It's a silly thought but you might consider the method used to charge
caravan batteries. Using the ignition warning light to drive the coil of a
contactor (big relay). The signal is normally low when the alternator is
stopped and goes high when its running. When you turn off the ignition the
contactor drops out because the feed is taken from the switched +ve (it
needs two resistors and a transistor (tip121 or similar) 

Simple but effective. I use this method to charge a NP100-12 100Ah sealed
block for helicopter starting battery its been in the van for 3 years infact
it came from the old van so its about 5 years old now.   

Regards

Steve


-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Micro Brix
Sent: 20 January 2007 18:50
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Help with boosting powersupply.

>
>
> After the car shuts off, it won't stay at 14-ish volts for very long, at
> most a few seconds, and that won't hurt anything.


True, and if I used a voltage comparator I wouldn't have to worry about the
pull-in / drop out differential being too large, which it always is if the
contacts can pass any real current.

But I was also concerned about the charge side. These batteries aren't
designed for use in a car, and I want to compensate the charge system for
the temperatures involved.. -30 to 150F isn't an unreasonable span in this
environment.
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