What is the method to measure the state of charge of a battery when it is :- a) Idle b) Connected to Load c) Charging. Is there a device like a multi meter which can achieve this?


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    \$\begingroup\$ There are fuel gauges of various complexity. \$\endgroup\$ – Eugene Sh. Jan 18 '17 at 15:07

Short answer: NO, it depends on your application and budget but there are smart chargers that measure this.

SoC Methods are diverse depending on;

  • application load variance, from C/50 to 50C, pulse and/or steady
  • battery chemistry, many Lipo types as well as NiMH, SLA, etc
  • battery array size, 1 cell 1S or 2S to array xS yP
  • temperature curve
  • Coulomb counter non-linearity
  • Battery ESR vs SoC correlation
  • Resting V, charge and Discharge V load V/V100% curves vs chemistry
  • user interface ( analog, digital ( serial and parallel )
  • package level of complexity, (Discrete, IC, PCA, Unit, Unit with charger)
  • Digital integrity with authentication and encryption
  • history with aging analysis

Consult battery university.com for other explanations. Precision measurements of voltage, temperature, and current, along with a cell characteristics table and application parameters, are used for capacity estimation calculations. The capacity registers report a conservative estimate of the amount of charge that can be removed given the current temperature, discharge rate, stored charge, and application parameters.

Below is an example of Maxim's Fuel Gauge (one of many many solutions)


A method for determining state of charge (SOC) from a simple measurement is the Holy Grail of the battery industry, all major companies are trying to do it, and it has still not been done well. This is why 'gas gauge' ICs are integrated into battery packs.

That said, a simple voltage measurement can get you some of the way. It's fairly easy to say with confidence that a battery is fully charged, or totally discharged, it's not possible to be at all sure about figures between those extremes.

Different battery chemistries have characteristics that are more or less amenable to simple voltage measurement. Lead and Nickel have a fairly flat voltage to SOC curve, and are very difficult to assess by voltage. Lithium has a much stronger voltage to SOC curve (as do alkaline primary cells) so you can estimate somewhat better with those.

The voltage difference between a battery being on charge, idle, and discharging, can be quite profound. The problem is polarisation. Concentration gradients of different states of chemicals within the battery, set up by a charge or discharge, can take hours to settle down fully when idle, resulting in a terminal voltage that changes over that period.


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