How to choose the discharge time of a rechargeable battery

[CharanV]

I am working on a project which involves rechargeable batteries. To calculate the variations in the internal resistance of the battery, i used data from the existing experimental results to establish an approximate value for the internal resistance as a function of the SOC, discharge time and the temperature (effectively a 3D lookup table). At any given instant, the power demanded from the battery is known and the current profile (w.r.t time) is also known. The SOC can also be calculated. But I am not sure how to calculate/choose an appropriate discharge time w.r.t which the resistance can be determined. Can someone please help me understand it or point me in the right direction? Thanks in advance.

 

[anorlunda]

how to calculate/choose an appropriate discharge time w.r.t which the resistance can be determine

I don't understand your question. Internal resistance is not a single number, it is a nonlinear function of temperature, current, and SOC. In what sense do you mean "appropriate"?

 

[CWatters]

You could calculate the discharge time from the SOC and time. If it's taken 10mins to discharge to 50% then the likely discharge time is 20min. If it takes 2 days to discharge 25% then the likely discharge time is 8 days.

 

[CharanV]

I don't understand your question. Internal resistance is not a single number, it is a nonlinear function of temperature, current, and SOC. In what sense do you mean "appropriate"?

View attachment 129442

Hey anorlunda! I understand that the internal resistance is a function of temperature, current and SOC. It also happens to be a function of the discharge time. For example, for a given value of temperature, SOC and current at any instant, the value of the IR will vary depending on whether the discharge time is taken to be 1 second, 2 seconds, or any other time. My question was whether we have any method to determine the value of the discharge time that we need to use for this purpose. That is what i meant by "appropriate".
Thanks

 

[CWatters]

What do you mean by "discharge time" ?

a) The time for which you have been discharging the cell or
b) The time for which the cell can supply a constant current (eg related to it's capacity).

 

[CharanV]

What do you mean by "discharge time" ?

a) The time for which you have been discharging the cell or
b) The time for which the cell can supply a constant current (eg related to it's capacity).

Hey CWatters. By "discharge time", I meant the time for which i have been discharging the cell.

Thanks

 

[CWatters]

Doesn't that answer your question then?

 

[CharanV]

Doesn't that answer your question then?

It shows how to calculate the total discharge time of the cell for a given current. But my question essentially was how long should the cell be discharged to get a particular power (no need to discharge completely). Basically,the requirement is to choose a value from a look-up table of how long a cell should be discharged to calculate the corresponding internal resistance

 

[CharanV]

Doesn't that answer your question then?

I think I should rephrase the question a bit.

My question was not to just decide the time it will take for it to be discharged. I wanted to understand if i can incorporate varying current into choosing an appropriate value of the discharge time. For example, if the current is constant over time (say, 20A), for the first instant, i can consider the discharge time as, say 0.1 sec (assuming that is the least value of time that can be measured) and the discharge time to be 0.2s for the next instant and so on. But in the case of a varying current, i can still choose the discharge time as 0.1 for the 1st instant(20A). But for the next instant (say the current required is 25A now) i cannot choose 0.2s as the discharge time as that would mean that a current of 25A has been flowing for 0.2s when actually, it is 20A for the first 0.1s and 25A for the next 0.1s. I wanted to know if there is any method to determine a "equivalent discharge time" taking this variation into account.