# battery protection circuit - identify discharge current rate that will trigger protection

We are considering to add a battery protection circuit to our system in addition to the protection circuit that already exists on the 4.2V Li battery pack itself.

I know that the batteries have maximum allowed discharge current rate specified. Let's say it is 1000mA current. Hence I need to make sure that the battery protection circuit will disconnect the battery once the load draws more than 1000mA. Now, in the all datasheets of battery protection circuits, this kind of protection is called "discharge over current", and its range is not given in Amps, but rather in Volts. For this device to be compatible, it should trigger at 1000mA, but I cannot figure out the current value from the voltage values provided in the datasheet. For example, this is the device that I am currently considering to use - AP9221: https://www.diodes.com/assets/Datasheets/AP9221.pdf

Discharge Over-current Detection Voltage: 0.025V to 0.2V (10mV Steps) Accuracy ±12mV

I can order this device with specified voltage. Let's say I choose 0.025V. What is corresponding discharge current?

In the datasheet there is also following information:

RSS(ON) 120 mΩ

So, my understanding is following: I = V/R = 0.025/0.12 = 0.2A -> current rate after which the protection circuit will fire.

But in this post, there is different formula(with factor of 2): Over current protection for a 1-cell battery

What is proper way to calculate it?

I would appreciate any help.

## 1 Answer

The Diodes part refers to $$\R_{ss,on}\$$ as the source-to-source resistance. This would be the sum of the on-state resistances of the two internal MOSFETs.

The DW01, on the other hand, uses two identical external MOSFETs, where each one would have the same on-state resistance, so each individual FET would have $$\R_{ds,on}\$$ rated to be equal to half of the effective $$\R_{ss,on}\$$.

• Thank you very much. It makes more sense now. But still, how should I calculate the discharge current rate for the diodes part? Since Rss,ON=120mOhm is overall resistance, and there is no info on individual resistances, as I understood we have to guess resistance per each MOSFET as an average (60mOhm), and I = 0.025/60mOhm? Am I correct? Commented May 20, 2021 at 14:20
• No, the resistance of one FET doesn't matter. The resistance it cares about is the total of the two, which is why it doesn't give you individual ones. The two FETs should be identical, though, so 60 mΩ would be the best estimate for their individual on-state resistances. It just doesn't matter. Commented May 20, 2021 at 14:23
• So, the the threshold current for discharge is 0.025V/120mOhm for the diodes model? Commented May 20, 2021 at 14:25
• @UserRR Yes, as long as you have the threshold reference set to 25 mV. Commented May 20, 2021 at 14:25
• I understood it fully now, Thank you very much! Commented May 20, 2021 at 14:29