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Are battery specifications for open circuit or closed circuit conditions?

I understand that a battery connected to a load will have a multimeter reading of low voltage, due to the voltage dropped on the internal resistance of the battery. Thus, if I have an open circuit voltage Vbat = 3.6v and connected a load, I could measure a lower voltage. I also read that the current draw and temperature affects this.

I am particularly interested in Lithium ion cells. Most datasheets I read specify: Nominal voltage: 3.6v Max charging voltage: 4.2v Cut-off voltage: 3.0v

Are these voltages in open circuit (and *rested) condition or loaded condition?

*I read that loaded batteries, when removing load will have their voltages rebound and stabilize after a few hours or so. Say I have a loaded battery with Vloaded = 3.0v. When I remove the load, the open circuit voltage becomes higher (3.3v) and will continue rising and stabilize only after a few hours.

Background/Reason for asking: I want to test a certain charging circuit to see if it can charge healthy fully discharged batteries. By healthy, I mean batteries that were discharged up to their cut-off voltage only (not to 0v). In order to this, I am going to discharge the battery cell (taking into account the max discharge current from datasheet) up to their cut-off voltage by monitoring the loaded voltage. Once I see that the loaded voltage ~3v (cut-off), I am going to remove the load. But when I remove the load, I see that the open circuit voltage becomes higher and continues to go higher until it stabilizes after a few hours. I need to know if the specified cutoff = 3v is the open circuit rested or the loaded voltage. If its the former, then it means I would need to keep discharging/removing/resting the batteries until I finally get a rested open circuit voltage of 3.0v.

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The max and nominal voltages are open-circuit voltages. Note that lithium ion batteries have smaller series resistance for their capacity when compared to lead acid batteries (all variations of lead acid). So the difference between open and closed circuit condition is usually less pronounced unless it is a heavy load.

The cut-off voltage of 3.0 is pretty low. Also, typically when you cut-off, there is a load attached. So in practice, you can just use the closed circuit condition for the cut-off.

Lithium ion batteries "rebound" faster than lead acid when the load is removed. I think it may take a few minutes or 20 minutes or something like that. Rather than several hours. It would be a good thing for you to measure, if you feel like it. Just take a volt meter reading every 30 seconds for a while, then every minute, then every 5 minutes and graph it all out.

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  • \$\begingroup\$ When I have attached a load to drain the battery cell to 0% SoC (ie., Vbat goes to Vcutoff = 2.8v, I measured 2.8v loaded), Vbat goes back to >3.0v on 4 of 6 samples after 2 days. Is this an indication that I haven't really drained the batteries to 0% SoC? Does the open circuit/rested voltage needs to be equal to Vcutoff or is it enough for Vloaded voltage to cross through the Vcutoff to say that I have drained the batteries? \$\endgroup\$
    – user139731
    Mar 27 at 23:37
  • \$\begingroup\$ If the open circuit voltage is close to 3 V after 2 days, the battery can be considered to be at 0% SOC. If you try to put a load on that battery, it will drop extremely fast. There is basically no more energy available. \$\endgroup\$
    – mkeith
    Mar 29 at 7:13

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