ages or if they have been damaged before, then they are not matched. Mismatched cells will quickly become
unbalanced when the pack is cycled. This is one reason why you should pay for good grade-A cells.
I bought 4 of the very cheapest low grade garbage cells from Aliexpress (See Figure C.4 below), just for
experimenting. I balanced them several times, but after even 1 cycle of charging and discharging, they are way
out of balance. This is because they are not matched at all. Some cells have a high internal resistance, so they
get hotter than the better cells, and this puts them at a lower state of charge. If you are trying to use crappy
cells like this, you will only be able to charge them up to ~80% to avoid constant cell over-voltages. This might
be good for a big cheap solar storage bank, but it can cause big problems for a pack that you cycle daily, or
use with large loads.
Figure C.4: Experimenting With Cheap Cells
Q: I’m still not convinced.
A: Here’s a real-world scenario that happened. A vendor in China that won’t be named shipped four 280Ah
LiFePO4 cells to a customer in Florida. This vendor was nice enough to email a video of the battery voltages
being measured before they were shipped. Here were the voltages, screen-captured :
● Cell 1: 3.3298V
● Cell 2: 3.2999V
● Cell 3: 3.3281V
● Cell 4: 3.3269V
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