Voltage Levels for Li-on batteries

[JerryM]

Somewhere on the forum I found a table that shows that for 18650 batteries
4.2V-100%
4.1V-90
4.0V-80
3.9V-60
3.8V-40
3.7V-20
3.6V-empty
3.5V-over discharged

I was glad I had the figures as I got an Ultrafire 501B that came with two Ultrafire batteries. When I checked them one was at 3.9V and the other at 3.6V. I'm waiting for a charger.I tried the 3.9V, but was disappointed in the light output. Hope it is better with a fully charged battery.

My question is, do other size li-on batteries have the same readings to empty or does a 10440 have different voltages at the various levels? Maybe there are charts similar to this one that provide the info, but I have not found one using the Search function.
Thanks for the help.
Jerry

 

[tandem]

For the most common li-ion chemistry formulations, the chart you've posted represents a good rough guide to the "state of charge" of a cell.

It doesn't matter how big or small the cell is, the voltage guide still represents the current state of charge - basically you are getting an idea of how much capacity remains before what we'd consider a cell depleted.

Cell capacity is not measured in volts but in Ampere Hours (or milli-Ampere-Hours or mAh). As the cell is discharged the voltage of a cell will drop. The guide you've posted refers to the voltage of the cell "at rest" - i.e., not straight out of the light after it has been used.

A high power light drawing significant current ("amps") can easily cause a cell voltage to drop markedly. Your Ultrafire light apparently uses a Cree XM-L LED; if the light was designed to draw as much current as the LED can handle, it is quite likely your Ultrafire cells aren't up to the job.

XM-L lights are bright and somewhat floody. If they are driven hard they certainly should look markedly brighter than any XP-G light you have; if they aren't driven hard you might not be as impressed with the visual difference. It may be your expectations are too high.

Or it could be the light simply is not well served by the Ultrafire cells. Often inexpensive cells are not able to deliver high current for very long without dropping significant voltage. Note that a light may not appear to be, and in reality may not be, "brighter" at 3.9V or 4.2V - how the light responds depends entirely on the driver circuit.

 

[JerryM]

Hi Tandem,
Thank you very much. That was very helpful. It will be interesting to me to see how the light works when I have the cells fully charged. I have a couple of Trustfire protected cells that should arrive with the charger.
Thanks, again.
Jerry

 

[tandem]

What a full charge should do is extend the period of time of perceived "full brightness", particularly if the driver circuit runs the light in a regulated fashion, meaning constant output at some predetermined level designed to optimize output and runtime. Whether this plays out or not does require a cell that can handle the current load while the light remains in regulation.

If the light runs in direct drive, the light ought to run brighter at first and at some point will become perceptibly less bright as the voltage drops. Here the quality of the cell will tend to show up sooner as better cells will hold higher voltages when current draw is higher than a poor quality cell.

It'll be interesting for you to see how your next set of cells compare to the first; hopefully your new light will be more satisfying.

 

[BVH]

The only point that I would edit is the "3.5V-over discharged" line. I do not think a resting Voltage of 3.5 indicates a cell has been pulled down to an over-discharged state prior to rebounding to this level.

 

[JerryM]

Thanks, I'll post the results.
Jerry

 

[loquutis79]

I am still a bit confused. For the most part I am reading 3.5 is the lowest voltage to safely run down to on my 18650, an empty battery.
But I am sure I have seen reference to 2.5 or 2.7 for higher cap. like AW 18650 1300mAh. Which is correct? Can I take these down to 2.7 safely all the time?

The T****fire which came with my Romisem actually has 4.2V as max. and 2.7 as min. on the label. So what gives?

 

[HKJ]

I am still a bit confused. For the most part I am reading 3.5 is the lowest voltage to safely run down to on my 18650, an empty battery.
But I am sure I have seen reference to 2.5 or 2.7 for higher cap. like AW 18650 1300mAh. Which is correct? Can I take these down to 2.7 safely all the time?

The T****fire which came with my Romisem actually has 4.2V as max. and 2.7 as min. on the label. So what gives?

There are two voltage, loaded and unloaded.
The data sheets for LiIon has a voltage between 2.5 and 3 volt as minimum loaded voltage, this depends on the actual battery chemistry (This varies between different LiIon batteries).
When you remove the battery from the light and measure the voltage some time later, you have the unloaded voltage and this will be higher. I cannot give you any numbers.

 

[tandem]

The T****fire which came with my Romisem actually has 4.2V as max. and 2.7 as min. on the label. So what gives?

In addition to what HKJ said above, the 2.7V lower limit may also represent the maker's lower limit set in the protection circuit. (See HJK's review of the TF2400 - that model tripped at 2.75V)

A protection circuit by my definition is something to protect against extra-ordinary events, not serve as regular backstop for daily use. I am admittedly conservative in how I use li-ion cells with flashlights, and except when testing cells, I've never tripped a protection circuit.

Budget cells may be made with budget components including cast off materials the better makers won't use in their products. To me it makes sense to avoid pressing the extremes when using budget cells, or any cells for that matter.

 

[45/70]

The only point that I would edit is the "3.5V-over discharged" line. I do not think a resting Voltage of 3.5 indicates a cell has been pulled down to an over-discharged state prior to rebounding to this level.

This may, or may not be true. Years ago when the suggested voltage/capacity charts first began to appear, it was noted that the accuracy of these figures begins to deteriorate somewhere around the 3.80 Volt level. The capacity remaining in cells from different manufacturers become far less clear at the bottom end of the scale. This is due to the specific formula used in manufacturing cells, being slightly different.

Also in my experience, for cells that are lower quality, older, have a higher IR, or do not hold voltage under load well, the lower end of the chart needs to be expanded downwards.

For a rough estimate of the remaining capacity of LiCo cells though, the chart is still useful. Just keep in mind that at voltages below ~3.80 Volt, or thereabouts, accuracy diminishes, and also these figures are just an "estimate", nothing is "carved in stone". Unlike nickel based rechargeable cells though, at least you can get a fairly accurate estimate. Evaluating LiCo cell's capacity by OC voltage is similar to doing so with Pb batteries, it's only an educated guess, but at least you can get some idea.

Dave

 

[HKJ]

This may, or may not be true. Years ago when the suggested voltage/capacity charts first began to appear, it was noted that the accuracy of these figures begins to deteriorate somewhere around the 3.80 Volt level. The capacity remaining in cells from different manufacturers become far less clear at the bottom end of the scale. This is due to the specific formula used in manufacturing cells, being slightly different.

When I get some spare time on my battery test stations, I hope to redo this table for a couple of different LiIon batteries.
It is fairly easy to do on one of the test stations, but like every battery test, it takes time.