The subject of this video is very small, so we'll just cut straight to the Chase and we'll go into the expanded image one moment. please. That's better Now we can see what we're dealing with, which is the charge and balancing controller for a two Cell Lithium battery pack. And it's interesting because it's divided into two distinct sections here. We've got the control section which monitors the voltage across two cells. The cells are connected with the extreme negative end to this pad, the extreme positive end to this pad, and then the center connection to this pad. and as soon as one of those voltages exceeds 4.2 volts or probably a bit higher or drops below the safe, so I cut off voltage. It will basically turn the output and these pads on or off via these mosfets here. And it's worth mentioning these are dual mosfets, and technically speaking, you could remove one of these off the circuit board and it would just make it trip at lower current on its protection. The reason they've used two is for a greater current handling capability and the other section on this circuit board. well, the other two sections because they're complete independent. Each cell has a balancing circuit and it's quite interesting the way it uses it. I think at this point? Uh, well, is there anything else worth mentioning here? And we've got the spot welded tab connections here. for the battery, we've got the power, the takeoff wires from the battery pack. we've got these big fat resistors and a mosfet to switch them, the Uh sensing circuit to detect when overcharging is about to occur. Uh, and then we've got the control circuitry here. Uh. One other thing worth of worth noting is this: uh screen printing. here. They have a track running under here because it's directly in the vicinity of these pads and shorting to either one would be pretty bad. Either way, one would short the battery completely, the other would defeat protection. So what they've done here: although the track runs under here, they've not just covered it in the solder resist, but they've also printed on Uh screen print as well to give it an extra layer of insulation. So when you terminate on, should a little splasher solder go across, it will not be so dramatic, right? I Think we go straight to the notepad now one moment, please. So to make this circuitry easier to follow, I Divided into two pages, let's Zoom down this. The first section is the charge and discharge control, so it's fairly typical. It looks like this classic single cell dw01, but in this case it's a high contrast. I Think it's a high con hy2102 I Found it very hard to find the exact chip as such. This one is labeled 7022 and only by typing in the set of the Sort 23 package and describing the specifics. I Came up with the icon chip which fits the Pinner and description. so the positive is connected directly to the end of the two Lithium cells here and the negative is connected to the end of the Lithium cells via two mosfets. This is a very common arrangement. The reason for the two mosfets is because each mosfet has an inherent diode in it, which you can't really avoid that I Believe they do most fats without diodes now, but the typical standard mosfet has this inherent distress part of its design. it diode and if you consider if you had a load connected to here, supporting you had a lamp, oh, that's very old-fashioned so push out a lamp and this battery pack was discharging with current flowing clockwise using conventional current, not electron flow conventional current. So currents flowing this direction through the circuit, which is the way diodes face. The current could flow through this diode, but as long as the two, most vets are not flow in the circuit. However, if you turned this mosfet off, even though it was off, current could still flow through that diode. But by turning this mosfet off, it will block the discharge. That's why this one is marked as the discharge prevention mosfet. But when you're charging it, if current is effectively flowing the opposite direction. So now this diode here is the wrong direction to stop that current flow. So now this is the controlling mosfet and in reality both are turned on, uh, simultaneously and current can flow through a mosfet in our Direction. The issue is that diode. but by basically controlling the two mosfets, you basically block current flow in either direction. This 2K resistor here is used to sense the voltage between the negative connection which is this end of the mosfets and this end the most fats and that's the over current protection. If it sees very high current, there'll be a high voltage across these mosfets because they have a given resistance and when it detects that it will actually shut it down, the other parts of the circuitry is for monitoring the voltage across the cells, so when you're charging, the voltage will gradually rise and it's filtered via a 330 Ohms resistor and a 100 nanofarad capacitor. You can use various values, but 100 nanofarad is very common and by measuring the voltage across these cells, it has a potential divider inside that can sense the first cell that reaches 4.2 volts in the case of charging and as soon as one of them reaches it doesn't matter which one. as soon as one of them reaches 4.2 volts, it will