Charging IOT Device(12V) by 36V battery only when it is low

Dear sir,
I have made an iot device consisting of gps,gsm and micro-controller, and this device is working on 12V Lipo battery.
And i will be using this iot device on a vehicle which has 36V li-ion battery, Now i need to charge my Lipo battery with the vehicles battery whenever it is low.
I am thinking of stepping down the voltage but their will be huge power loss and inefficiency,
So, is their any way to charge the 12V battery only when it is low, and disconnect charging when it is charged.
my iot device can work for voltages above 5V -12 V.
Any advice to minimise this huge power loss will be very helpful.

Thank you

You can use a Switching regulator for this and reduce it to 12V as needed. But you need specific Lithium battery charger. Hope you took care of it.

okk thanks for the suggestion

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