How to Measure IR of Samsung 50E 21700 Cells?

By – 17 Mar, 2022When doing anything lithium-ion battery related, knowing the internal resistance (or IR if you like) of the 50E cell (aka 2I01) is useful; it has a role to play when it comes to monitoring the cell as well as it’s life, especially in energy storage and electric vehicles. So how might one go about measuring the IR of a Samsung 50E 21700?What is Internal Resistance?As in literally what you find inside, although inside beware, people can become confused as to what exactly that is under most ordinary circumstances. If a given cell sports a relatively high internal resistance then esp. if we try to discharge it, a voltage drop is observed,which we would rightly expect to accompany heat, much to our chagrin. The new cell that warns of depletion peaks is the relatively high internal resistance, in general terms. So it’s a useful figure to have a handle on when someone proposes to let you use a cell in regular life.Why Measure the IR of Samsung 50E Cells?This gives us a useful hint as to how ‘well’ it/s a 50E 21700 and when it’s ready to go.Eye On Quality Control: When you set out crafting batteries, it’d may make sense to measure every actual physical internal resistance so you know they are good and within spec and ‘ready to rock’ before they’re all prettied up and packed up to ship off to whoever wants them.
Methods of Measuring the IR of Samsung 50E 21700 Cells
There are really quite a few methods of measuring the internal resistance of lithium-ion batteries and which of these you use will depend mainly on the gear you have and how accurate you require the measurement. Probably the most common are:
1. Pulse Method (DCIR Method)
Pulse Method is probably the simplest way of measuring the internal resistance of lithium ion batteries and is used with the Samsung 50E 21700 cells. The basic idea is to pulse towards a load of the Samsung 50E and then measure the drop in voltage.
How to carry out:
First charge your Samsung 50E 21700 cell fully to the rated voltage (3.6V for the Samsung 50E generally)
Discharge with constant current load (briefly, say bell 1 second)
Magnitude of drop (ΔV) measured
Then use the simple calculation to find IR. 𝐼𝑅=Δ𝑉𝐼IR=IΔV​This method sounds quick and simple although of course at the expense of very high degree of accuracy required for some makes and models.The EIS Method
This technique is an accurate way of measuring the IR of lithium ion cells, which ‘probes’ the cell with a small AC signal. The steps are:
Attach a Samsung 50E 21700 to an impedance analyzer
Put a small AC signal (typically say approx 1mV) across the battery.
Get the impedance across the device over some frequencies, as the impedance at low frequencies may be used to find out what the internal resistance is as effectively this is just the device itself.
The measurement is tighter and is often used for quality control for high end batteries, or in development.
The Constant Current Discharge Method
With this method you discharge the battery under a constant current and with a constant voltage for a period of time, and then through the analysis of its voltage and current and your can work out what its IR is.
The steps are:
Fully charge the battery.
Discharge the Samsung 50E 21700 with constant current load (say, 0.5C or 1C rate).
Watch voltage from that, and then calculate internal resistance from your results.
Things Affecting IR
These or some of these affect IR:
Temperature. The li-ion cell IR lowers with temperature and rises when temperature goes down, so do the testing at say 25C.
Age More ageing/cycling/use measures increases/increasing internal resistance, so keep an eye on that.
Charge level A battery’ IR can be in general dependant on its State of Charge too, so just getting a nice standard charge first (say, 50% SOC) is better than testing at a random SOC of a battery.
In Summary
Measuring the IR on Samsung 50E 21700 cells (and indeed other brands of cell) is very much part of doing battery pack tuning and battery cell tuning techniques. However you go about measuring it min/max methods, EIS and so on or some constant current method, then you no doubt will be long term treating your cells well indeed in doing that and getting an accurate IR measurement on. Testing IR routinely to you and your cells is recommended and can help avert issues out of the gate.