Sorry, me being novice. But I am willing to read a lot. Really glad there are people like you.
One that handles the voltage and current, and can turn fully ON with the gate drive you have.
By the way, I have not used MOSFET before, so I don't know what to choose. The LiPo battery will power the ArduinoMega (current consumption I cannot compute), 3 IR sensors (about 100mA max for the three), 2 servos (we checked a while ago, the consumption is about 250mA for the two), LCD (150mA including BKL), and vacuum pump (about 500mA max). What FET do you think will handle this?
cant say, as I dont know the purpose of this. to turn off +15V when the dock is empty? what will activate the microswitch or relay? can it be falsely activated? is +15v easily accessible? current limited? why does it matter?
I hope the image below will clarify what I am trying to say before. ^^
sorry it just makes more sense to me drawn this way.
Don't be sorry. You are the one helping me. The fact that you drew that thing is a big help to teach in the first place.
Since your VEXT is so much above your battery voltage, the body diode of the FET might conduct, so I added a diode after the FET, too. But since you have plenty of overhead voltage you no longer really need the FET, just a pair of diodes will be fine.
I don't understand.
This version also shows advice from the various gods, such as the heavy loads need to be wired with their own "home run" to the single point ground. The BRAIN is Arduino plus sensors, no heavy loads. If your other servos are substantial loads they can have their own ground too. This "star" ground ensures the charger and brain dont get confused by IR drop from common ground current.
So you are saying I need to connect the grounds of heavy loads direct to the negative of the battery?
For switching heavy loads use FET and include a few K of gate resistance. That way any "ground bounce" can get dissipated harmlessly. This might cause some ugly switching but you'd rather have it there than mixed in with your low level IO.
Do you think 2k ohms will do? If I am controlling the FET's gate directly via Arduino, then sending a HIGH will switch the FET? 5v 40mA to a 2k resistor will still switch the FET?
As the gods pointed out the Arduino battery monitor voltage will depend on loads. Best to monitor voltage when heavy loads are idle. You can test the battery under load, but you need to know the current and use different criteria for voltage vs charge.
I have not tried using heavy loads controlled by a uC before. If the vacuum pump + servo are running, will the voltage from battery decrease? By how much? Do you recommend other means of detecting charge of battery other than the voltage divider?