Vitaliy wrote:
> Dave Tweed wrote:
> > 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).
> 
> Was the time hard-coded, or were you measuring the voltage to determine the 
> state of charge?

We never actually built the system, but the idea was that it would simply
be hard-coded.

The actual requirement was that under most circumstances, the equipment
wanted to be in its "active" state for at least one hour *before* the next
drive. This requirement was to be met by simply leaving it active for up to
24 hours after the previous drive.

> So what would happen in the standby state? And how did you exit from it?

Pretty much nothing. The system would become active again the next time
the ignition was turned on, as indicated by the "switched" power lead from
the ignition key. Car entertainment systems generally have access to both
switched and unswitched power.

> > 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.
> 
> Hmm, I like the idea, but I think the implementation can get messy.

Yeah, we never really pursued this.

-- Dave Tweed
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