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.
Best Answer
Some more info on the charger would be useful (any specs written on the casing, in the manual, etc)
Hopefully it's a switching charger.
Anyway, some rough calculations:
Battery Wh = 1.1Ah * 3.7V = ~4Wh Typical car battery Wh = 50Ah * 12V = 600Wh
You say the device operates for roughly 6 hours on a full charge, so the average current and power consumption is:
1.1Ah / 6 = 183mA 4Wh / 6 = 678mW
If we assume the battery is in good condition and can supply, say 300Wh (half of it's capacity) and still start (I'm no expert on car batteries so this is a bit if a guess, but I'm pretty sure less than 50% charge is a bad idea) then the operating time is:
300Wh / 0.678W = 442 hours or 18 days.
Now the above does not include the charger inefficiency and battery drain caused by other electronics in the car, so it seems feasible that you could halve this figure quite easily.
So although making sure the charger is doing a good job is certainly a good idea, I think it is likely that just shutting off the charger periodically may not be the answer, and you need to look at lowering the devices consumption (e.g. transmit data less frequently) and/or installing a larger capacity battery. Also, if the car is not in a garage, one of the readily available dashboard solar panels would help to keep the battery topped up.
Simple Timer Circuit
Since there has to be many such circuits already out there, rather than draw it from scratch I had a quick look around to find the kind of thing I had in mind that didn't involve a microcontroller (sorry it's a bit late, some urgent stuff came up)
Anyway, this circuit at http://www.electronics-project-design.com/electronictimerswitch.html seems to almost fit the requirements quite nicely, and is based on easily swappable and cheap components. It can be upgraded if necessary (e.g. the oscillator could be crystal based for better accuracy)
The IC is a 14-bit binary counter, and is clocked by the RC oscillstor made from pins 9, 10 and 11. The values of the RC components set the clock period (more info in the C4060B datasheet)
To set the on/off period, you need to AND the correct pins together for your desired ratio. Say you set the timing so the the count reaches 2^14 in 60 minutes, this means the clock is 2^14 / 3600 = ~4.55Hz.
Now you want the timer on for 7.5 minutes of this hour, so you need it on for 2^14 / 8 = 2048 counts. So to calculate the bits we need to AND together:
2^14 - (2^14/8) = 14336 to binary equals:
11100000000000
So bits 13, 12 and 11 need to be ANDed together (the same as the schematic above just without D1)
This is just a rough example, other timings can be achieved by calculating accordingly and you could use proper AND/OR gates, or a comparator IC or cascade another counter IC or... if you want to improve the design. Hope this helps to get you started.