battery internal resistance measurement products

[abdulbadii]

Anyone experienced in products of battery internal resistance measuring device/instrument or on building it?

Which is/are the best value performance to cost trade-off, preferably just as a feature embedded in another main purpose device eg. battery charger (is there one such) ?

May even better how is the circuit principle or method in creating / building it ?

[IanB]

Internal resistance is not a constant value. So measuring the value depends on the application.

For example, in DC there could be voltage drop under load, or voltage rise under charging. And either of these could change with time, with the state of charge, with the magnitude of the current, or with temperature (or any combination of these).

Another standard measurement is the AC impedance, which is measured by looking at an AC waveform superimposed on the cell.

So before measuring, you need to know why you are measuring, and what purpose you will use the measurement for.

For DC, you could look at two different currents and measure the two different voltages, and then compute R = ΔV/ΔI

For AC, you could pass an AC waveform through the cell, block the DC, and compute r = v/i.

[bateau020]

Indeed, there is not 1 value, but many values. Indeed, the most simple model of a battery is a voltage source and a resistance. As mentioned by IanB, most battery testers test that resistance via either a DC method (apply load, look at voltage drop), or a 1kHz load signal. Most slightly more serious devices do the latter.
But most battery chemistries are much more complex than that.
Hence methods like EIS (google "eis battery", or look at https://www.sciencedirect.com/science/article/pii/S2352484720303103). But that may be too complex for most goals, as EIS you will be able to give you many values (Look at figure 2 of the above mentioned article) and you may not know where to look. Some values are however fairly good indicators for age or safety. For a potential implementation of a EIS measurement device, look at CN0510 at analog.com. 
A derived and simplified version of EIS is DCIS, which is easier to implement than EIS and tries to focus on the "useful" values.
Soon I hope to publish a mixed "classic" 1kHz IR and DCIS pen style handheld device here. The device works, now polishing the UI and fitting it in the case.

[jwet]

AVO Biddle is old British based company that is still around (part of the Megger companies now) that produced a lot of this type of equipment and still does.  They make good stuff for a wide range of applications and are available on Ebay pretty commonly.  They also have app notes and manuals that can get you up to speed.  The overall field is pretty broad going from single cells to large banks of cells used in power plants, etc.  Fluke also make good stuff for these measurements.