RadioMaster Pocket balance charger defect
I recently bought a RadioMaster Pocket. I was suspicous of the balance function, as one of the batteries exceeded 4.2V on first charge. This took me down a rabbit hole to figure out what is going on exactly. It turns out that the balance part of the built-in charger is not working.
Suspicion
I got my RadioMaster Pocket together with two Lithium-Ion 18650 cells. Before I plugged them in, I checked the voltage on both batteries. They were quite far apart, so I was extra careful with the first time charging. I continuously measured the voltage on both cells while the USB charger was active. The cell with the highest voltage started exceeding 4.2V, which can be quite dangerous! I unplugged the charger when it read 4.23V.
How can this be the case? This is one of the most loved radio's currently on the market with very good reviews. I'm probably overreacting?
Teardown
Since this errorenous charge behavior made me distrust my brand new radio, I wondered if I was the first one to run into these issues. To my surprise I could not find anyone else on the internet reporting a similar problem. I also could not find any pictures of the back-side of the motherboard. But that's what I needed to see to understand the charge circuitry. So the only sensible thing to do next is tear it apart and see what we are dealing with.
The motherboard is relatively simple and after running some chip numbers through google, I found that the charge IC is the IP2326. On the picture, it is the one in the middle towards the bottom. Just above it is the balance resistor. The only available datasheet is completely in Chinese.
Functionality according to the datasheet
According to the datasheet, the IP2326 is capable of balance charging a 2S battery. It does this by connecting a resistor between the two cells of the battery and the BATM pin. Internally, this connects to two mosfets, which allows it to put the resistor in parallel with either cell. Basically it burns off some energy of the cell with the highest voltage.
The recommended resistor value is 100mOhm, which is what I also found on the motherboard. For a 4V cell, this results in a discharge current of 40mA. This is similar to what I've seen on other balance chargers.
What it actually does (or not)
I should be able to detect any balancing action by measuring the voltage over the resistor. This remained zero almost all of the time. There where some weird quircks where I would occasionally see a small voltage over the resistor, but I never managed to capture this. Most of the time I measured 0V. Also the voltage difference between the batteries didn't decrease.
If my interpretation of the datasheet is correct, the balance function is only active when one of the cells is above 4.1V. That did not make a difference. I also tested in a situation where both batteries where above 4.1V, but with a large difference (4.12V and 4.19V). Still nothing...
Other chargers with this chip
It got interesting when I started searching for other chargers that are built using this IP2326. The most notable example is the LX-LISC-V1 or LX-LISC-V2. There are some videos on YouTube that talk about these modules. You might need to enable captions with auto-translate to follow.
Most interesting are some of the comments on the first video, that after auto-translate read as follows:
@olegmet - Received 3 2S boards. I installed identical 18650 batteries with a voltage of 3.3V and 3.31V for charging. The result after charging with a 5V power supply from a smartphone: 4.35V and 4.04V. There is no balancing. I made several attempts, pre-leveled on imax, took 18650 regular, high-ampere... The result is the same. Overcharge, undercharge and no balancing.
And this user also reports no balancing happening at all:
@user-qe6ex4og8p Has anyone checked the operation of the balancer? At what voltage difference should it turn on? There is a 50 Ohm resistor on the board ( connected to the balancing wire), but even when there is a 0.1 volt difference between the banks, I did not notice that it would work. I measure the voltage on it - no! Judging by the datasheet, the transistors inside the microcircuit should connect it to minus or plus, but this does not seem to be happening.
So it turns out I'm not the only one with these issues after all! I have seen enough, we cannot trust this built-in charger.
Balance modification
I also recently purchased a digital battery checker with balancing function. All I need is an easy way to connect this to the radio. In my spare parts was also a 3S balance connector, which works fine for 2S when removing one cable. The leads were long enough to reach all three battery connection points, and the connector sticks out through one of the ventilation holes. It's not the prettiest creation, but it works!
