Internal resistance of Li Ion 26650 cells?

[Flyback]

Hi,
It sounds unbelievable, but its impossible to find on the web, a 3.7V 26650 Li Ion cell with a datasheet that gives its internal resistance.
Why is this?
We need to supply the attached current from two of such cells in series.
its 10.1A (Av) and 10.4A (rms)

 

[audioguru]

Li-Ion cells are sold at the storage voltage of 3.7V which is half a full charge. They are 4.2V when fully charged and yours in series will produce 8.4V at 10.1A for a few minutes if they are high quality new cells and are Name Brand.

 

[Flyback]

Thanks, do you know how to find out the internal resistance?....the datasheets never say...thats if there is even a datasheet.

 

[Nigel Goodwin]

Thanks, do you know how to find out the internal resistance?....the datasheets never say...thats if there is even a datasheet.

It's not something that's really tangible, and will vary greatly depending on many factors - which is why batteries don't generally give such information.

That's really little or no need to know it anyway.

You just need to start from how long you want the batteries to last.

 

[Diver300]

My guess is that the internal resistance varies enormously with state of charge, temperature and degradation of the battery. I don't know in detail how internal resistance varies with a Li-Ion battery, but I've seen some indication that it goes up as the battery degrades, which is probably no surprise.

Lead acid batteries have a big increase in internal resistance as they discharge. The internal resistance of electrolytic capacitors increases a lot as they degrade with age.

Internal resistance is measured by taking a large current, normally for a short time, and observing the voltage drop. An ac current can be used.

The logging on my EV shows around 80 mOhms on a 90 kWh 400 V battery.

 

[Flyback]

Thanks, we cant measure it, as we need to know it before buying it.
We are powering low resistance heaters with high current, so the battery internal resistance is crucial for us to know.
I have, many years ago, seen Li battery datasheets with internal resistance figures in them...but cant find them now.

Theres going to be i^2.R loss so we need to know it......havign said that, maybe youre just expected to look at the rated continuous supply current figure...hmmm..

 

[alec_t]

Does each cell have its own BMS? If so, the effective internal resistance of the combo may differ from the actual internal resistance of the cell itself.

 

[Tony Stewart]

You will never find decent specs on Ebay, Amazon etc. Go to the OEM site in China.
Only use brands certified by major companies.
Never assume ESR to be constant over 500 cycles unless specified. Never make any assumptions for that matter, unless verified.

XTAR quotes <=11 mohms for the 21700 with Ic=4A max

I would not expect the charge rate to increase with battery size due to temp rise of the inner most layers may increase with cell diameter, even though AH and ESR can improve. So do not expect any improvement, and in fact derate ESR for larger cells until proven otherwise.

https://www.xtar.cc/product/INR-21700-4200-High-drain-Li-ion-Battery-120.html

other
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Beware of partial dentrites and know how they are detected, created and hopefully screened out at factory.


ESR is the bottom line that when it has a NTC and sufficient energy transfer beyond Imax and above Tmax can trigger a thermal runaway event. Thus higher ESR means higher safety risk.

ESR is also a quality measure or Figure of Merit, FoM that correlates with cell size or diameter, Parallel Np arrays and weight of each cell. Quality varies with Process Design improvements and Cost Reductions which will be in constant change till the next generation.

ESR is an indicator of aging and State of Charge (SoC) and temperature if other variables are constant.

ESR can be estimated from max current ( assuming some temp rise and power thermal resistance and max temp) or from max charge rate relative to Ah capacity.

In the end if you intend to buy alot, you will have a DVT plan and qualification process and have a safety margin and/or thermal sensor in your design.

I

 

[Amit Bajpayee]

The internal resistance of Li-Ion 26650 cells varies depending on several factors such as the age of the cell, the temperature, the state of charge, and the manufacturing quality. However, the typical internal resistance of a new, high-quality Li-Ion 26650 cell is around 20-50 milliohms (mΩ) at room temperature. It's important to note that the internal resistance of a Li-Ion cell increases as it ages or undergoes numerous charge and discharge cycles. The resistance also increases at low temperatures and decreases at higher temperatures.

 

[Tony Stewart]

I agree with Amit.

