Our car sharing club is working to implement a smart system in our vehicles which will allow access via RFID-cards, and which will record milage for the booking and billing system.
We are using a tracker unit (http://flextrack.dk/en/products/tracking-devices/lommy-pro-ii.html which has a relatively low power consumption, but unfortunately not low enough.
We have found that if a car is left unused for 4-6 days the car battery is drained such that we cannot start the engine. This is not acceptable.
The tracker is in contact with our server over GSM/GPRS and we cannot switch it off since it must be available to receive information about new reservations and to act on RFID signals from users.
The tracker has a an internal 1.1Ah 3.7V Lithium-ion battery which will keep the unit running for around 5-6 hrs, so I suspect that our main problem has to do with the charging circuit being "greedy" and not energy saving.
Without trying to hack into the tracker, my plan is to put a timer switch on the external power supply from the car battery to the tracker. I would like a simple circuit which, when the ignition is turned off, would connect/disconnect power for e.g. 10min/60min.
I will do some tests to establish the actual on/off durations required to keep the tracker alive. My expectation is that this will reduce the overhead consumption of the tracker charging circuit, and ensure that it feed most of the power from the car battery into the internal battery in the tracker.
If possible, I would prefer to use simple discrete components tuned to the on/off periods instead of a microcontroller.