Another charger for assembling 3S Li-Ion battery packs

Another review of another small charger for 3S (12.6 V) battery packs. Not long ago, I reviewed the 3 A version; today it’s a simpler 1 A model.
Unfortunately, things didn’t go as planned, but I won’t get ahead of myself—the details are in the review.

It all started when I ordered five small chargers for a friend. Actually, no—I ordered about three times as many, but the others are from a more powerful series, and I’ll tell you about those another time. For now, let me show you the “little ones.”

I had no complaints about either the delivery or the packaging; the seller handled the job responsibly. Everything was packed tightly in a cardboard box, with a sheet of polyethylene foam on top.


Each unit was also packed in a small bag. Cardboard boxes would have looked better, of course, but this was fine too.


There were two plug options, and naturally I chose the European one. Each charger has a load cable about a meter long, with the familiar 5.5/2.1 connector at the end.
The stated specs are 12.6 V and 1 A, just as listed on the product page. It’s also explicitly described as a charger.


The case isn’t glued shut, so we just remove the single screw and take a look inside.


The circuit board gets a solid C. Even at a glance, you can see that it’s missing at least an input filter, and the transformer looks a little undersized for the stated 12.6 W output. Taking into account the losses in the diode and shunt, it’s probably closer to 13 W, but that’s not important—we’ll test it in operation later.
I’ll note that it does have a fuse. Given the overall build quality, I wouldn’t have been surprised if it didn’t.


1. It uses a KTG207C PWM controller with a built-in high-voltage transistor. According to the datasheet, it’s rated for 12 W in an adapter and 18 W with an open case. Here we’re dealing with an adapter (a power supply in a small, enclosed case), so it will run hot.
2. The input capacitor is rated at 15 µF; I measured 13.8 µF, with an ESR of 1 Ω. There’s no headroom, but that’s fine for a charger.
3. It has a proper Y-type interference suppression capacitor. I talked about these in one of my videos.
4. The output has a 3 A Schottky diode, a 16 V 470 µF capacitor, and a two-color LED. I do have some reservations about the capacitor. A capacitance of 470 µF (500 µF measured) is fine here; this isn’t a power supply, and ripple is harmful only to the capacitor, not the load. But 16 V is too low.


The soldering quality gets about the same grade as the top side of the board. There are some large solder blobs on a few contacts. The output wires are soldered on the underside, even though the board has holes for them. The wires aren’t particularly thick either, although, again, that’s not critical for a charger.


The primary side is of little interest to me; the secondary side is much more important.
It’s already clear that this is a “dumb” charger, and the familiar LM358 is used as the op-amp. You can also see that another resistor has been soldered on top of one of the resistors, apparently to adjust the output current.


Since it’s not very convenient to figure out how everything works from the PCB, I redrew the schematic in a more familiar format.


As expected, this is a simple charger. I was going to call it primitive, but no—there are much simpler designs out there.
I’ve highlighted the main sections in the schematic.
1. Blue: voltage regulation. This effectively sets the charge termination voltage.
2. Red: current regulation. This sets the charging current.
3. Green: reference voltage source. It ensures stable measurement of the charging current and stable indication.
4. Orange: indicator circuit. Since charge termination for lithium batteries is generally understood to mean that the charging current has fallen below 1/10 of its initial value, this circuit is similar to the current regulation circuit, but with different switching thresholds.
Note that the indicator circuit has no hysteresis, so the full red-to-green transition can take 10–40 seconds, depending on the battery capacity.


The standard initial test.
1. The charge termination voltage is 12.67 V, meaning each cell will be charged to 4.22 V rather than 4.20 V. That’s a little above the recommended level, though still tolerable.
2. With a battery connected and the power off, it draws 14 mA, which is rather high. The LED also stays lit.
3. The charging current is 1.05 A, slightly above the stated value. Interestingly, I showed the PCB above, and it had an extra resistor added. If you desolder it, the current drops from 1.05 A to 1.00 A, as calculated. Why they soldered it in is a mystery.
4. The current at which the indicator switches is 70 mA, which is below the standard value (100 mA).
5, 6. Out of curiosity, I checked the current 5 and 10 minutes after the indicator switched. After 5 minutes, the current had dropped to 35 mA, and 5 minutes later it was down to 20 mA. This charging mode isn't recommended, but it is acceptable. The advice is simple: don't leave it charging for a long time (several days).


Now we can move on to the load tests.
Since my electronic load can't operate in CV mode, I connected it before the charger's shunt and loaded it with 1.05 A to simulate a real-world situation. The charger was connected to the mains with a separate cable and covered with its original lid. You can see this in the photo. Of course, this differs from actual operating conditions, but not significantly.

The first test measures how much the charge termination voltage drifts as the unit heats up. There is some drift, though not much. Also, the temperature drops toward the end of charging, and the voltage returns to normal. Still, I run this test to get an overall idea of the device's quality.


But during the test, I got an unpleasant surprise. After about 20–25 minutes, the electronic load went quiet—in other words, its fan switched off. Usually, that means it has shut down automatically.
I set the shutdown threshold to 12 V for this test because I wanted to check the device, not burn it out.
I let the device cool down a little and started the test again. After 17 minutes, it shut down again due to the voltage dropping.


The cause was obvious as soon as I opened the lid: plain old overheating. At first I was worried about the transformer overheating, but the IC overheated first. During operation, it reached at least 115 degrees; in reality, it was hotter, since I measured it about 5 seconds after shutdown.


Since I did need the chargers, and they couldn't be used as-is, I decided to reduce the output current.
Below, I've highlighted the components that affect the output parameters.
1. Green: the shunt. It affects both the output current and the indicator. The effect is proportional, so halving the output current also halves the indicator switching threshold.
2. Red: the reference voltage divider. It affects the output current.
3. Blue: the second reference voltage divider. It affects the indicator switching threshold.

I had two options: change the shunt value or change the value of the reference voltage divider (red). Since it's easier to reduce resistance by connecting another resistor in parallel, I chose the second option and changed the divider values.


Of course, I could have worked everything out with a calculator, but it was much easier to do it in an old, tried-and-tested circuit simulator.
First, I recreated the original circuit and checked the voltage at the divider output (it will differ slightly from the actual value). It came out to 116 mV.
Then I calculated what voltage I needed to set to get the desired output current (I decided on 700–750 mA, with 725 mA as the midpoint).
Since the original current is known, we calculate: 116/1.05 × 0.725 = 0.79.
Then, by adjusting the added resistor (the one at the bottom right of the schematic), I got the voltage down to 80 mV. In my case, I needed to solder a 10 kΩ resistor in parallel.


Next, find the corresponding divider on the board; the component numbering matches the schematic. While I was at it, I straightened a crookedly installed resistor. Then solder the new resistor in parallel. I used an 0805-size resistor.


Check the result. It roughly matches the calculations, so we can leave it as is.


I ran it for about another half hour, and the controller temperature dropped from 155 to 85. That's pretty good, in my opinion. To improve things further, you could reduce the current to 700 mA, but there's no point going lower.


Now I'll briefly sum up what I think of this device.
The overall build quality isn't very high, and the circuit is simple. If you reduce the output current to 700–750 mA, it should work.
I strongly recommend against using it without modification. The controller will keep overheating and periodically shut down to cool off, and it may fail.

That's all. I hope you found the review useful. I should also mention that I have another 12.6 V, 3 A charger lying around, but this one is "branded".