An Interesting but Slightly Strange Battery Protection Board

While preparing a couple of upcoming reviews, I needed to put together a battery pack. Since the cells are lithium-based, they usually need a protection board. I originally bought these boards for another battery pack, but they came in handy here too.

As usual, a reminder: to keep up with new posts on the blog, I recommend following me on Instagram. I’ll post notifications about every new article there, and might also write about something interesting from time to time — link to my account

I actually bought the boards from another store, but they’re no longer in stock there, and the product page itself is gone too, so I had to pull the pictures from my order snapshot. I later found them from other sellers at roughly the same price, about two dollars per board.

So, the original plan was to add standalone lighting in the kitchen—or, more precisely, make the existing lighting work during a power outage. I don’t think I need to explain why. The lighting consists of a pair of 1-meter LED strips, each rated at about 14 W, so the current is around 1.2 A per strip, 2.4 A total. I want to power them from six of the well-known 26650 LiFePO4 cells from A123.
That means I need a 6S protection board, but most of the ones I came across were monsters. Then I spotted these small, neat boards with plenty of current headroom. So I decided to make a pair of 3S battery packs and connect them in series. To make things more versatile, though, I ordered 4S boards. There was a discount for buying 3 or more, so I ordered exactly that many.

The first thing that stands out is their size. Even all three together are about the size of a matchbox, and individually they’re tiny.



The package includes a connector with wires, about 15 cm long.


On the sellers’ pages, you’ll probably see six board options. They fall into two groups: boards for “regular” lithium-ion batteries and boards for LiFePO4. Each group includes three boards: 3S, 4S, and 5S. But in fact, the only differences come down to two things:
1. The default jumper configuration
2. The number of connector pins, and therefore the number of wires.

Yes, the boards themselves are essentially identical. What’s more, even the 3S board has five LEDs and five balancing circuits. So if you want a universal option, get the 5S board; you can always reconfigure it for any setup.


The board is built around the BM3451 chip, familiar to many from the “white 100-amp” board. In other words, it’s essentially the same white board, but “stripped down.” The key difference is the protection trip current and, accordingly, the number of transistors.
By the way, the transistors here are AON7202, rated for 30 V and 40 A, with an on-resistance of 5 mΩ. Based on my estimates, 20 A is the maximum short-term current; for continuous use, no more than 10 A, and 8 A would be better. There are two transistors; one is hidden under the connector. The 5 mΩ shunt is there too, which seems to match a current of 20 A according to the controller’s datasheet.


The back of the board has instructions for setting the jumpers, but it’s easier to look at the pictures on the product page.


Since I bought three boards and only need two for my project, I’ll have one left over for experiments with another UPS. It’s a 4S board for LiFePO4, too. I couldn’t have planned that coincidence better if I’d tried.

For this pack, I used 32700 cells, which I reviewed recently. I did have to buy a couple of double cell holders on OLX, though; I had no desire to wait for those to arrive from China as well.


Before assembling the pack, I decided to check the cells’ self-discharge out of curiosity. They’d been sitting around for a couple of weeks after a full charge.
And what can I say? The results look great to me. The largest amount needed to top up a cell was 88 mAh; the smallest was 55 mAh.



After this test, the cells sat on the table for a few more days. I checked the voltage, and it was 13.76 V, which is also excellent. In the UPS, they operate at 13.80 V, so the current needed to compensate for self-discharge will be tiny.


The wiring diagram is on the product page, but everything here is simple and clear even without it.


The only problem wasn’t how to connect it, but where to stick it :)

While I was wiring it up, I noticed that the corresponding LEDs lit up as I connected the balance wires. But after I connected the last wire, they all went out.


At first, the output was around 7 V, but while I was poking around with the multimeter, the full voltage appeared. By the way, this is quite critical: some UPS units require the battery to be at full voltage to start.


So, this is the setup I ended up with. It’s a pretty awful hack, of course, but it’ll do for now.


I put the pack on a top-up charge and ran into the first glitch. The battery tester was set to 4S LiFePO4 mode, which corresponds to 14.6 V. The current was low, but the pack went into overcharge protection almost immediately.


Strange... I thought, and decided to check the discharge first. I set it to discharge down to 2 V per cell, but the board cut out at 2.5 V. That’s not really good for batteries like these, since the UPS should always be the first to shut down; the protection board should only cut off when things are really dire.


The transistors’ temperature rose to 70 degrees in just a minute. It was still 70 half an hour later, and then reached about 74 degrees after another 15 minutes, at the end of the discharge. Looks fine to me: the discharge current was 8 A, and I think the maximum is 10 A, as I mentioned above.


Oddly enough, the batteries also got noticeably warm, even though they’re rated for currents up to 30 A.


Now let’s see how it handles charging.
First, I fully charged the pack, and the protection board cut out at the end of the charge. The charging current was 0.5C, which is perfectly normal.


Then, to speed up the experiment, I discharged it a little and charged it again.
This time, though, I set the charging current to about 1 A and the cutoff voltage to 14.40 V, i.e. 3.60 V per cell; previously it was 3.65 V. The protection board cut out again, but I noted the cutoff voltage: 14.303 V!


And yes, when I tried setting the cutoff voltage to 14.2 V, the board let the charging process finish.
Honestly, this makes no sense. The usual cutoff voltage for LiFePO4 is 3.65 V, sometimes 3.60 V, but definitely not lower. What’s more, not once during all the tests did even one of the balancer LEDs blink, so you can forget about balancing.
Sure, I understand that balancing wouldn’t work with a UPS anyway, but not everyone needs that, and UPS units vary.


I knew this chip came in a bunch of variants with different protection voltages and with or without balancing, so I thought they might simply have installed the wrong chip.
But after opening the datasheet, it turned out that installing the wrong chip is fundamentally impossible: not only is this chip rated for a minimum voltage of 3.65 V, it’s also the only version available—there are no other variants, even with the same 3.65 V rating.
There are, however, a couple of chip variants for the 2.0–3.85 V range, which would make more sense for this board.


So, the board seems to work, but its overdischarge and overcharge protection thresholds are... strange.
The overcharge threshold prevents proper charging in cyclic use and keeps the balancer from working.
The overdischarge threshold won’t work very well with a UPS, since 10 V could well be the UPS’s own low-voltage cutoff.
By the way, the balancer should start at 3.405 V, or about 13.62 V for the whole pack, but it doesn’t start even at 14.3 V. And yes, this balancer’s operating mode would be just right for a UPS, where the battery is charged to 13.80 V.

Well, what can I say? The board basically works—well, almost, and almost all of it—but it behaves strangely. On the other hand, the main issue is that the balancer never turns on. As for the thresholds, those are highly application-dependent, so you could use it as is, though it’s still odd.

I did like the boards themselves, especially their size. They’d work well in compact packs that aren’t designed for high discharge currents. The strange behavior probably applies specifically to boards for lithium iron phosphate batteries; I’d guess the “regular” boards work fine.

That’s all for now. I hope you found this useful.