A pair of almost nameless Chinese protected batteries

These unknown batteries happened to come my way. A friend brought them over and asked me to check, measure, sniff, and taste them—which is exactly what I did.
The batteries are almost unbranded. Our intermediaries brought them in, but I wouldn’t be surprised if you can find them on Taobao or AliExpress, so this review may still be useful.

One is a standalone battery called 18650C5; the other is a 2P pack made from two 18650 batteries.


1. The first battery is rated at 2500 mAh, and the second at 6400 mAh (2 × 3200).
2. The dual battery has slightly thicker wires than the single one, but in either case they’re not designed for high current. I doubt it makes sense to load them with more than 4–5 A.
3, 4. The ends of both batteries are protected with cardboard inserts.


Both batteries have a built-in protection board. On the single battery, it protrudes slightly like a “hump”; on the dual battery, it’s tucked between the cells and isn’t visible.


Here’s the total weight. I’d guess the cells themselves weigh around 44–45 g.


I later found datasheets for these batteries.
18650C5 2500 mAh — link


And the “half” of the 2P 6400 pack, i.e. a 3200 mAh battery — link.


I tested the batteries using my main electronic load, the EBC-A10, with the standard connection—that is, with the protection board and wires in the circuit.


Test parameters:
Charge current: 0.2C (500 mA) up to 4.2 V, with cutoff when the current drops to 100 mA.
Discharge current: 0.2C (500 mA) down to 2.75 V. If I remember correctly, that was the voltage specified in the datasheet.

First I ran a discharge cycle, then charged the battery, and only then ran the measurement discharge cycle. The capacity came out to 2290 mAh, or 8.579 Wh—91.6% of the rated capacity in mAh and 95% of the rated capacity in Wh, compared with the claimed 2500 mAh and 9 Wh, respectively.
The protection board has a noticeable effect on the result, so there may be some measurement error.


The discharge curve has an almost linear section, followed by a sharp voltage drop at the end.


Testing the protection board. The battery was fully charged before the test.
At 200 mA, the voltage was 4.123 V. The cutoff threshold was 4.4 A, at which point the voltage was 3.62 V.


I tested the second battery under the same conditions, with a charge/discharge current of 1.28 A, which is 0.2C for 6400 mAh.
The result was 5957 mAh, or 21.48 Wh—93% of the rated capacity in mAh and 91% of the rated capacity in Wh, compared with the claimed 6400 mAh and 23.61 Wh, respectively.


The discharge curve is quite different from that of the first battery: the voltage drops almost linearly.


The protection kicked in at 6.7 A, with the voltage at 3.6 V. As before, the battery was fully charged before the test. At 100 mA, the voltage had dropped to 4.168 V.


I decided to take the small battery apart for a closer look and an additional test.
Inside, I found the same marking as on the outside: ICR18650 2500 mAh 3.6 V.


The protection board has one transistor and extra contact pads with welded tabs. I recently received some similar boards.


1. The internal resistance of the single battery with its protection board is almost 55 mΩ.
2. Without the board, it’s 20 mΩ—not bad at all. In my opinion, the battery is designed for significantly higher load currents than the protection board allows.
3. The dual battery without its protection board has almost the same resistance as the single one. I’d guess it uses a similar protection board, and that the batteries themselves have a resistance of around 30–35 mΩ.


Just as an experiment, I measured the capacity of the single battery without its protection board. It came out slightly higher, which was to be expected. The results on the right are from the test with the protection board.


That’s it. They’re ordinary Chinese batteries: capacity is a little below the rating, and the protection works.