# Lithium Ion voltage during discharge

I'm trying to understand how lithium ion batteries work during discharge, looking at the following batteries https://www.nkon.nl/sk/k/hg2.pdf.
My question is how do you determine the current voltage of the batteries during discharge? Looking at the chart showing cell voltage over discharge capacity, from what I understand it's made by testing the batteries, not from calculating.

Is there any easier way to model it/approximate it? I'm trying to calculate the power of a motor as the battery discharges, and how much difference that does do the power. I have a varying discharge current from 0-20 A, so ideally I'm trying to find some simpler models for the current voltage depending on discharge current and capacity left.

An example, If I were to run a 3.5 v motor with a total "consumption" of 3 Ah, and a varying load varying from 0-20 A, would the "safest" way be to put 10 batteries in parallel so the cell voltage always stays above 3.5? Or would you take the average, 10, and put 3 in parallel?

Is there perhaps any easier way to calculate the power of the motor as the battery discharges?

I also have a question about nominal voltage, from what I understand, the nominal voltage is the average voltage when discharged, in this case 3.6 v. Does this mean that the battery can deliver 3000 mAh with a discharge current of 3 A and a voltage above 3.6 v? Or is it with a current of 0.6 A?
And what do you actually use the nominal voltage for? Thanks

• Too many questions. Start from visiting batteryuniversity.com – Ale..chenski Apr 19 '18 at 21:43

## 1 Answer I created an equation for Vcell as a function of Amps (A) used and capacity [mAh] consumed.

The 3000 mAh rating at 0.2C means 0.2*3A=0.6A = 600mA for 5 hours or 11 Wh.

This degrades as Amps increase as shown in table.

Is that what you need?

• Hey, thanks. I'll do something similar to your equation, I figured I'll just take the average Amps, and create an approximation similar to yours. I still have a question regarding cycles. If I look at the chart showing 10A cycles, am I correct in thinking that after 400 cycles, the battery will have a voltage going from 3.7-2.5 and a capacity of 2400 mAh? As for the nominal voltage, what I don't understand is if that's related to discharge in any way? I read somewhere that if I discharge with a 0.2C current, the voltage will stay above 3.6 for the entire discharge, is that correct? Thanks – Blomma Apr 21 '18 at 19:55