I went to a Samsung service store today with my S9 phone to ask about replacing my battery.

The people there plugged the phone into a USB port (I don't know where it was connected) and after a minute or so the program in their PC finished reading my battery capacity. They told me it was 70% (after 3 years.)

Now I don't think my phone has any built-in spectrometer for the battery (although with all the €€ it cost it should.)

From reading through the literature like here, I've seen the typical way to do this is via cycling the battery fully and Coulomb counting.

How did these people diagnose the battery so quickly?

  • 3
    \$\begingroup\$ A spectrometer won't tell you much about a battery unless you set the battery on fire. The most likely answer to how the store came up with that value is that Samsung phones track the capacity and that Samsung dealers have a way to access that data. \$\endgroup\$
    – JRE
    May 7, 2021 at 15:21
  • \$\begingroup\$ The phone likely keeps a record of this "coulomb counting" and the assistant simply read the latest record of it, and compared to the initial. \$\endgroup\$
    – rdtsc
    May 7, 2021 at 15:56
  • \$\begingroup\$ The phone can track battery health as it charges/discharges the battery. There are apps like AccuBattery that allow you to see the battery status. \$\endgroup\$
    – user16324
    May 7, 2021 at 18:35
  • \$\begingroup\$ @JRE eis has been used to estimate battery capacity like here: doi.org/10.1038/s41467-020-15235-7 The phone may be coulomb counting and storing it in a FCC register,but why would this take 2 minutes to be read? Now i'm not sure it just read this value. The employee in the service store told me they were doing "battery and charging diagnostics" \$\endgroup\$ May 9, 2021 at 11:13

2 Answers 2


The phone is Coulomb-counting all the time. It keeps track of all the charge it puts into the battery and all the charge it gets out, correlates against the cell voltage, and uses that information to constantly update its own estimate of the usable battery capacity (with an appropriate amount of filtering, of course, so that one noisy reading doesn't throw everything off). When you go into the shop, they don't have to test anything directly; they just ask your phone to hand over the number that it already has somewhere in memory.

  • \$\begingroup\$ this makes sense but why would it take 1-2 minutes to read the battery Full Charge Capacity (FCC) register? I saw the PC program scanning. \$\endgroup\$ May 9, 2021 at 10:51

I think spectrometer doesn't mean what you think it means.

The phone does measure the charge in the battery all the time; that is how it knows and displays you the battery percentage. When your battery completely dies and then you recharge it, the charge that went into the battery is the total capacity of the battery. The phone does measure this and it does store it. Then it measures the charge used when you use the phone and displays the percentage remaining. You can enter a special code into your phone, just as if you were making a proper phone call (not Facebook or WhatsApp etc) and you will then enter into the service menu where you can find this info yourself. This code can be easily found on the internet for whichever particular model you have using Google.

  • \$\begingroup\$ Thanks for the service menu,didnt know about it. However it holds no battery data for my phone. Regarding eis battery capacity estimation,it has been done,see here doi.org/10.1038/s41467-020-15235-7 \$\endgroup\$ May 9, 2021 at 11:10

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