I'm making my final project for monitoring the power of wind turbine using arduino. The energy from wind turbine will be stored in accu. I must have a planning calculation of the component that i need to make my final project, one of which is 'internal resistance of accumulator'. I using luminous accu 12V 100Ah. Specification doesn't tell the information about 'internal resistance' of it. My question is how can i know the internal resistance of luminous accu 12V 100Ah or maybe how can i measure 'it' ? (maybe there is a book or a theory) Thank you for your help

  • \$\begingroup\$ If needed you can measure it or use the value provided by the manufacturer. Anyway, do you really need this value? What if it's 0.1 ohm. What if 0.01? What if 0.001? \$\endgroup\$
    – Lelesquiz
    Commented Jan 3, 2017 at 15:00
  • \$\begingroup\$ Provide a link to the spec please. \$\endgroup\$
    – Andy aka
    Commented Jan 3, 2017 at 15:06
  • \$\begingroup\$ batteryuniversity.com/learn/article/… \$\endgroup\$
    – User323693
    Commented Jan 3, 2017 at 15:17
  • \$\begingroup\$ battU article is false for all suppliers but true for some. Read my explanation. Also ESR changes with frequency due to chemical reaction so Pulsed loads affect differences from DC incremental measurements of ESR. Same for Caps, although small enuf deviation to neglect for most uses. Same method of spectroscopy ESR(f) is used in oil filled Power Transformers to locate flaws after shipping damage. This can be used on lead acid batteries too to detect warped plates. \$\endgroup\$ Commented Jan 3, 2017 at 16:04
  • \$\begingroup\$ This article may help battcon.com/papersfinal2003/korinekpaperfinal2003.pdf. The question I would be asking myself is given the time I could spend on this, how many marks would I lose if I just stated/used a reasonable ballpark figure based upon an academic reference? Probably none. \$\endgroup\$ Commented Jan 3, 2017 at 16:36

2 Answers 2


To get the internal resistance of a battery, measure its voltage at two different currents. The resistance is then the change in voltage divided by the change in current between the two operating points.

However, the internal resistance of a battery is not a fixed quantity. It varies greatly by state of charge. It also varies somewhat by operating point. For example, using 1 A and 2 A discharge may result in a different internal resistance value than using 1 A and 5 A discharge, or 1 A and 3 A charge, or ...

At best, you can get a rough idea.


Batteries are always rated for cold climate starting conditions, although I am not 100% certain if the initial condition is 50% or 100% SoC or 12V,12.3,12.5V or 12.8V , I do recall the test condition for CCA is 0'C Current at 7.5V after 30 seconds.

Thus ESR = (12.5-7.5[V])/CCA[A] = 5000/CCA[mΩ]

We also know cold ESR is higher than room temp or hotter so CA or Crank Amp spec is never published but known to be 1.1 to 1.2 x the CCA ( depending on supplier) But then ESR degrades with SoC and also over time with aging. So let's stick with CCA.

e.g. 12V bat

 CCA[A]  ESR[mΩ]
 ---    --- 
 500   10.0
 750    6.7  ( approx 50 Ah,  ESR and Ah are not always related )
 850    5.9
1000    5.0

remember this after 30 second. Initial ESR can be <10% of this in first second. 
This is due to model of battery having 2 parallel caps with diff.ESR and C value
So short circuit currents > 10kA are possible on new batteries.  *N.B.*


e.g. 3.7V 18650 LiPo 3200mAh
ESR[mΩ] = 5 good , 8 typical, 10 poor or approaching low SoC
ESR balance requirement <2% ideal  < 10% acceptable  >30% poor

Similarly TELCO maintenance staff follow Eng Specs for 2V cell balance in 48V arrays for CO within 1%~2% cell matching before removal from service for reliability. These are 200~250 kg 2V cells. ( recall from memory)

The reason ESR and Ah ratings are not always easy to correlate is the difference in construction between suppliers. ESR is a property of the electrodes while Ah is a quality factor of the electrolyte.

But within the same supplier brand or family good correlation exists (not withstanding cost reductions and quality improvements)

ESR * Ah = k = some konstant

However Tesla battery recycler's have experience reclaiming bad packs with one bad cell and found they have to sort by ESR for parallel arrays within a few % ESR or full charge Voc, , I believe because active balancer will fail or fuse will fail (from overtemp shunting power during CC phase or blown fuse in one grid of each grid in the array _S_P , from weak link theory...)

The same is true for Capacitors within one family where RC=ESR * C= k where k < 10us for ultra low ESR < 100us for low ESR and>1ms for very large e-caps and > k seconds for batteries. ( although due to I^ESR losses you cannot charge up a battery in seconds with CV=Q charge in a few seconds even if Ceq*ESR is in seconds due to thermal runaway poof)

I recommend you test each battery like a Bode plot with 1A pp cc using LDO circuit and sweep from 0.01Hz to 1MHz to learn effects on SMPS assumptions of source Ri or ESR.

You will find ESR and Specific gravity s.g. correlates very well on Lead acid while ESR and Ah capacity does not due to aging or electrolyte contamination from charged conductive particles shorting out cell when aligned . My theory is this from excess aging and excessive charge/discharge currents, oxidation and antimony breakdown in electrolyte. This is why old batteries appear to have good crank amps for a second then die suddenly, then quick 10 second charge and repeat .. non-repairable. You may have had similar experience.

  • \$\begingroup\$ thank you for all your response, now i have a new problem and i hope you can help me with this one too. GBU \$\endgroup\$ Commented Jan 13, 2017 at 5:00
  • \$\begingroup\$ did this answer any questions? \$\endgroup\$ Commented Jan 13, 2017 at 6:25

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