I've found several answers on switching between an Adapter and a Battery. But I need to autmoatically switch between two batteries. One of them is 'hotplugable'.
Problem: I've got a Solarcell which charges a ~16000mAh Li-Ion battery (BATa). Once in a while, the Solarcell doesn't manage to charge the battery enough. It's crucial that I have 3.0 to 4.1 Volts on my VSYS but the voltage will drop below this value if the Solarcell wasn't able to do its job (cloudy weather, vulcano eruption [joke], dirty cells, etc).
My Plan: I've come up with the idea to feed the VSYS with a additional, hotplugable backup battery (BATb) which does get connected to VSYS if BATa drops below ~3.1v.
So basically I need a Low-Voltage-Cutoff (using a Z-Diode with a Mosfet?) which, when cutoff happens, opens another Mosfet..?
simulate this circuit – Schematic created using CircuitLab
I'm not sure about the selection of M2 and how to wire it, so it will be conductive if M1 won't forward power anymore.
And I know about another electrical problem: Just using a Z-Diode (D1) with M1 will render M1 to something like a resistor, not a switch. I've read, that I could use something like a Shut Reference (i.e. TI's TL43xx). Again I'm not sure how to chose the correct one and how to wire it.
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