BLF Q8 Batteries in set have different capacity?

Hi there,

About a year or two ago (in 2018 back when you could still buy and ship batteries from banggood to the US), I purchased a set of 30Q button tops specifically for use only with the BLF Q8, I received. Since that time, they’ve been married and lived in the BLF Q8 and have never been used for other lights or separated out. There was one occasion where I kind of forgot about the Q8 and I think that the batteries drained out completely as the green switch was no longer glowing—although I am not sure if the batteries indeed drained out or if it was due to LVP (?). The batteries were recharged fine.

Anyways, I recently put the 4 30q set into my Opus charger and did a capacity test. Surprisingly, ONE of the four batteries had a slightly less capacity (2884 mah) than the others as seen in the photo below. This test was performed again with batteries in the same position and still, the battery sat at around ~2880 mah.

Sorry for being too long winded but my questions are:

- Are the batteries still OK to be used?

- Should I replace with new button tops?

- Should I replace the single slightly lower battery?

- Is the difference or change in the battery capacities too minuscule to make a difference?

  • One of the batteries, sits every so slightly lower than the rest when in the Q8, could this be the cause for the change in capacity? Should I fix the spring?

Thank you!

Less than a 1-1/2% difference between the max and the min.
Nothing to worry about especially because the cells are in parallel. Use the light, have fun.

Or buy new ones and mail the used cells to me. I’ll be sure they are safely disposed of.

The slight difference in height means nothing as long as each cell is making contact. You can test that by removing all the cells and then installing one and reassembling the liht If it works the cell contacts okay. Then remove the tube and move the cell one position clockwise. Reinstall, turn on, if it works repeat for cell position 3 and then 4.

Could also be variation between the slots in your BT-C3100. In any case, you have been given good advice about it, mostly the bit about not worrying.

In fact, some of your readings look a bit high, even if you are using a very low discharge rate. HKJ’s test only managed 2908mAh from the 30Q even at the lowest 200mA rate.

Edit: Here is the link (copy and paste into the browser):

dont see this as a problem, if u only use them in that light u should be fine. u can always buy a new pair of 30q or whatever they are cheap like 5$ each…

No cause for concern.

Contrary to what a lot of others might warn against, you can parallel pretty much any cells, a 30Q with a VTC6 with a panny-B with an old laptop-pull, and they’ll still work together happily.

When the runt of the litter is starting to sag, the other cells will take up the slack. Voltage all evens out, as they’re all crowbarred-together in parallel.

If you had a MT09R or any light that had a series arrangement, then yeah, they’d better be balanced as best as possible.

The capacity test measurements you got are very close to each other!

2884mAh vs 2926mAh is very small difference.
I get similar differences from new batteries of the same batch (I try to capacity test batteries when I get new ones, and I sometimes observe around 70mAh capacity test difference from 18650s that are rated 3000mAh).
I would worry slightly more if the capacity differs by maybe 150mAh (for a 3000mAh, which is 5% difference) battery, but I think still OK.

Thanks for the replies everyone.

If you’re still concerned, you could perform the capacity test again, but switch the batteries around to see if the 2nd slot is still lower. It may just be the charger. But if that particular battery is still lower, then you would know it’s the battery, which should still be fine.