What do cell ratings actually tell us? Part 1B: Resting vs Under-Load Voltages

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Electron Wrangler
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May 13, 2015
Expanding on my earlier post on cell voltage ratings. I see a lot of confusion regarding advice on how far down we can discharge a cell. The differences between these two voltages are important.

These voltages show us how the cell reacts to being used, or not used. They aren’t ratings or specs but they are important concepts for being able to best use the datasheet information.

Resting Voltage:
This is the voltage of the cell or battery pack when it has had a chance to rest after being used or charged. All of the datasheet specs are based on the resting voltage.

Under-Load Voltage:
This is the voltage of the cell or battery pack while it is being used, while it is “under load”. It is always lower than the resting voltage because of the voltage drop (“voltage sag”) caused by the internal resistance of the cell/pack.

This is not the true cell/pack voltage! It is just a lower temporary voltage seen only when it is being used. The voltage rises back up some once you stop. How much it rises depends on on how hard you were using the cell/pack.

Why is the difference between these two important?

The decisions we make about how hard we can use a cell need to take into account the differences between the resting and under-load voltages. It’s also important to mention which one of these is being discussed when talking to someone about cell performance or safety.

For example, the 2.5V low voltage spec for a “standard” li-ion cell is the resting voltage. If we are using a cell and the voltage sags down to 2.5V but bounces back up to 2.8V (or some other voltage) then we have not discharged that cell to 2.5V. The voltage sag that caused the temporarily lower voltage only made it seem like we discharged it that far. In this example we have only discharged it to 2.8V.

But if we used a cell and then stopped for a while (at least several minutes) and the voltage only rose back up to 2.5V then we should not discharge it any further. This cell has had time to settle to its true (resting) voltage and it’s down at the 2.5V low voltage rating for the cell. We should stop using this cell and charge it.

When someone says “never go below 3.0V per cell”, or some other level, it’s important to know if they are talking about the resting or under-load voltage.

Going to 3.0V/cell while being used is a lot less stressful to a cell than taking a cell down to the point where it only rises back up to 3.0V after a couple of hours. Using a cell until it drops to 3.0V means it might bounce back up to 3.3V or even a higher once you stop. But that cell with the 3.0V resting voltage might have been discharged to 2.8V or even a lot lower. This can create more heat and age the cell faster.

Charging Voltage Sag:
This is a third example of a “phantom” voltage. While charging a cell or pack you will see the opposite of what happens during discharge. The internal resistance of the cell cause a voltage rise instead of the voltage drop (sag) seen during discharge.

This is what causes the cell voltage to almost always read lower than 4.20V after charging to that voltage. The resting voltage might be pretty high, 4.19V, or it could be a lot lower, 4.15V, if the cell was older and had a lot of internal resistance or if you were charging very quickly.

This usually has very little effect on how completely the cell was charged since most chargers keep going until the current has dropped to a certain level, they are not fooled by this phantom voltage rise.

But if your cells are dropping down to around a 4.15V resting voltage, or lower, then I recommend charging slower to minimize the voltage rise in the cell. Some chargers are more sensitive to this voltage rise that than others and all chargers are affected by it a bit at high charging current levels.

Examples of Resting and Under-Load Voltages:
Examples of a cell at its resting voltage are easy to find, just don’t use the cell for a couple of hours. You can get a good idea of its resting voltage after a few minutes, perhaps just a couple of minutes, but it takes up to a few hours to come to its final resting voltage.

The under-load voltage can be seen any time a cell is being used. The harder you use the cell the bigger the difference between the resting and under-load voltages.

If you are a vaper you might have noticed this happening while taking a puff with a regulated device (with a display). The cell voltage/percentage or the number of “battery bars” will drop when you take a puff but then go back up when you stop. The higher the power setting, the larger the voltage drop will be.

If you have ever checked the voltage of your battery pack when riding an electric skateboard or e-bike you might have noticed the voltage dropping whenever you accelerate and then going back up when you are just cruising. This is the voltage sag difference between low power cruising and the high power being drawn from the battery pack when accelerating.

The under-load voltage effect isn’t only visible when comparing it to the resting voltage. The under-load voltage difference can also be seen when using a cell/pack at two different power levels (like when cruising or accelerating with an e-bike).

Using Resting and Under-Load Voltages Properly:
If we wanted to always stay above 3.0V/cell (or some other voltage) to extend cell life then we should be paying attention to the resting voltage we bring the cells down to, not the under-load voltage.

This means we can discharge our cells to under 3.0V (or whatever you chose) as long as the voltage rises back up to at least 3.0V in a couple of minutes.

If you stopped when the cells reached 3.0V/cell under load that means you might have only discharged them down to a 3.2V-3.5V (or even higher) resting voltage. This is certainly better for the cell, helping to extend its life even more, but you are missing out on the additional run time you could get by using the cells down to a 3.0V resting voltage.

How much a cell’s/pack’s voltage rises back up when you stop using it depends on how hard you are using the cell/pack. A powerbank user might have to stop at 2.90V to be sure the voltage rises back up to 3.0V. But a high power vaper or PEV rider might be able to run the cells down to 2.5V each and still have them rise back up to over a 3.0V resting voltage. You’ll have to experiment to see how your setup responds.

High power battery pack users should be very careful though if bringing the voltages down low to maximize running time (while still having a decently high resting voltage).

If the cells in the pack are unbalanced or aging, and you do not have a BMS monitoring each cell for low voltage, then you could force one or more cells down to a very low voltage. This can possibly harm the cell.

I recommend monitoring the voltage of each cell, instead of just the pack voltage, to make sure you are not overdischarging any of them. I think 2.5V under load is a good minimum to stay above to give you a bit of a safety margin.

That should give you a little idea of how a cell responds when used and at rest. Knowing how the resting and under-load voltages are different, and how to best use them, is important for getting the most out of your cells. This is true if you are staying well within the cell’s ratings or going far beyond them.

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