stop any current flow through the circuit the charging current because it wants to protect the Uh, the fully charged cell from being pushed too far. Likewise, when they're discharging, it monitors the voltage across the cells and as soon as one reaches probably if it's like the DW and it'll be 2.5 volts it cuts off at but as soon as one of them starts discharging to the point of it, the voltage drops that low it will once again turn the mosfets off, and that's what protects the cells from overcharge and over discharge. It's quite unusual in the the case of the Dwl1. it's just a simple. It's just basically one of these sections, but in this one, because you get two cells in series, it monitors them independently with different voltage thresholds on the input. Uh, the next bit. The circuitry is kind of interesting. I could have drawn this bigger, but I didn't. It's another high corn chip as far as I can see. what's this one called. This one was called Um Bb3a L Double O Eight Bb3a Battery Balancing 3A I'm not really sure, but either way it's a Sword 23 package. It's a six pin package, but it only uses three pins and there's one of these across each cell. and it monitors the voltage across the cell with a 471 resistor and capacitor to provide filtering. so it gets a nice stable reference voltage. and if the voltage across this cell reaches the upper threshold, it could be just above 4.2 volts. This mosfet will turn on and it will effectively shunt this resistor across that Lithium cell. And if there's current flowing through the circuit, say the trickle charge at the end of charge. it wouldn't deal with amps, it would only deal with there typically less than 60 milliamps, but if the current is flowing through, that cell has reached its full. Voltage This mosfet turns on and it will just basically bypass this cell with that current and keep the voltage across the cell at 4.2 volts so it drops below 4.2 volts. The mosfet turns off. If it goes above 4.2 volts, the mosfet turns on. This appears to be how it works, but this does mean I'm not sure how you're supposed to use this in this configuration. I Would guess that maybe the battery pack if it was intelligent and it was applying the charge voltage to these pads here when it reached say it might detect I'm not sure to take would it detect the upper voltage because there will another beer. Can think of using this because effectively as soon as it reaches the full charge, it cuts power to the cell so that trickle charging wouldn't happen. All I can really think is that maybe once it comes to about 8.4 volts it, the charger would have to drop the current down to the trickle charge and just finalize them just balancing the cells until um, until they're all equal. I Don't know if they'd ever really reach the point that this thing would cut off at that point. Then the other option. this would be much easier. I Suppose this is just a theory. This is just a theory because this is connected directly to the batteries. Because this is connected directly to positive, you could have a three terminal charger and the bulk of the current above 8.4 volts. Would, uh, go via these protection mosfets and it would give the bulk of the charge until one of the Lithium cells reached the 4.2 volts and this thing turned the mosfets off. At that point, you could have a resistor trickling designed to actually connect between this pad and this pad so that then it trickle charged it via a direct connection to the battery and that would allow the balancing circuits to actually control that it would with that lower currency, a cap of a resistor designed to cap it to 50 milliamps or less. It doesn't have to be 50 milliamps, it could be 10 20 milliamps that will then allow the current to flow continually and these will just keep cycling on and off just to basically keep that voltage across the cells at 4.2 volts making sure they all do charge up to that level. And when you see the larger protection sockets. uh, the balancing circuits, it's basically the same circuit just repeated over and over again. all the cells in series and all these circuits just sat across those cells in series. It's very, very simple. It's kind of simple and I was expecting it means you'd have to be careful and not just trust this to basically balance them if you just supplied it with an amp because that wouldn't work if you supplied it with one amp continually at a higher voltage in the cells and you applied it directly to the cells. Then the at that current, the voltage across there's six to eight ohm resistor would be much higher. I Calculated the 60 milliamps as I equals the voltage required 4.2 volts divided by the resistance and it gave about 60 milliamps. So anything more than that, there is still a risk of overcharging the cells, but that is how it works. It does require a bit of Intelligence on the charger side or that sneaky resistor which I think would work. but uh, interesting stuff. So that answers the question. I've looked at these circuits and I've always wondered what is it doing to balance the cells and it turns out it's basically just dobbing a resistor across the cell when it reaches the required voltage and just trickling the current past it. So that answers that question. Quite an interesting technique.