Calculate 18650 mah: Is my reasoning correct?

I have an Xtar VC4 charger that calculates mah into the battery during charging. If 3.8v is 50% discharge for an 18650 battery can I charge the battery when it's at 3.8 resting voltage and then multiply the mah that went into the battery x 2? Will this give me a reasonably close capacity of the battery? Thanks.

hammick said:

I have an Xtar VC4 charger that calculates mah into the battery during charging. If 3.8v is 50% discharge for an 18650 battery can I charge the battery when it's at 3.8 resting voltage and then multiply the mah that went into the battery x 2? Will this give me a reasonably close capacity of the battery? Thanks.

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No. The discharge curve is not linear.

So then is the only way to take it below 3v to the voltage cutoff? That doesn't sound good for the battery.

hammick, what kind of 18650 battery have you got?

no need to use VC4 or calculate anything! on good batteries the capacity is written in mAh on the wrapper, or it is listed in the PDF datasheet.

hammick said:

So then is the only way to take it below 3v to the voltage cutoff? That doesn't sound good for the battery.

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Every 10 charges or so, I do exactly that... discharge fully to measure mAH capacity. This lets me know when a cell starts wearing out and not taking as large of a charge as it used to. I keep a log of all my cells, number of charges, and capacity measurements.
I don't think it is necessarily bad for a LiIon cell to occasionally be discharged to its lower limit, cell phones and laptops do it all the time. Might shorten life of the cell a little, but they last so long anyway - and I'd personally rather know if a cell is starting to fail. The only way I know to do this is capacity measurement.
If you want to do capacity measurement to the halfway mark, I don't think that is necessarily bad - just don't think that the result will be an accurate "50%" of full mAH capacity. Again, it's not linear.

I to just got the awesome VC4 and was also wondering if there is an equation to figure out what the full capacity the charger charged the cell to figuring 3.6v starting voltage and the mah added between that and 4.2v. Guess Ill just have to get an analyzer. Am working with 3400mah Orbs

I have Olights. Three 3,400mah and a 2,600mah. I know the specs. Just trying to figure out actual capacity. More curiosity than actual need.

hammick said:

More curiosity than actual need.

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lovecpf

I was curious about some of my cells too. Just wanted to check them for fun.

Have the Opus BT-C3100 coming which can automatically measure resistance and capacity (mAh). $35. Can't beat that! Plus it can charge all 4 batteries at 1000mA, or even two at 2000mA.

hmph! interesting looks like its time to get an Opus! Does the Opus do NiMH as well?

CelticCross74 said:

hmph! interesting looks like its time to get an Opus! Does the Opus do NiMH as well?

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It does. HKJ did a review: http://www.candlepowerforums.com/vb/showthread.php?389732-Test-Review-of-Charger-Opus-BT-C3100-V2-1

He had a couple concerns but I think it will be fine for what I'm doing. Also the v2.2 update may have addressed any "issues"

In the reviews I've seen, testing the resistance doesn't seem to be very reliable, but it doesn't hurt.

Testing the resistance also tells you if the cell is going bad/not the same as other cells, etc...so you don't mix cells with different resistances, etc.