A123 Nanophosphate Li-Ion Cell: How Does Internal Impedance Change During Charging?

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#1 21686093 03 Apr 2023 09:48

John Weiss John Weiss

Anonymous

Post #1
21686093 03 Apr 2023 09:48

What impedance does a lithium cell present to the charger PS over the charging cycle?

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

https://www.buya123products.com/uploads/vipcase/844c1bd8bdd1190ebb364d572bc1e6e7.pdf

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
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#2 21686094 04 Apr 2023 08:55

David Ashton David Ashton

Anonymous

Post #2
21686094 04 Apr 2023 08:55

I'm no great shakes on this, but it seems to be saying the internal impedance is 8 milliohms. That's pretty low, but as Li batteries can supply a LOT of current it might be right. Most lithium charghing uses constant current up to a certain voltage, then tapers the current off till full charge voltage is reached (usually around 4.2V) and then shuts the charge off, trickle charging is not recommended.

I'd refer you to Battery University

https://batteryuniversity.com/

Which has a wealth of information which might help you.
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#3 21686095 18 Apr 2023 12:45

Saracote Saracote

Anonymous

Post #3
21686095 18 Apr 2023 12:45

From empty to fully charged, lithium-ion's internal resistance is comparatively flat. Battery power drops asymptotically from 270 mW at 0% state-of-charge to 250 mW at 70%. Between 0% and 30% SoC, the biggest modifications take place. Lead acid's resistivity increases with discharge.
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#4 21686096 21 Apr 2023 17:27

upbeatlimb upbeatlimb

Anonymous

Post #4
21686096 21 Apr 2023 17:27

Battery state of charge affects the battery's internal resistance. Nickel-based geometry dash lite batteries have undergone the most dramatic transformations. Battery life, charging voltage, and charging current are all factors.
#5 21686097 23 Apr 2023 09:33

DA Tony Stewart DA Tony Stewart

Anonymous

Post #5
21686097 23 Apr 2023 09:33

We expect Internal Impedance (1kHz AC typical, 8 mΩ) is the ripple V/I at some equivalent slew rate such as a half cycle of 1 kHz = 0.5 ms.

If we applied 40A for 0.5ms we might expect a voltage change of I*R= 320mV.

Yet over a longer time constant of some % of the total energy cycle time, this resistance is actually greater at DC, neglecting thermal effects where voltage may rise if it heats up fast with 2 watt-hours used up in 3 minutes at 40 amps.

Consider 0.5V drop @ 30A after 15% of the 3 minute life cycle.
That is 500 mV/30A=16 mΩ or twice the impedance at 1kHz.

This is much lower than most Li Ion cells.
#6 21686098 19 May 2023 12:51

stephendaves stephendaves

Anonymous

Post #6
21686098 19 May 2023 12:51

To determine the charging load resistance for a lithium battery, you can follow these steps:

1. Identify the battery specifications.

2. Determine the desired charging current.

3. Calculate the charging load resistance.

4. Consider additional factors.

It's important to note that charging lithium batteries requires careful attention to safety and proper charging methods. It is recommended to follow the guidelines and recommendations provided by the battery manufacturer to ensure safe and efficient charging. If you are unsure or unfamiliar with the charging process, it is advisable to seek expert advice or consult professionals in the field.
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#7 21686099 03 Jul 2023 09:32

Tim Taylor Tim Taylor

Anonymous

Post #7
21686099 03 Jul 2023 09:32

You can measure the actual battery impedance pretty easily. Simply apply a known voltage 1khz sine wave across the battery with a series DC blocking capacitor and measure the series current. The rest is a simply ohms law calculation. There are several Youtube videos that show the process.
#8 21686100 09 Oct 2023 10:19

Lwayla Lwayla

Anonymous

Post #8
21686100 09 Oct 2023 10:19

The internal resistance of lithium-ion batteries remains relatively consistent throughout the charging cycle, showing a gradual decline in power from 270 mW at 0% state-of-charge to 250 mW at 70%, following an asymptotic curve. The most significant changes occur between 0% and 30% state-of-charge. In contrast, lead-acid batteries experience an increase in resistivity as they discharge.
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Topic summary

The internal impedance of A123 Nanophosphate High Power Lithium Ion cells is typically around 8 milliohms at 1 kHz AC, indicating a very low resistance that allows high current flow. This impedance represents the ripple voltage-to-current ratio at high frequency and is lower than the DC resistance observed over longer charge cycles. During charging, lithium-ion cells generally maintain a relatively flat internal resistance profile, with minor variations primarily occurring between 0% and 30% state-of-charge (SoC). Unlike lead-acid batteries, lithium-ion internal resistance does not significantly increase as the cell charges. Charging protocols usually involve constant current up to a set voltage (around 4.2 V), followed by current tapering until full charge, avoiding trickle charging. Measuring battery impedance can be done using a 1 kHz sine wave with a DC blocking capacitor and applying Ohm’s law to the measured current and voltage. Proper charging requires adherence to manufacturer guidelines to ensure safety and efficiency.
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A123 Nanophosphate Li-Ion Cell: How Does Internal Impedance Change During Charging?

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