Roman, Jeff,
You have damn good car batteries there if you consider only
12.8 to 14.5 voltage variations !
Here a battery having 10...11V with load is considered excellent.
And I'm not talking about measurement at -15C
Also a defective charging relay could charge your battery up to 16V
I think ( with batteries from here ) you have no chance with linear
regulators. Just my personal opinion, please don't kill me...

best regards and succes,
 Vasile

On Thu, 10 Jan 2002, Roman Black wrote:

> Byron A Jeff wrote:
> >
> > On Thu, Jan 10, 2002 at 08:44:58AM +1100, Roman Black wrote:
> > > because of the storage capacity of the
> > > battery you can regulate the output voltage up
> > > to the PEAK input voltage. Not the MINIMUM
> > > input voltage like a normal series regulator.
> >
> > Thanks! It was the solution to the starter dropout voltage when I was driving
> > this equipment with a laptop power supply I got surplus from Marlon P Jones.
> > Unfortunately they don't carry it anymore and I succeeded in shorting out
> > the two rather fragile units I had.
> >
> > > You should aim for your battery to be kept at
> > > about 12.8v, and full, so it can be charged
> > > anytime the car is above 13.0v with a simple
> > > series regulator transistor. The only times the
> > > car will be under 13.0v will be when you start
> > > it, or maybe idling with the headlights on. Anytime
> > > the motor is "driving" the car will produce about
> > > 14.2v to 14.5v.
> >
> > Correct. But the problem is getting the 12V line to regulate when only the
> > UPS gel cell is on line. While it'll float at 12.8V to 13V, it quickly heads
> > towards 12V when put under load.
>
> No drama there, low dropout series regs are not that
> hard to do with a decent low-sat PNP device and a
> 7805 as a zener.
>
> You need to determine your max current and typical
> current for the 12v rail, if it's only 2A or so this
> will be an easy task. If it's 4A continuous the task
> gets slightly harder.
>
> > I'm trying to ensure that the drives will not brown out during starting and
> > when the car has short stops in the 5 to 10 minute range.
>
> That should not be a problem, most modern floppies
> i've seen either don't use the 12v anymore, (and I
> strip 5v motors from them) or the 12v motors are
> 150mA or so. Even a floppy starting up will not draw
> more than 0.5A from your 12v supply. