A small protection board for assembling 2S lithium battery packs

I should say right away that I don't have a specific use for these. I bought them as an add-on to an order to qualify for a discount coupon, and just in case they might come in handy. Though at the end I'll show one way they can be used.

I ordered them quite a while ago, and the seller no longer has them in stock. They cost about 85 cents each. I found the closest similar listing, and that's the one the link points to.

The little boards came in neat bags, with the part number on them. I also suspect they originally came from the Banggood store.



They look very neat, and all the contacts are labeled.



The board is quite compact, even though part of it is just empty space.
It's 35 mm long and 6.2 mm wide.



The controller, its supporting components, and the transistor assembly are on top.
The marking on the bottom is ZYT240 2S 3565.



I couldn't find anything by that marking, only by the part number. I initially found more specs, but I haven't been able to find the page again.
Voltage: 7.2 V/8.6 V
Operating current: 3 A (4–8 A peak)
Color: green
Weight: 2 g
Size: 35 × 6 mm

I won't weigh the board, and you can see the color anyway. I've given the dimensions above, so let's check everything else :)

But first, a look at the board itself.
As I mentioned above, the board has a controller and a MOSFET.
1. The controller is presumably an equivalent of the S-8252. Here's a link to the datasheet.
2. An assembly of two N-channel MOSFETs. The specified maximum continuous current at 70 °C is 5 A. Here's a link to the datasheet.



The controller's circuit is almost the same as that of its single-cell version, except that it has an extra pin to monitor the second battery. By the way, some single-cell versions have a thermistor for overtemperature protection, but this one doesn't, since all the IC pins are already in use.



A brief description of the components on the board.
1. MOSFET assembly. It switches the load or charger on and off from the battery.
2. Protection resistor. It protects the controller from damage during charging. There is no current-sense shunt on the board; instead, current is measured by sensing the voltage drop across the MOSFET assembly.
3. Controller. It measures the battery voltages and controls the MOSFETs.
4, 5. Protection circuits for the measurement circuits of the upper and lower batteries in the pack. The capacitor protects against voltage spikes.

There's a footprint for capacitor C3 next to resistor R3. If you install a small-capacitance capacitor there (find the right value experimentally), the IC will be less sensitive to load-current spikes. This can sometimes be useful with loads that have a low static current but a high dynamic current.



One small but interesting observation: the board's length is equal to the width of a pack made from two 18xxx-size batteries.

I connected the board as follows, just to be safe.
First, prepare short wires with a cross-section of 0.5–0.75 mm² and solder them to the board pads. This is safer than soldering them to the batteries first and then to the board, since there's less chance of accidentally shorting the battery terminals.
Solder the outer terminals of the pack.
Solder the middle terminal. The wire gauge doesn't matter here, since it doesn't carry much current.



An alternative way to install the board. This lets you connect everything neatly and then insulate it with heat-shrink tubing. The result is a very convenient battery pack.



Sometimes, when you connect the batteries to the board, it starts up in a locked state, i.e. there's no output voltage. That's normal: just connect the pack to a charger, and the voltage will appear.
In my case, the board started up on its own.



Now let's move on to the tests.
I'm connecting the battery to a battery tester, which in this case is acting as a board tester :)



Next, I ran a program that gradually increased the current from 0.2 A to 5 A. At 3.6 A, the protection circuit disconnected the load.



After that, I moved on to testing the low-voltage cutoff threshold. But first, I checked how much voltage was lost across the board at different currents.
1. 0.75 A
2. 1.5 A
3. 2.25 A
4. 3 A.
5. 3 A; after half a minute, the voltage had risen slightly.
6. 3 A; after about 3–4 minutes, the voltage stopped rising.

The voltage rise as the board warms up is normal. Copper, which the circuit board traces are made of, has a positive temperature coefficient of resistance, so its resistance increases as the temperature rises.



A little later, I measured the board temperature at 3 A. The hottest component was the transistor array, at almost 70 degrees. 70 degrees isn't all that high, but cooling will be worse when the board is packed into a battery pack, and the temperature will rise. So I wouldn't recommend running the board at high currents for extended periods in this configuration.


The battery pack's total voltage at cutoff was about 5.35 V.



But since the board monitors the voltage of each cell individually, I first found the cell with the lower voltage and then measured its voltage.
The board disconnected the load when the voltage dropped below 2.41 V. At that point, the voltage on the second cell was noticeably higher.
This is the key feature of the protection: the board monitors each cell separately.



After that, I repeated the test, this time while charging.
Cutoff occurred at a pack voltage of about 7.49 V.



But the cutoff was actually caused by the battery becoming "unbalanced" because the cells installed have noticeably different capacities (that's just how it worked out).
That's why I don't recommend connecting cells with different capacities in series, no matter how tempting it may be. People usually do this when they want to reuse old laptop batteries.
The cutoff was actually triggered by one of the cells exceeding its voltage limit.
The overvoltage cutoff was at 4.28 V, which is a bit high; 4.25 V would be better. But the voltage may have been rising quite quickly.



After the emergency cutoff, I started charging again. The board correctly disconnected the battery, while the input voltage was about 12 V.



If you "balance" the battery a little, charging works just fine: the battery is at 8.4 V and continues charging.



There's a fairly widespread misconception online that the board described above (and similar ones) is responsible for charging the battery.
Almost all boards like this are responsible for just three things:
1. Preventing battery overdischarge
2. Preventing battery overcharge
3. Protecting against excessive load current.
Sometimes the board can also monitor battery temperature.

That's it! The board can't charge batteries. This also applies to larger boards designed for power tools and radio-controlled toys. Boards with built-in chargers do exist, but they're so rare you could say they don't exist.
You also can't use the emergency overdischarge cutoff as a charging function. It's an emergency protection feature!

As I promised at the very beginning, I'll show you what you can use a board like this for.
For example, a couple of years ago I modified radio batteries, replacing the nickel cells with lithium ones. Back then, I used protected cells; with this board, that protection isn't needed.



By the way, I also made an active balancer and charger on a single board back then.



That's about it. I can say that this is a decent board, works perfectly, and can save your battery for very little money :)