Rds

See...

But I don't think RDS(ON) is going to be a very reliable value.

...Jim Thompson

Rds is a function of a variety of things including process variations, current and temperature. There is not a single temperature coefficient on a FET but many which could result in significant variations. I can't see why you'd want to use one over a resistor thats explicitly designed to do a good job at this application.

I'm also not clear how you would use a MOSFET as a "blocking diode". If you are using a diode connected MOSFET with the gate and drain tied together you are still taking a Vgs drop in the FET which may be more than a Schottky diode drop.

My project has the same requirement, monitoring the battery charger current and isolating it from the charger to verify that the battery is still present. I did use a mosfet to isolate the battery from the charger and I chose one with a small Rds to limit power disipation as the charger current can be as high as 10A.

To monitor the current, I used a four terminal resistor of .005R. The one I used costs a couple of dollars as I chose a high precision device. The net result is that I can read the current with my 12bit ADC (0 to 10A) both positive and negative direction with better than .1Amp accuracy.

I agree that Rds is too variable of a parameter if you wish a measurement with accuracy. If you are looking for a crude measuremetn of wheter the charger is charging, it may be sufficient for your needs.

I would also recommend a snubber circuit across the mosfet!.

If you are interested in implementation details, contact me at the email address listed in the post header and we can discuss.

I'd use a p-channel mosfet with the source connected to the positive terminal of the battery. With the mosfet turned off, it would act like a diode, blocking the battery from discharging. Connecting the gate to ground (battery negative) during charging would turn the mosfet fully on and reduce the power dissipated in the mosfet, assuming I(charge) x Rds is less than Vf of the body diode The mosfet would be conducting in the opposite direction of usual.