Bob Blick wrote:
> Vitaliy wrote:
> > Let's say you have a device that you leave permanently plugged in to the 
> > vehicle, and that draws about 100 mA. How do you calculate (or estimate)
> > the  time it would take for the device to drain the battery to the point
> > where it  won't have enough power left to start the engine?
> > 
> > To clarify, I am more interested in determining the number of hours it is 
> > safe to leave the device plugged in, rather than the number of hours it 
> > takes to kill the battery for sure (although both values are useful). :)
> 
> It's all relative, but my rule of thumb is that car batteries are sized
> such that there is a 50% reserve when the battery is new. A medium-sized
> car has a battery that is about 50AH, so you can take 25AH out of it and
> still start the car. So your 100mA load can go 250 hours and still leave
> enough to start the car.
> 
> But after the car battery is two years old in a rough environment, the
> battery capacity has dropped to perhaps 30AH so 50 hours is all you can
> expect to run your device and still leave 25AH to start the car.
> 
> These are just my rules of thumb because cars and batteries are real
> systems and not calculable. The reality is that it's not the AH that
> start the car. But this simplification is as good as any other.

I once was doing some system-level design of a car entertainment system,
part of which needed to be running even when the engine was off. On the
assumption that most cars are started at least once a day, I decided that
I could consume 10% of the nominal battery capacity over the course of 24
hours, after which, the equipment would put itself into a very low power
standby state. This should leave enough reserve for even an older battery
to start the car after sitting for potentially weeks. Therefore, if a car
battery is, say 48 A-H, I could consume 4.8 A-H over 24 hours, or 200 mA
(2.4 W @ 12 V).

I also toyed around with ideas like having the system "learn" the usage
patterns of the car over the course of multiple weeks, and have it power
itself up in time for the next anticipated drive. 

-- Dave Tweed
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