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To calculate the battery capacity in mAh of an 18650 battery I understand you can use something like an iMax B6 in discharge mode on a fully charged (4.2V) battery to get the mAh it provides. The iMax B6 stops at 3V.

So my understanding of this is that the reading shown is the mAh that were provided by the battery going from 4.2V (or whatever it charged to) down to 3V.

I understand that running an 18650 Lithium Ion battery below 3V causes chemical instability and shouldn't be done. So what I am unsure on is if the value shown on the iMax B6 is the "rated capacity" for this batteries (the ones shown on the specs and printed on some batteries) or if you need to do some other calculation to get the full capacity of the battery going down to 0V?

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    \$\begingroup\$ The datasheet should list what conditions the values are measured under. \$\endgroup\$
    – SomeoneSomewhereSupportsMonica
    Commented Dec 28, 2017 at 14:52
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    \$\begingroup\$ Discharge capacity of 18650 cells is the integral of the area of the discharge curve bounded by seller greed and purchaser naivete \$\endgroup\$
    – Chris Stratton
    Commented Dec 28, 2017 at 16:59
  • \$\begingroup\$ The capacity on the battery is based on charge and discharge rates specified by the manufacturer. It is not based on discharge to 0. I have never seen such a thing and it would be of little value. The cutoff voltage will be something like 2.7 or 3 or 3.2. Generally, there is very little life left in a cell once it gets to 3.5V or so (depending on discharge rate). \$\endgroup\$
    – user57037
    Commented Dec 28, 2017 at 18:08

3 Answers 3

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The rated capacity of a battery is measured between its rated full charge and rated end of life voltages. With lithium polymer, these are usually 4.2 V for full charge, but the end of life quoted varies between manufacturers. I have seen 2.5, 2.7 and 3.0 V all used as the end of life voltage. There is no 'right' voltage, as higher end point voltages extend the cycle life of the battery, at the expense of capacity, leaving the capacity/lifetime tradeoff as your choice.

If you are lucky, the manufacturer quotes the life of the battery, the number of expected charge/discharge cycles, between the same voltages. If you are unlucky, the number of cycles will be quoted to a higher voltage than the Ah capacity.

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  • \$\begingroup\$ Thanks @Neil_UK So for this LGDBHG21865 the Discharge End Voltage is 2V and the Charge End Voltage is 4.2V so if I charged it to 4.2V then used something like the iMAX B6 to discharge it to 2V the mAh that it records should match the 3000mAh when the battery is new? nkon.nl/sk/k/hg2.pdf \$\endgroup\$
    – GazB
    Commented Dec 28, 2017 at 14:11
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    \$\begingroup\$ @GazB Whoa! 2v is verrrrry low. As Olin points out, there's very little capacity between 3v and 2v. But, there's an awful lot of cycle life between the two. Regular cycling down to 2v will trash your battery compared to only taking it down to 3v. Read the data sheet and do your own sums, but it would have to be a very weight-critical high maintenance application before I would cycle a battery regularly down to 2v. But to answer your question, yes, testing it using the same conditions the manufacturer uses should result in the same measured capacity, +/- reading errors. \$\endgroup\$
    – Neil_UK
    Commented Dec 28, 2017 at 14:42
  • \$\begingroup\$ Thank you. Don't worry I didn't mean regular I just wanted to get an accurate test once. :) \$\endgroup\$
    – GazB
    Commented Dec 28, 2017 at 15:09
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    \$\begingroup\$ That particular cell is rated for 20A continuous discharge. The higher the discharge rate, the lower the cutoff can be without damaging the battery. Also, some cells (such as this one) are specifically designed for rapid discharge. They are used in applications where the battery must supply high peak loads, or where the battery might be discharged very rapidly (let's say in 20 minutes or less). \$\endgroup\$
    – user57037
    Commented Dec 28, 2017 at 18:13
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    \$\begingroup\$ Yes, in fact, Tesla limits the depth of discharge on their packs to extend pack life and reduce their exposure to warranty claims. This summer, they blasted out a software update allowing deeper discharge for people fleeing hurricanes. \$\endgroup\$
    – Harper - Reinstate Monica
    Commented Dec 29, 2017 at 0:42
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I understand that running an 18650 Lithium Ion battery below 3V causes chemical instability and shouldn't be done.

Correct.

So what I am unsure on is if the value shown on the iMax B6 is the "rated capacity" for this batteries (the ones shown on the specs and printed on some batteries)

No, it's the actual capacity. Rated capacity is whatever the manufacturer decides to put on it. This figure may be wildly 'optimistic'. Reputable manufacturers rate a bit lower than typical capacity, though some discharge below 3V (knowing that the cell may suffer damage) in order to get a higher rating. Others just think of a number and double it or worse.

Here's a graph of actual capacity curves for many 18650 size Li-ion cells. Most had very little capacity left below 3.3V, and all had virtually nothing left below 3.0V.

enter image description here

or if you need to do some other calculation to get the full capacity of the battery going down to 0V?

Going down to zero volts turns your rechargeable battery into a non-rechargeable battery, which makes the calculation of subsequent capacity very easy - 0mAh!

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First, finding capacity of a battery has nothing to do with its physical size and layout. "18650" refers to the size and shape of the battery, in this case the shape is cylindrical, 65 mm long and 18 mm in diameter. It says nothing about the battery chemistry, or any electrical parameter.

Second, the real answer to your question is It doesn't matter. There is so little energy left after you get the type of lithium cell you seem to be assuming down to 3 V that it won't change the whole much.

Usually, the capacity spec is what you get under optimal conditions discharging down to a specific voltage listed in the manufacturer's datasheet. In this case, the capacity is what you get by discharging to 3 V.

The correct answer is, of course, to read the datasheet and see exactly how the manufacturer defines their capacity spec.

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    \$\begingroup\$ The remaining capacity at the end of discharge voltage, together with the leakage+self-discharge rate, determines the safe standby time of your product prior to which a recharge is required to preserve the cells. Unfortunately in my experience getting the manufacturer to commit to this type of detailed cell data, or really anything else beyond the nominal operating limits, is easier said than done. \$\endgroup\$
    – doynax
    Commented Dec 28, 2017 at 14:40

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