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Internal resistance of Li Ion 26650 cells?


Well-Known Member
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)


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Well-Known Member
Most Helpful Member
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.

Nigel Goodwin

Super Moderator
Most Helpful Member
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.


Well-Known Member
Most Helpful Member
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.


Well-Known Member
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..


Well-Known Member
Most Helpful Member
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

Well-Known Member
Most Helpful Member
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.

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.

Last edited:

Amit Bajpayee

New Member
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

Well-Known Member
Most Helpful Member
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.

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