How much current would flow if 2 x 18650 batteries are shorted due to fault?

Hello,

I want to calculate how much current would flow if a short happens between battery terminals. I am using two 18650 batteries.

Battery specifications:
Product name: NCR18650-34B Rechargeable Li-ion Battery
Voltage: 3.6v-4.2v
Capacity:3400mAh
Output: 4.2v 600mA
Input: 110-240v
Over charging:4.2v
Maximum charge: 1.5C
internal resistance: below 50 milliohm
Charging time: 1000 times
Weight: 48g
Size: 69.5mm x 18.5mm (length x diameter)
The PCB use 2*MOS protection board, maximum power voltage is 4.2V, the minimum voltage of 2.5V.

I have come up with two methods to calculate the current:

  1. Using the formula, V=I * R, when the battery is fully charged (4.2 V) and internal resistance of 0.05 Ohms, then the current should be: 4.2/0.05 = 84 A?? Then, for 2 similar batteries, it would be 84x2 = 168A?? That just seems unrealistic to me.

  2. But i'm also thinking that maybe another way to calculate the short current: since this battery is rated at 3400mAh, so if it's shorted, then for one battery, i should expect 3.4 A to flow??

I don't want to experiment to find out, in case my batteries get damaged or explode!

It will be very high, certainly the 100s of amps is not unrealistic, and the batteries will get very hot very quickly.
Do not do this .

DryRun:
Using the formula, V=I * R, when the battery is fully charged (4.2 V) and internal resistance of 0.05 Ohms, then the current should be: 4.2/0.05 = 84 A?? Then, for 2 similar batteries, it would be 84x2 = 168A?? That just seems unrealistic to me.

It sounds about right to me. I wouldn't be surprised if it was even higher. I have a very small 240mAh 20C LiPo cell (that's 1/14th the size of your cell). That means it is designed to produce 20 x 0.24 = 4.8 amps.

Lithium cells can produce a huge current - that's why they explode or cause fires.

Make sure to operate them within their C rating both for charging and discharging.

...R

Depends on what you short them through. A thick piece of copper, the battery will likely change shape as it overheats. I had a tray of 3.7V button batteries get jammed and had to throw half of them out, they deformed badly even w/o fire.

Short them through a jumper and the jumper becomes a fuse. Short them through a 1 ohm 1/4W resistor and it is already known to be used as a fuse, hint-hint-hint.

A capacity of 3.4Ah means it could supply 3.4A for one hour, not that 3.4A is the most it can supply instantaneously.

If you short a battery it discharges in a lot less than one hour. So obviously the current is higher.

Your calculation of 168A is about right if you have a perfect zero resistance short. But it would be sustained only for a very short time because the battery voltage immediately drops so reducing the current.

In real life you don't see perfect short circuits. It's normally wires connected to the battery that short together so you have to add the resistance of the wires to the internal resistance of the battery. But you still get high current and normally the wires will burn out.

I wouldn't recommend trying it at home!

Steve

DryRun:

  1. Using the formula, V=I * R, when the battery is fully charged (4.2 V) and internal resistance of 0.05 Ohms, then the current should be: 4.2/0.05 = 84 A?? Then, for 2 similar batteries, it would be 84x2 = 168A?? That just seems unrealistic to me.

Probably about right for a short while.

A lot of people using lithium batteries dont appreciate how much current they can supply, its one of the reasons they can start fires so easily.

Lithium batteries that are actually designed to be used at 50C (50 X mAhr rate) are not unusual, so if your 3400mAh battery were rated at 50C it could supply 170A, and probably a lot more under short conditions.

It also depends on the battery. When I short my 18650 LiPo batteries (I have accidentally done this), nothing spectacular happens. The battery's built-in protection circuit shuts it down (no voltage produced any more!) until I place it in the charger, which resets the circuit. I've never used them for more than about 2A of current - powering two small DC motors so no idea where the limit is before the protection kicks in.

For unprotected batteries, yes the current can be huge.

Note that the specs say "below 50 mΩ" for the internal resistance. As that's apparently a guaranteed value, the actual value could be as low as 10-20 mΩ. LiPo battery applications include RC boats and planes with high-powered motors that draw tens of amps continuously.