Documentation sections

Battery Internal Resistance Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug battery-internal-resistance. Calculate battery internal resistance from open-circuit and loaded voltage measurements, determine power loss and maximum short-circuit current.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of battery-internal-resistance
Name	Unit	Range	Default	Description
vocvnumber	V	0.1 … 1000	12.6	Battery open circuit voltage with no load applied
vLoadnumber	V	0.1 … 1000	11.8	Battery terminal voltage under load current
iLoadnumber	A	0.001 … 10000	4	Load current applied during the voltage measurement

Retired, still accepted and ignored with a warning: temperature (Temperature, 2026-10-12).

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of battery-internal-resistance
Field	Unit	Description
rinternalheadline	mΩ	Battery internal resistance: R = (Vocv − Vload) / Iload
powerLoss	W	Power dissipated inside the battery: P = Rint × Iload²
efficiency	%	Energy delivery efficiency: Vload/Vocv × 100%
maxCurrentShort	A	Theoretical maximum short-circuit current: Isc = Vocv / Rint

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "battery-internal-resistance",
    "inputs": {
      "vocv": 12.6,
      "vLoad": 11.8,
      "iLoad": 4
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("battery-internal-resistance", {"vocv": 12.6, "vLoad": 11.8, "iLoad": 4})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "battery-internal-resistance",
    "inputs": {
      "vocv": 12.6,
      "vLoad": 11.8,
      "iLoad": 4
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "battery-internal-resistance",
	  "inputs": {
	    "vocv": 12.6,
	    "vLoad": 11.8,
	    "iLoad": 4
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "battery-internal-resistance",
  "values": {
    "rinternal": 199.99999999999974,
    "powerLoss": 3.1999999999999957,
    "efficiency": 93.65079365079366,
    "maxCurrentShort": 63.000000000000085
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:battery-internal-resistance",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vocv": {
          "min": 0.1,
          "max": 1000,
          "unit": "V"
        },
        "vLoad": {
          "min": 0.1,
          "max": 1000,
          "unit": "V"
        },
        "iLoad": {
          "min": 0.001,
          "max": 10000,
          "unit": "A"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vocv": 12.6,
      "vLoad": 11.8,
      "iLoad": 4
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vocv"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageBattery Internal Resistance Calculator, at these values