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#### john-boom

Mar 31, 2023
5
I'm trying to understand the load-resistance of an A123 Nanophosphate High Power Lithium Ion cell.

My understanding is that resistance increases as the cell charges.

The datasheet says "Internal Impedance (1kHz AC typical, mΩ) 8". I don't understand what that means.

Here's my (probably incorrect) attempt to apply Ohm's Law, at the low-SOC and full-SOC of the cell:

Low SOC
charging at 4A
R = V/I
R = 2.5/4
R = 0.625

High SOC
charging at 0.00001A
R = 3.7/0.00001
R = 370K

#### Bluejets

Oct 5, 2014
6,595
Use a charger designed for the job and move on.
There would have to be gazillions already out there to choose from.

#### crutschow

May 7, 2021
763
You never use the internal resistance of a battery to determine the charge current.
The charger regulates the charge current to a safe value for the battery it is designed to charge.

#### crutschow

May 7, 2021
763
The charging requirements from the data sheet:

So your charger must be designed to limit the charge current (CC) to either of the above values and time, and limit the charge voltage (CV) to 3.6Vdc.
The internal battery impedance does not have any affect on generating those charge characteristics as, at those currents. the voltage drop across the battery internal impedance is negligible.

This is how a typical (somewhat idealized) battery charge cycle would look:
Yellow trace shows the battery current (limited to 3A during most of the charge) and the green trace shows the battery voltage (limited to 3.6V at the end of the charge).
The charge is shown terminated at 45s (horizontal axis).
(Real time would be 45 minutes).

#### john-boom

Mar 31, 2023
5
Thanks for all the awesome information.

But none of it answers my question.

I fully understand how standard CC/CV charging works. Not asking that.

#### john-boom

Mar 31, 2023
5
Behold, the holy grail:

But why do they use AC for this test?

#### Bluejets

Oct 5, 2014
6,595
But why do they use AC for this test?
That's what impedance is.........AC resistance.
If you read it all, it will make sense.
If difficult to understand, you'll need to dig further into all applied theory.

#### Delta Prime

Jul 29, 2020
1,226
I fully understand how standard CC/CV charging works. Not asking that

Yes you are asking" that" & you do not completely understand, in order for the charging phases, that is, trickle charge, constant current ,constant voltage, are implemented you must take into consideration the simultaneous effects of charging factors, such as (SOC) state of charge , (SOH) state of health , (SEI) solid electrolyte interface, C-rate, and rest period is essential. the SOC on the ohmic resistance and the cathodic charge transfer resistance depends on the C-rate. The charge-transfer processes, both from the electrolyte into the SEI and from the SEI into the active material of the anode/ cathode,
it is important to take into account that a battery is a nonlinear, time-invariant system, and therefore a long rest time is needed to ensure that the battery is in electrochemical equilibrium
Electrochemical impedance spectroscopy has long been used to characterize the condition of a battery and for the description of the electrochemical characteristics.
That's what impedance is.........AC resistance

I like using the imaginary numbers with my impedance measurements

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,338
Here's my (probably incorrect) attempt to apply Ohm's Law,
You can't apply Ohm's law to a bettery as a battery is not a resistor. Ohm's law applies to resistors only.
Example:
measure the voltage and current of a battery at no load, then you have
V = some arbitrary value depending on battery chemistry and state of charge
I = 0
R = V / I = infinite
or measure the voltage and current of a battery with short circuited poles, then you have
V = 0
I = some arbitrary value depending on battery chemistry and state of charge
R = V/I = 0
Makes no sense in terms of resistamce because Ohm's law is not applicable.

But why do they use AC for this test?
The impedance of the battery is a differential resistance (note: not a esistance as per Ohm's law) which indicates the change in battey voltage due to a change in battery current (again, note the "change in", no absolute values are used). This differential resistance is frequenca dependend and thus called an impedance. Consequently AC is used to measure it.

#### melvinlcotto

Mar 29, 2023
3
It depends various couses,such as battery performence,charging voltage and charging current.

#### john-boom

Mar 31, 2023
5
measure the voltage and current of a battery at no load

How can there be current when there's no load?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,338
Current = 0 A in that case

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