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What is the cutoff voltage for 18650 3.7V Li-ion batteries?

Since there is an internal regulator board on these how can they accurately be measured? When they reach their cutoff voltage will this circuit simply shut them off providing 0 volts to the load from that point?

I would like to debunk some of these Ultrafire batteries coming with crazy high maH capacities. I have a dummy load to drain them down, just not sure which voltage level qualifies officially for "depleted". enter image description here

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    \$\begingroup\$ There's very little capacity below 3v on an unprotected cell, only a few percent of the total, so it's safest to stop there, to avoid damaging the cells. On a protected cell, the type you have there, you'll have to see what voltage they've been programmed to cut off at. Put them on discharge, and see when they stop. \$\endgroup\$ – Neil_UK May 2 at 15:05
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    \$\begingroup\$ The datasheet should tell you the terminal voltage that the manufacturer used to specify the cell's capacity. The datasheet will also provide another critical piece of information, the discharge rate that was used to specify the capacity. Right now you are shooting in the dark. Please provide a link to that datasheet. \$\endgroup\$ – Elliot Alderson May 2 at 15:14
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    \$\begingroup\$ Naming your lithium battery brand "ultrafire" doesn't seem like a very smart move.... \$\endgroup\$ – Hearth May 2 at 15:22
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You don't need to do the debunking, it is already done, see the Lygte website

enter image description here

As one can see, the measured capacity even at lightest load of 200 mA yields just about 1200 mAh instead of advertised 6000 mAh.

And yes, running a protected (in a standard way) cell down will result in 0 terminal voltage, somewhere at below 2.5V. The cell will disconnect under load. However, after a few seconds, the voltage will re-appear, due to small no-load recovery. In any case the cell will happily assume charge if plugged into a charger, and no harm will be done to it. This is what the protection is for. More, there is not much energy left in the cell when its voltage is below 3V, a percent maybe or less, so the difference between discharging to 3 V and down to protection limit can be safely ignored in capacity measurements.

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If you want to verify the cell capacity specified by the manufacturer then you must test the cell in exactly the same way the manufacturer does. In particular, you need to know the discharge current and final voltage that the manufacturer used when determining capacity. Those values should be in the datasheet for the cell.

Using arbitrary values that someone on the internet suggests will not give a valid test.

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  • \$\begingroup\$ You also need to charge the cells in accord with manufacturer's rules. Fast charge will yield less capacity. \$\endgroup\$ – Ale..chenski May 2 at 18:09

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