Battery Output

Introduction: Hello everyone! I'm new here, ( this being my first post ) and I would like to kindly ask you for some support on my little arduino project.

Question: How can you determine the maximum A output of a battery?

Details: I have 4 Lithium-ion batteries wired in parallel ( 18650 3.6V 9Wh - SZNS UN4M151EJ ) and I want to know what is the maximum current draw this pack can safetly support. More importantly, I'm interested in the way this is determined/calculated.

Thank you in advance and have a nice day!

Wiring batteries in parallel is generally not recommended (because a higher-charged battery can discharge into a lower-charged battery).

For current, you'll have to check the manufacture's specs. If you try to measure it, you can usually 'pull' more current than is safe.

More importantly, I'm interested in the way this is determined/calculated.

I assume its done experimentally while monitoring the temperature... Maybe the internal and external temperature. And, it's probably done in a fire-proof enclosure.

And for that kind of test, the manufacturer is going to test many batteries and take the worst-case as the spec.

Battery voltage 3.6v
Battery capacity 9Wh
Battery capacity in Ah = Wh/V = 9/3.6 = 2.5Ah
4 batteries in parallel capacity = 4 x 2.5 = 10Ah

Battery discharge rate is normally specified in terms of C (battery capacity in Ah)
Li-Ion can be discharged at a nominal 1C so you should get 10 Amps
Note that they are however capable of higher discharge rates (2C or 20 Amps) and if short circuited will endeavour to produce extremely high current, with associated risks.

By "parallel" I presume you mean all + terminals are connected together and all - terminals are connected together. By that method the stack voltage remains the same as a single battery but the Ah capacity increases to the sum of the individual batteries.

'Junior' posters on this site sometimes don't understand the difference between serial and parallel connection.

Battery capacity in Ah = Wh/V = 9/3.6 = 2.5Ah
4 batteries in parallel capacity = 4 x 2.5 = 10Ah

1. There are in fact 3 cells each 1.2V (similar to Ni-Cd).

2. Each cell can be safely discharged to its final voltage of 1.0V.

3. If we draw 10A current, the battery pack will supply energy for approx. 1 hr (10 Ah/10A = 1 hr).

4. Step-3 is the rule of thumb according to Industrial Handbook on batteries.

Here's an example data sheet (note that specifications vary by manufacturer and model, so this may not apply to your particular batteries): 2144243.pdf

There are several things of note here.

1. The quoted nominal capacity (3.1) is at a 5 hour discharge rate, that is 440 mA for this 2200 mA/hr cell. This is common practice. The capacity will be reduced at higher discharge rates.

2. They specify a "standard discharge rate" (3.8 ) of 0.5 C or 1100 mA for this cell. This is the point where the battery lifetime (number of charge/discharge cycles) is tested.

3. They specify a "rapid discharge rate" (3.9) of 1 C or 2200 mA for this cell. This presumably costs battery lifetime, but is within the reasonable operating range for the device.

4. Finally they specify a "maximum discharge rate" (3.10) of 2 C or 4400 mA for this cell, beyond which the manufacturer considers use of the cell to be imprudent.

For this particular cell, 1 C or 2200 mA is the maximum discharge rate following normal engineering practice. There exist Li-Po batteries designed specifically for high discharge applications (e.g. quadcopters). I don't know that these are commonly available in the 18650 package.

Finally, no name battery specifications are often wildly overstated. If you're pushing the envelope, do so with batteries from a reputable manufacturer who provides a clear data sheet.