So I got the circuit design I will need for one of my more fun projects. I need another circuit to go along with it but the job of this circuit will be to automatically shut off the power outlet in my Saturn when it reaches a chosen voltage. This way I can leave something plugged in all night such as to charge, and have peace of mind the car will start up in the morning.

I will be modifying it some more to fit my needs but the base design is simple. Only requires one zener diode, two relays, two transistors, four resistors, and a variable resistor.

The final design of the complete circuit I use will be available to share should anyone want to make their own. I just might build a few raw circuit boards of it just for fun, maybe give to someone who wants to have it.

 

I will likely install one that will control the outlets and another at a different trigger voltage to warn me of a low battery should anything happen on the road like the alternator failing.

Does anybody know what voltage the battery needs to be to start my cold 94 SL1 SOHC engine? I know about the safe range, but I also know it is not always true because it varies by engine size and temp and everything.

 

I believe the charging light comes on on the dash if the charging system Output V drops below some voltage level. Should start w battery voltage near 12 (don't know limitations -- but what you're really worried about is whether the battery can deliver the required CURRENT to start the car, no?

 

Yes. I want to make sure I know at what point is the battery reaching the bare minimum power left to start a cold engine.

 

a battery can theoretically measure 11.5 V at rest (cold) but if it has a marginal cell that cannot supply its share of the total current, you won't know anything's wrong until you turn the key.

You'll then confirm by watching the V on the battery nose dive when you turn the key to start it that the battery cannot supply the required current at the requested V. So the battery (which measured ok V at rest) is actually history.

So to try to specify what voltage on the battery will still allow you to start the car seems a bit pointless.

I know it'd be nice to be able to make such a generalization, but personally I just don't think that is possible

This dude's opinion, anyway......

 

That is a very good point. So then the circuit cannot be relied upon for a guarantee, nothing can. All it would do is help. Still, I'm going to build the circuit.

I value everyone's opinion, after all these posts are for discussion. There is only one thing I do not want to hear from anyone...