If they do not specify ESR, then there are perhaps good reasons ;either technical or marketing related.

"battery internal resistance is crucial for us to know." This should be true for any designer.
Don't guess and turn your product into a "battery heater!" say after 200 cycles of aging.

Once critical design parameter is ESR*I and the cutoff voltage then knowing how ESR rises with DoC or falling SoC in a nonlinear fashion. More conservative cutoffs or current limiting below some threshold ought to be considered.

 

[MrAl]

Hi,
It sounds unbelievable, but its impossible to find on the web, a 3.7V 26650 Li Ion cell with a datasheet that gives its internal resistance.
Why is this?
We need to supply the attached current from two of such cells in series.
its 10.1A (Av) and 10.4A (rms)

Hi,

Really all you have to do is look for high quality cells that can do maybe 20 amperes discharge. That's really the key to getting cells to power devices that are very high power.
I have a special LED flashlight that is rated for about 70 watts and to get it to run I had to buy high quality cells, which of course cost much more than the run of the mill cells you find in various places.
It is possible that they will be the flat top type not the button top type.

 

[Tony Stewart]

The measure of ESR has a spectral density or frequency response that is affected if you use a PWM rate with harmonics . But fundamentally you can estimate it from the charge ratio C which is chosen to limit the I^2*ESR =P for a known by the designer a thermal rise from P*Rja for the useful life. Naturally aging accelerates ~100% for every 10’C internal junction temperature rises conservative over-design extends charge cycle lifetime as well as time duration spent above ~4V during CC to CV final charge. Note that’s because of this fact, the duration of time above for volts in this transition. Is small which also equates to the amount of charge that is retained. Therefore, when a battery is fully charged to 4.20 V and a small load is applied. It will quickly decay below 4 V, and that is your hundred percent charge voltage which is closer to 3.9 V
therefore, I reject the notion that 4.2 V is the 100% charge voltage

Bottom line

if you have old battery specifications for ESR, like 80 to 100 millI ohms for a 1C battery you can estimate the reduction and ESR by the C ratio for the same physical size battery and similar Rja thermal response.

So a similar chemistry and size 1C battery may have an ESR 20x that of a 20C cell. But then std thresholds for allowed temp rise and power transferred may cause correlation errors.

 

[audioguru]

Bbatteryuniversity.com shows that a Lithium-ion battery cell is far from fully charged when its low current room-temperature voltage reaches 4.2V, it is still charging.

 

[Nigel Goodwin]

I reject the notion that 4.2 V is the 100% charge voltage

Reject what you like, it's still true

However, it's not as clear cut as that - when it REACHES 4.2V it's not fully charged (only about 70-80%), and will be switched to constant voltage charging instead of constant current for the remaining time.

Then once the charging current has fallen to the required fully charged current (often 20% of full current current), then it's considered 'fully charged'.

But 'fully charged' itself is open to interpretation, and both the voltage and the lower current current limit are decided by the designer of the charger - based on safety factors, and how fast you want it to charge - essentially fast charging will leave the battery considerably below full charge, while slow charging will take longer, but will get closer towards 'full charge'.

But any charging scheme isn't likely to reach 'maximum possible' charge, for safety reasons - apart from the Samsung incendiary phones

 

[Diver300]

The state of charge of a lithium ion battery when it reaches 4.2 V per cell (or whatever the limit has been set at) depends on the charge rate. On a slow charge rate, the state of charge will be very near 100% when 4.2 V is reached.

When I'm charging my car at around 6.2 kW, full charge takes around 12 hours, and the charging power drops and it goes into a constant voltage mode about when the state of charge is only about 1% short of its final value. That is about 20 minutes before it shuts off.

At a higher charge rate of around 20 kW, the charging slowed down and the maximum voltage was reached at about 6% short of it's final value.

 

[MrAl]

Hi,

Remember we also have the cutoff current level to consider. If i remember right it's around C/20. That means in some cases we will be terminating the charge when the current drops to 50ma.

The battery models are fairly complex, and the models secondary capacitor is the one that discharges first, and that is not the capacitor responsible for the majority of the charge level (SOC). That discharge could take the voltage down faster, then the major capacitance takes over and becomes the dominate component that is responsible for the SOC when fully charged.