Hi everyone, I am new to the forum
I have a Rechargeable Li-ion battery that nominal voltage is 7.2 vdc with maximum charge voltage is 8.4 vdc. I want make battery discharge auto cut off when battery is 7.00 vdc its automatically stop. I have included an image that my currently simple circuit using resistor 10 ohm with 50 watts. Do you have a recommendation for component that going to be use and also schematic drawing? currently I've been thinking using TL431 as voltage regulator and IRF9540 mosfet. thank you

 

Welcome to AAC.

First, are you certain the pack doesn't already contain protection circuitry? It would be unusual for a pack like that to have no protection against overcharge, overcurrent, and overdischarge.

If it doesn't then it would make the most sense to use something like the venerable DW01 chip and a pair of appropiately rated MOSFETS for each cell. This protects against balance issues causing overheating and fire.

You can purchase purpose made modules designed to add protection to an unprotected 18650 cell simplifying things. Note that your overdischarge threshold is extremely conservative and ~2.5V is more normal.

Alternatively, you could use a 2S1P BMS module to protect the entire pack, though this doesn't address the imbalance risk.

 

hi k21,
This is one simple option.
E

 

This is one simple option.

The ±5% tolerance of that Zener makes the circuit problematic for this application.
Better to use a circuit with a TL431.

 

With a simple circuit like that what happens after the battery "rests" a little while?

 

Welcome to AAC.

First, are you certain the pack doesn't already contain protection circuitry? It would be unusual for a pack like that to have no protection against overcharge, overcurrent, and overdischarge.

If it doesn't then it would make the most sense to use something like the venerable DW01 chip and a pair of appropiately rated MOSFETS for each cell. This protects against balance issues causing overheating and fire.

You can purchase purpose made modules designed to add protection to an unprotected 18650 cell simplifying things. Note that your overdischarge threshold is extremely conservative and ~2.5V is more normal.

Alternatively, you could use a 2S1P BMS module to protect the entire pack, though this doesn't address the imbalance risk.

The battery doesn't have protection circuitry, I think it's just 2 pcs 18650 wired series connection.

 

hi k21,
This is one simple option.
E
View attachment 362499

okay eric thanks for feedback, but I don't know on my place there's still have IRF7220 because based on article this component is discontinue. do you another suggestion what's component that can replace IRF7220 and also replacing Zener diode?

 

With a simple circuit like that what happens after the battery "rests" a little while?

hello Spidey, when battery still plugged in with 10 Ω 50watt around 2 hours it will be depleted 6.5 volts and battery stop discharge. That measurement is measure when battery unplugged from resistor. So, the discharge it's out of requirement that I want.

 

what's component that can replace IRF7220 and also replacing Zener diode?

Okay.
Below is the LTspice sim of a cutoff circuit with the common TL431 programmable 2.5V reference used as a comparator for a more accurate/stable/sharp trip point, and a P-MOSFET that disconnects the output at 7V:

Positive feedback from R4 provides about 250mV of hysteresis to the trip point so the circuit doesn't oscillate around that point.

The TL431 stops conducting, which shuts off the MOSFET, when its REF input voltage drops below 2.5V, as determined by the voltage divider R2 and R3.

The transistor can be just about any logic-level type P-MOSFET (max Vgs threshold ≤2V) with an on-resistance that doesn't dissipate more than about ½W I²R power at the maximum load current.

If you want to tweak the trip point to an exact value, change R3 to 11.5kΩ and add a 1kΩ trimpot between R2 and R3 with the wiper going to the REF input.
That will give about a ±½V adjustment around the 7V nominal trip point (bottom sim showing the trip points for pot wiper settings of 0%, 50%, and 100%).



-------------------------------------------------------------------

 

The battery doesn't have protection circuitry, I think it's just 2 pcs 18650 wired series connection.

hello Spidey, when battery still plugged in with 10 Ω 50watt around 2 hours it will be depleted 6.5 volts and battery stop discharge. That measurement is measure when battery unplugged from resistor. So, the discharge it's out of requirement that I want.

So the battery does have over-discharge protection, meaning there is a board in there doing it. Depending on which chipset they chose for the job you might be able to simply replace a resistor to change the threshold.