# The battery equivalent internal impedance equation

#### Marius Balandis

Joined Apr 26, 2016
2
Hello, long story short: I need to create a model of Li-Ion battery and i'm stuck on calculating the battery equivalent internal impedance. The thing is, I don't know the equation or even how to make one. However, I have a circuit drawing: Could someone explain how to calculate or write down an equation for battery equivalent internal impedance

#### Kermit2

Joined Feb 5, 2010
4,162

#### Marius Balandis

Joined Apr 26, 2016
2

#### WBahn

Joined Mar 31, 2012
26,398
What is your starting point? You calculations are going to use what information as inputs?

#### Papabravo

Joined Feb 24, 2006
18,099
What makes you think there is a way to calculate it? Maybe you can't pin down enough information to make a meaningful calculation.

#### MrAl

Joined Jun 17, 2014
8,868
Hello, long story short: I need to create a model of Li-Ion battery and i'm stuck on calculating the battery equivalent internal impedance. The thing is, I don't know the equation or even how to make one. However, I have a circuit drawing: Could someone explain how to calculate or write down an equation for battery equivalent internal impedance
Hello,

If all you have is that circuit then it is quite easy to do. You do have to know how to combine impedances that's all.

If we number all the resistors as R1, R2, R3, R4 from left to right, and the two C's as C3 and C4 from left to right, then we can follow this procedure:
1. Combine R4 and C4 into one impedance, Z4.
2. Combine R3 and C3 into one impedance, Z3.
3. The total impedance is then R1+R2+Z3+Z4, except you may want to simplify the result.

To combine two parallel impedances into one, we can use:
Z=Z1*Z2/(Z1+Z2)

If you have never done this before then we'd have to go into that next.