Difference between two values of different measurements

InfluxDB 2
1

Hello. I have the voltage measurements of 16 cells that make up a battery. I would like to ask a question to represent the difference between the cell with the highest voltage and the cell with the lowest voltage. Do you know how this can be done? Thank you.

2

Hi @s118 Are you using Flux or InfluxQL? Have you attempted any queries thus far and if so, can you share here (even if not working or giving incorrect results)?

3

Hi. Obviously I’m referring to Flux (we’re on an influxdb 2 forum).
If we were talking about a single table I would solve it this way: How to calculate difference between min and max with Flux?
The problem is that I want the difference between the maximum and minimum of 16 measurements.

4

That was my guess, I have been wrong to assume that before as InfluxQL is still being used by people who are on v2.

Since you have only 16 cells, I would think creating a tag called cell number would be logical (rather than separate measurements for the 16 cells). Assuming the measurement would be called voltage, then I think something like this should work (did not have an opportunity to create fake data and test).

import "math"

data = from(bucket: "s118-bucket")
  |> range(start: -1h)  // Adjust the time range as needed
  |> filter(fn: (r) => r["_measurement"] == "voltage")

maxVoltage = data
  |> max(column: "_value")
  |> findRecord(fn: (key) => true)

minVoltage = data
  |> min(column: "_value")
  |> findRecord(fn: (key) => true)

diff = math.abs(x: maxVoltage._value - minVoltage._value)
5

Tanks. This is the code:

import "math"
data = from(bucket: "energy")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r._measurement == "VOLTAGE_CELL")
maxVoltage = data
  |> max(column: "_value")
  |> findRecord(
    fn: (key) => true,
    idx: 0,
    )
minVoltage = data
  |> min(column: "_value")
  |> findRecord(
    fn: (key) => true,
    idx: 0,
    )
delta = math.abs(x: maxVoltage._value - minVoltage._value)

and this is the error obtained:
“error in query specification while starting program: this Flux script returns no streaming data. Consider adding a “yield” or invoking streaming functions directly, without performing an assignment”

6

Can you add this to the very end and see what happens?

|> yield(name: "voltage_difference")
7

Error:

expected stream[A] but found float (argument tables)
8

Hi @s118 Let’s break this down into chunks and try to hone in on the error.

Does this give you a single value?

data = from(bucket: "energy")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r._measurement == "VOLTAGE_CELL")
maxVoltage = data
  |> max(column: "_value")
  |> findRecord(
    fn: (key) => true,
    idx: 0,
    )
  |> yield(name: "max_value")
9

Other error:

error @9:6-9:13: expected stream[A] but found { B with _value: A, _time: time, _stop: time, _start: time, _measurement: string, _field: string, } (record) (argument tables)
10

any help to solve my query?