Whatever he's doing, he needs the pull up voltage to be high enough to turn the MOSFET on. 1.25V isn't high enough to guarantee the device will turn on. The threshold voltage can be as high as 1.45V. 4V will give a typical on resistance of 20-30 milliohms.

The fuse was connected to the battery (the note BAT1). Why is the fuse unnecessary?

Also the pullup is connected right to the battery so the voltage on the gate of the fet should be 4.2V - 3.8V

 

The fuse was connected to the battery (the note BAT1). Why is the fuse unnecessary?

The maximum current from the battery will be less than 1A, so the fuse serves no useful purpose.

Also the pullup is connected right to the battery so the voltage on the gate of the fet should be 4.2V - 3.8V

Since you have 5V available, why are you reluctant to use it so the MOSFET will operate at the lowest possible on resistance?

The information about the threshold voltage was irrelevant. I kept thinking the voltage divider voltage would be applied to the gate. Regardless of that, it's still better to strive for having a lower on resistance.

EDIT: Since you're learning, you should get in the habit of using MOSFET to describe the transistor you're using. There are dozens of types of FETs and whenever I see it unqualified, I think of JFETs because they were the first to be manufactured.

 

The maximum current from the battery will be less than 1A, so the fuse serves no useful purpose.
Since you have 5V available, why are you reluctant to use it so the MOSFET will operate at the lowest possible on resistance?

The information about the threshold voltage was irrelevant. I kept thinking the voltage divider voltage would be applied to the gate. Regardless of that, it's still better to strive for having a lower on resistance.

There isn't any 5V, the batteries are powering the whole circuit itsel

Also the lm339 is open collector and the pull up resistor is pulling the gate to 4,2v -3,8V

 

The maximum current from the battery will be less than 1A...

And we know this how?
The TS wants short circuit protection. I agree that there is essentially zero chance of that if the circuit is assembled properly.

 

And we know this how?

The discharging current is about 0.8A but it's there if something goes wrong

 

The discharging current is about 0.8A but it's there if something goes wrong

Kind of the definition of a fuse.

 

And we know this how?

Ohm's Law. Since the OP hasn't stated nominal battery voltage, I've been using is 4.2V:
\( I = \frac{V}{R} = \frac{4.2V}{5\Omega} = 0.84A\)

The TS wants short circuit protection. I agree that there is essentially zero chance of that if the circuit is assembled properly.

What can short the battery? Presumably he's going to use a battery holder and there is no chance of shorting a battery. If the battery was shorted, how would he guarantee that current would go through the fuse?

You can only go so far to make something idiot proof. It'd be less bother to train the idiots...

 

Kind of the definition of a fuse.

Yeah it is so it's not useless right

 

Yeah it is so it's not useless right

It's useless.

 

Ohm's Law. Since the OP hasn't stated nominal battery voltage, I've been using is 4.2V:
\( I = \frac{V}{R} = \frac{4.2V}{5\Omega} = 0.84A\)
What can short the battery? Presumably he's going to use a battery holder and there is no chance of shorting a battery. If the battery was shorted, how would he guarantee that current would go through the fuse.

You can only go so far to make something idiot proof. It'd be less bother to train the idiots...

Ok I get it why the fuse is not nessesary, thanks for explanation

 

In my opinion too much of your circuit is superfluous. Use Abel 431, a p channel get in the positive line and the load resistors. Nothing else is neede to make a simple discharge circuit that cuts out at any voltage you choose.