was also fine. Below are the results at 25, 30, and 35A; at the maximum current, they reached 82 degrees.
As a reminder, all temperature measurements are taken at the very end of the discharge cycle, and the maximum temperature is recorded.


The second battery behaved much the same, although there were a few minor differences.


The temperature results were similar to the previous one.


After discharging at 35A and cooling to room temperature, the remaining capacity was 63 and 74mAh, which is very good.


I do have some questions about how closely their characteristics match. The graph shows that at low currents, one battery has a higher average voltage, while at high currents the situation is reversed. It feels like these are similar cells, but either from different batches or perhaps entirely different cells.


The key feature of these cells is their impedance. The product page specifies "less than 6mΩ," but in reality I'd say it's below 4mΩ, probably closer to 3mΩ, which is excellent and could be useful when using these cells in high-current power tool batteries.


The product page compares them with other batteries. Unfortunately, I don't have those, so I'll have to compare them with ones I've tested before.


I'll compare them with these previously tested models:
1. Vapcell T39 21700, the subject of this review
2. Vapcell T42 - review
3. Molicel INR21700-P42A - review
4. Samsung INR21700-40T - review
5. Vapcell G45 - review

Well, the G45 is clearly ahead of the pack in capacity, which is only to be expected since it starts out with a higher capacity. But its voltage drops significantly under a heavy load.
The cells under review are more in the middle of the pack, but their rated capacity is 3900 mAh, compared with 4000–4500 mAh for the others. They make up for it mostly when operating at high currents.


The closest competitor from Vapcell is the T42, rated at 4200 mAh. But I have a bonus here: I tested it at currents up to 35 A, so I can compare them under those conditions, which is more interesting. Here are the results comparing the T39 under review with the T42 at 0.2C, 1C, 20 A, and 35 A.

I can say with confidence that at high discharge currents, in the 20–35 A range, the T39 actually performed slightly better than the higher-capacity T42. I suspect this is because of its lower impedance; the T42's is exactly twice as high.


Overall, I'm quite happy with these batteries. Almost everything gets top marks, except for one thing: consistency between cells. I wish that were better.
As you can see from the review and comparison graphs, these batteries can easily compete at high discharge currents not only with 4000 mAh cells, but even with 4200 mAh ones.

That's all for today. I may put the batteries aside for now and work on a review of an electronic load. As always, I'd be glad to hear your comments.