Lithium ion charged at 5V 2A, why internal resistance is not 2.5 Ohms?

Thread Starter

MrsssSu

Joined Sep 28, 2021
120
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Lithium ion cell are charged at 5V and 2A max. Following Ohm's Law, 5V/2A would mean at least a 2.5 Ohm internal resistance of battery.

However, Lithium ion internal resistance is only about 0.28 Ohms. How is that possible :)?
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
120
The charger adjusts it output to obtain the correct charging current.
The charger adjusts it output to obtain the correct charging current.
Hi, assuming constant current 2A, so the voltage to charge the battery is 2A*0.28Ohms=0.6V ? The charging voltage must be higher than battery. Seems confusing:). Or is it current voltage of battery+0.6V to charge?
 

AlbertHall

Joined Jun 4, 2014
11,704
The battery, its internal resistance, and the charger form a series circuit. So the voltage across the series resistance is the charger voltage minus the battery voltage.
 

BobTPH

Joined Jun 5, 2013
4,297
The internal resistance if a battery is not a constant. It changes with state of charge and other factors.

Bob
 

MrSalts

Joined Apr 2, 2020
850
View attachment 254171
Lithium ion cell are charged at 5V and 2A max. Following Ohm's Law, 5V/2A would mean at least a 2.5 Ohm internal resistance of battery.

However, Lithium ion internal resistance is only about 0.28 Ohms. How is that possible :)?
Because, if you connect the battery to a load of 2.5ohms, the battery would appear to be 2.5v.

A battery doesn't follow ohms law because "internal resistance" is the measure of imperfection of a battery (which ideally should be a clean voltage source with zero internal resistance. Also, the current put into the battery is converted to chemical energy as a reduction/oxidation potential.
To measure internal resistance, assume a resistor is inside of the battery in series with the anode. Measure the voltage from anode to cathode and you'll get the nominal voltage of the battery. Now connect a resistor across the battery and measure the voltage. The voltage drops to some new level. You've made a voltage divider with one unknown resistor value. So, just like in high school physics class you'll have to solve for the unknown value. In your case, for a new battery that is fully charged, it should be 0.38ohms.
You can do the experiment with a good 4 to 6-digit ohm meter and a 100ohm resistor (premeasure the resistance so you can use that exact value). Or, if you use a fairly small resistor, make sure it is rated for the wattage that you'll get (do the math).
finally, as mentioned above, the internal resistance posted for a battery is usually for a fully charged battery. The internal resistance increases as the battery discharges, or gets colder, or ages.
 

Audioguru again

Joined Oct 21, 2019
4,129
A Lithium-Ion cell has a maximum do not exceed charging voltage of 4.20V. It will explode and catch on fire if your charging circuit gives 5.0V and does not limit the charging voltage to 4.20V.
A lithium-Ion cell will also explode and catch on fire if your charger circuit does not limit the charging current.

Maybe the power supply to the charging circuit produces 5V at 2A? Many cell phones have the charger circuit inside the phone and uses a 5V at 2A power supply.
Maybe the battery has a protection circuit inside it that limits the voltage and current?
 

MrSalts

Joined Apr 2, 2020
850
A Lithium-Ion cell has a maximum do not exceed charging voltage of 4.20V. It will explode and catch on fire if your charging circuit gives 5.0V and does not limit the charging voltage to 4.20V.
A lithium-Ion cell will also explode and catch on fire if your charger circuit does not limit the charging current.

Maybe the power supply to the charging circuit produces 5V at 2A? Many cell phones have the charger circuit inside the phone and uses a 5V at 2A power supply.
Maybe the battery has a protection circuit inside it that limits the voltage and current?
There are buck/boost "lids" on some newer lithium packs and single cells that allows them to be charge by simple USB chargers and discharged as 5v usb supplies. I think he does not have a raw Lithium battery.
 

Audioguru again

Joined Oct 21, 2019
4,129
Then it is called a "power bank" not a lithium battery.
I have a power bank with a USB 5V at 2.1A input and it has a voltage boosted output of 5V at 2.1A output. Its rating is 16Ah.
The power bank has a charger circuit for its Lithium battery and a voltage boosting circuit for its output.
 

MrSalts

Joined Apr 2, 2020
850
Then it is called a "power bank" not a lithium battery.
I have a power bank with a USB 5V at 2.1A input and it has a voltage boosted output of 5V at 2.1A output. Its rating is 16Ah.
The power bank has a charger circuit for its Lithium battery and a voltage boosting circuit for its output.
I agree but the question was about internal resistance of a power source, not the semantics discussion over "of what is a battery". Most people (like many new members on this site) call any chemically stored energy a "battery".
 

Audioguru again

Joined Oct 21, 2019
4,129
My little 280mAh Li-PO batteries have such a low internal resistance that they can easily produce 6A with only a small voltage drop. I measure 0.5V drop for each series battery cell then the internal resistance is only 0.5V/6A=0.083 ohms per cell.
My little batteries charge at 300mA.
 
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