I am looking for some assistance with options in isolating 2 parts of a circuit.
I have a 4G hotspot that I am using in the car, powered via the cigarette lighter socket (rated at 12V/10A) and a 5V USB regulator. The hotspot is intended to be always available even when the car is off. Due to the fact that the car is off for far longer than it is running, the device discharges it's internal battery in a few days as it is not charging long enough.
To this end I have a circuit that will charge several (in my case 3) 12V 2.2Ah Sealed Lead Acid batteries in parallel and operate the hotspot device while the vehicle is operational. The batteries alone will power the device while the vehicle is not operating (ie the Ignition switch is off).
However, I am stumped on how to isolate the rest of the car's accessories within the accessory circuit (as isolated by the ignition switch) from the charge circuit when the ignition is off. Once activated, the charge circuit feeds power back into the vehicle accessory circuit.
One version I have has a relay to enable the circuit when the ignition is on, but the power feeding back from the batteries through the closed relay energises the activation circuit and keeps the relay closed when there is no power from the ignition. The dashed line is roughly the boundary between the part of the circuit that needs to be permanently powered and the part that should only powered when the ignition is on.
I have another that replaces the relay with parallel diodes in the power path, but the ~1V voltage drop and heat dissipation from regular silicon diodes is undesirable, and I tried several Schottky Diodes in parallel (because they have a smaller/acceptable voltage drop of ~0.13V each (measured)). However the Schottky diodes leave ~1.8V on the 'dead' side circuit that could put extra drain on the SLAs when the ignition is off. The LED on the plug interfacing with the 12V car socket indicates power is there when the ignition is off and I measured the voltage with a multi-meter across the terminals of the plug with it disconnected from the car.
Is there a way that I can have a low or no voltage drop, low impedance circuit, fairly high current (but < 10A) that will sense when the ignition is off and isolate the external batteries and charge circuit, without modifying the car's built-in and under warranty wiring?
I have seen mentioned elsewhere use of a MOSFET(s) and a sensing circuit to control it to replace diodes to make an 'ideal diode' but the references are mostly in the context of rectification of AC current not this circumstance and I haven't worked out how to apply the theory to this situation.
The hardest part in anything I have dreamt up is the fact that once the circuit is first powered from the ignition, you cannot tell when the ignition has been switched off as the batteries in the charge circuit maintain the power on both sides.
Any pointers would be appreciated.
EDIT [2018-05-30 - 22:51 AEST [UTC+10]:
Would something like this work to have the battery circuit connect to the car supply if the car supply voltage is over 13V (ie the car is running and alternator is operating at 14.2V). As the SLAs are only about 12.5V fully charged then no current would flow thru the zener diode and the transistor would be off. If the voltage was above 13V, current would pass thru the zener, limited by the resistor and switch the transistor on to activate the relay. My zener theory is rusty and transistor theory even rustier, but would something like this work? Calculating the resistance values would be the next challenge (for me at least...).