If you’ve ever bought 18650 cells in bulk, you’ve probably noticed the prices can be all over the place. And yeah, the cheaper ones? There’s a good chance they’re what people call “second-sorted.”
Basically, second-sorted cells are the ones that didn’t pass the factory quality check. Maybe a tiny defect, maybe just a little under spec. Instead of scrapping them, they get pulled out, sometimes lightly reworked, and then sold again. For big orders, these can be sneaky troublemakers—lower capacity, uneven internal resistance, shorter life span. Here’s how to catch them before they bite you.
Start with a close look
Honestly, appearance tells you a lot.
Labels & batch codes: A real 18650 has a clean, consistent label. If the label is a bit off, colors slightly off, or looks like it’s been peeled and stuck back on, that’s a red flag.
Welds & seals: Run your fingers over the welding points and seals. Uneven glue, tiny scratches around the weld, stuff like that usually means it’s been handled after the fact.
Surface scratches & color: Minor scratches, dents, or slightly different shades in the same batch? Suspicious.
Quick tip: lay out a batch side by side—you’ll notice odd ones immediately.
Don’t skip testing
Looking isn’t enough, you need numbers.
Capacity: Pop a few cells in a charge/discharge tester. If the capacity is lower than spec, that’s a sign.
Internal resistance: Cells should be pretty close to each other. If some are way off, it can cause trouble in a pack.
Voltage: Same story—voltage should be consistent across the batch. Big gaps? Something’s up.
For higher-capacity cells, even tiny differences matter when you string them into packs.
Cycle life matters
Even if the numbers look okay at first, the real test is how they perform over a few cycles.
Second-sorted cells might have already been cycled. Capacity drops faster than fresh ones.
Grab a few random samples, run a couple cycles, see what happens. If capacity takes a hit, you’ve probably got second-sorted cells.
Watch the source
Where you buy is just as important as what you test.
Go official if you can: Factory direct or authorized distributors = way safer.
Ask for proof: Batch numbers, QC reports, invoices—anything that confirms what you’re buying.
Be skeptical ofcheap prices: If it seems too good to be true, it usually is.
Some practical tips
Start small: Don’t throw down on a huge order right away. Buy a small batch, test it, then scale up.
Make a checklist: Track appearance, capacity, IR, voltage, cycles. Check it off as you go.
Lean on suppliers: If they can provide testing reports and support, that’s a huge plus.
Bottom line
Spotting second-sorted 18650s comes down to:
Looking closely at the physical details
Checking capacity, IR, and voltage
Doing a few simple cycle tests
Buying from suppliers you can trust
Do these, and you’re far less likely to get stuck with sneaky, second-sorted cells. You’ll know what you’re getting, and your packs will behave the way they’re supposed to.
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