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# Connecting batteries in parallel, suddenly?

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#### opampsmoker

##### Member
Hi,

Hi, supposing I suddenly connect in parallel two 18650 size Lithium ion cells….one with voltage of 4.2V and the other of voltage 4V.

What will happen? How much current will flow between the cells and for how long?
What is the lowest voltage 18650 Li ion cell which can be placed in parallel with a 4.2V Li ion cell and not cause damage to either cell?

How much does it depend on internal resistance, and how much on chemical effects?

Lithium Ion battery

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Anyone's guess. Try it and see or use a current limiting resistor until they're both at the same voltage. The current that will flow depends on the internal resistance of the two cells and could be very high and destroy the cells.

Mike.

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The current will be mainly determined by the batterys' internal resistance.
And the only way to accurately know that is to measure the internal resistance of both batteries (which would be in series to limit the current).
For an upper limit estimate, you could use manufacturer's data for that battery.

Thanks,
Is There a close model of an 18650 Li-ion cell for LTspice so I can simulate suddenly paralleling two Li-ion cells? Or simulate say suddenly putting a Li-ion cell in parallel with a layer of Li-ion cells which are at a higher voltage than it.
I think the cell model that’s “shipped” with LTspice is just an ideal voltage source, and so isn’t like a real cell. For a real cell, I think the internal resistance doesn’t exactly apply when a cell is charged. This is because the electric current is kind of formulating the cell chemistry, so in fact the internal resistance is much less when charging, would you agree?....i don’t think the LTspice cell model exactly and correctly simulates the electrochemical situation of a cell?

Also, i suspect the internal resistance of a Li-ion cell is even more complex...

.....The Samsung “INR18650 25R” Li-ion cell datasheet says that the 1kHz internal resistance is 13 milliohms, but the DC internal resistance is 22 milliohms….one then extrapolates that at 1MHz the internal resistance would be well under 13 milliohms…..would you agree?

INR18650 25R cell datasheet
https://www.powerstream.com/p/INR18650-25R-datasheet.pdf

….Now suppose a sudden pulse of current suddenly flows into the INR18650 25R cell. Now, the rising edge of a sudden “step” pulse current has extremely high frequencies associated with it…potentially well above 1MHz….so one would conclude that the internal resistance to a very fast rising edge pulse would be extremely low….well under 13 milliohms…..would you agree?

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Mike.

Sorry pommie, i forgot to say that we arent allowed to use any resistive balancing.

For the INR 18650 25R Li ion cell, what would you say are the parameters in the attached Li-ion equivalent circuit?

INR 18650 25R Li ion cell datasheet
https://www.powerstream.com/p/INR18650-25R-datasheet.pdf

Regarding about your advice that the cells can be destroyed....thanks, and i am seeking hard evidence of this.

As i described, the internal resistance value is a complex one, it is mingled in the academia of electochemistry......and i cannot find out the specific internal resistance value as i describe above.

#### Attachments

• Li ion cell _equivalent cct.jpg
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Use the DC internal resistance value for your simulation, since those are the conditions you are concerned about.
That value would be about the same whether you are charging or discharging.

In the neighborhood of 7 amps. Wow!
0.2 volts / internal resistance of two 18650's

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