BYD FC46120P 24Ah LiFePO4 batteries

The second test subjects from the batch I received—and this time, these are batteries I hadn't tested before. On top of that, they're simply more interesting than the previous ones.

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In the title, I identified these batteries as BYD FC46120P. I'm not entirely sure about the manufacturer, but searching online for this model number brings up batteries from that company. In any case, the model marking is what really matters.

There are four test subjects here. I'll run the main tests on a pair, and test the other pair in a shortened version. I'll explain why later.




I found some information on a Chinese website—not a datasheet, but at least something. In any case, all I really needed to know was the lower discharge cutoff, since the tests are mostly conducted according to standard procedures:
1. Charge current: 0.5C unless otherwise specified
2. Charge termination voltage: 3.65 or 4.2 V (for LiFePO4 and “regular” batteries, respectively)
3. Charge termination current: 1/10 of the standard 0.5C charge current
4. Capacity measurement at 0.2C of the rated capacity.
5. Overdischarge cutoff, usually 2.5 V, but sometimes 2.0 or 2.75–2.80 V. Again, this is specified in the datasheet.
6. Capacity measurement at 0.2C, 0.5C, 1C, 2C, etc.


The description specifies discharge down to 2.5 V at temperatures above zero. Since it's clearly warmer than that indoors, we'll discharge to that voltage.
Maximum charge current: 1C; maximum discharge current: 3C.


The heat-shrink sleeve is marked with the manufacturer's name, rated capacity in Ah and Wh, and operating voltage.
The terminals are flat and presumably (with a very high degree of confidence) aluminum. The positive terminal is simply flat, while the negative one protrudes slightly and has a copper core. There's also a safety vent.
The positive terminal also has a laser-etched serial number.



The terminal configuration turned out to be a problem. Usually, a nickel strip, stud, or “nut” is welded onto them. Here, it wasn't at all clear how to make a connection. I figured out a way to handle currents up to 40 A, but I'd also like to test them at the full 3C (72 A), especially since I have the equipment. I just couldn't come up with a way to connect to the cell, so the tests will go up to 40 A at most.


As for the dimensions and weight, there's nothing particularly interesting: 120 mm long, 46 mm in diameter, and just under half a kilo.


The impedance was specified as “less than 2 mΩ,” but I measured quite a bit less—about 0.8 mΩ, which is very good. I expect the cells to perform just fine at 3C as well.

I'd like to say a huge thank-you to my reader and sponsor, who sent me batteries to test in the spring and later gave me a long battery holder. It originally had some small contacts, which I replaced with the highest-current contact set I could buy; I had one stashed away. This holder came in very handy.



The cells were initially charged to about 20%, which is typical for new LiFePO4 batteries. Each battery went through a simplified “conditioning cycle”: first a full discharge, then a full charge.


Charging at 0.5C took 2 hours and 6 minutes—a very good result.


At 1C, the CV phase ran a little long, but that's perfectly normal. Even at this rate, charging took just over 1 hour and 8 minutes.


Battery case temperature during the CC-CV transition while charging at 24 A.


Tests: discharge rates of 0.2C, 0.5C, 1C, and 40 A. Only in the last test did the cell deliver slightly less than 24 Ah; at a 0.2C current, its capacity fully matched the rated value.


The second sample produced practically identical results.


At a discharge current of 40 A, the temperature rise was about 21 degrees above ambient, which is perfectly normal for this operating mode.


As always, the test comparing the cells' characteristics reveals a lot, and I have no complaints here either: the difference is within the measurement error and variation in air temperature.


And now a few comparisons with the second pair.
1–3. The first, second, and third cells at 0.2C
4–7. All four cells at 1C.

Only at 40 A did the fourth sample deviate slightly from the others, which was actually rather surprising. Though looking at the graph, the difference only shows up right at the end of the discharge.


Conclusions.
In my opinion, these cells are noticeably better than the ones shown in the previous article, and they should definitely be "kept in mind" when choosing batteries for "a harsh winter."
Of course, I'm not going to say, buy these and you'll be all set, since there are many manufacturers of cells like these. But I'd be more inclined to suggest looking at something like this or even larger, mainly for the higher capacity, easier assembly, and the possibility of a more compact solution.

So, my verdict: perfectly decent cells. The only way to learn more is through cycle-life testing, and I could do that, but unfortunately, first, it takes a long time, and second, it costs a lot :(