Battery Voltage vs Time

[hkBattousai]

I am designing a circuit which will contain a rechargeable battery. The circuit will be a microcontroller based one. The microcontroller has an ADC input which I'm going to use for monitoring battery voltage level.

I need a data/look-up table, or a graph, or a mathematical relationship between battery voltage level and state of charge of the battery.

I haven't decided what battery type to use, but probably it will be a Ni-Mh or Li-Ion battery.

Is there any information you can share with me?

 

[squishy36]

At first blush, I'd suggest contacting the battery manufacturers -- that's what their application engineers are there for. Of course, if you're only buying one, sometimes they don't give you the time of day.

If you can get your hands on a DC load instrument (or make something suitable), you can measure this functional relationship yourself. No doubt someone on here is experienced enough to tell you whether monitoring the voltage is a good strategy for what you want to do -- I would think integrating the current flow might be a better "gas gauge", especially for digital loads that don't have steady current draw.

 

[crutschow]

I need a data/look-up table, or a graph, or a mathematical relationship between battery voltage level and state of charge of the battery.

I haven't decided what battery type to use, but probably it will be a Ni-Mh or Li-Ion battery.

The only battery type that has a reliable relationship between voltage and state of charge is a lead-acid type. All the rest require monitoring of the charge current/temperature or rate of voltage change for proper charging. You can go to the battery manufacturer's website for more detail on that.

 

[RCinFLA]

All batteries have a 'kinetic' factor when current flows in or out of the battery. This kinetic factor is energy used in the movement of ions and/or molecules within the battery chemistry and resulting in a voltage drop during discharge and voltage rises during charging. There is also internal resistance that causes voltage drop when current flows. This also means the terminal voltage will be dependent on charge or discharge current levels. This makes battery voltage a poor indicator of state of charge.

Many battery chemistries do have a relationship between state of charge and open circuit terminal voltage for a rested (chemical reaction equalized), zero current flow state. Problem is you have to ensure there has been no current flow through battery for some period of time before taking voltage measurement.

 

[MrAl]

I am designing a circuit which will contain a rechargeable battery. The circuit will be a microcontroller based one. The microcontroller has an ADC input which I'm going to use for monitoring battery voltage level.

I need a data/look-up table, or a graph, or a mathematical relationship between battery voltage level and state of charge of the battery.

I haven't decided what battery type to use, but probably it will be a Ni-Mh or Li-Ion battery.

Is there any information you can share with me?

Hi,

The various chemistry types have their different requirements, and some require measuring the temperature of the battery to correct the voltage levels. There's kind of a lot to it, but you can get much more information here:
https://batteryuniversity.com/

 

[hkBattousai]

Thank you MrAl, this was what I was looking for.

 

[squishy36]

Oh, I forgot -- here's a spreadsheet I made up many years ago for my RV for lead-acid batteries. The rule of thumb after charging is to let the battery sit for at least two hours before measuring the voltage. As others mentioned, it takes time for the chemical cell to equilibrate.