Electronic – how to build a low-voltage disconnect switch for DC 12.4V 70A

Designing a power supply is not a trivial task. I've spent more than 10 years in the employ of professional companies which do this sort of work.

Some thoughts:

1. Input-side - that's huge power you'll be drawing from the mains. I'd estimate that you'll need at least 20A service. You'll also need some hefty EMI filtering to avoid polluting the mains with the switching noise from the converter. Don't forget about inrush limiting, surge suppression, fuse sizing and X-capacitor discharge circuitry.

2. PFC - Most designs need some sort of power-factor correction to ensure that sinusoidal current is being drawn from the mains. You'll most likely need a multiphase PFC to handle this sort of current effectively.

3. DC/DC converter: You'll definitely need a soft-switching topology to achieve any reasonable sort of efficiency on the primary side. Look into the zero-voltage-transition full-bridge converter, aka phase-shift full-bridge (Intersil, Texas Instruments and others make controllers for this sort of topology) and look into really rugged MOSFETS (I've used IRFPS40N50Ls for 3kW designs). You'll need really good transformer core material (consider Ferroxcube or Nicera) to keep the losses low. You may want to even consider running two 2kW converters, and sum the currents on the secondary-side with a current sharing circuit.

4. Rectification: There isn't an efficient way to rectify such high voltage. You won't be able to take advantage of synchronous rectifiation (in my estimation) so you'll be burning power in Schottky diodes.

5. Thermal management: You need to ensure that your magnetics and switching devices are all in their safe operating areas, and design heatsinks / install fans to ensure that they all stay out of any potential thermal runaway conditions.

6. Protections: Over voltage, over current, over temperature, short-circuit, line surge, ESD, EFT ... all things that you need to design protection against.

7. Regulatory / Safety: You'll need to ensure that you meet proper creepages and clearences, that your touch current is safe, that all the critical magnetics and power train devices do not result in the power supply becoming unsafe during any single abnormal event, that your thermal management is keeping the parts within their safe operating areas, that you're using regulatory-approved parts for safety-critical functions (X- and Y-caps, MOVs, optoisolators, etc.)

Are you sure you really want to try and design such a thing yourself? There are books out there that explain how to design, but becoming proficient is a life's work in itself.

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.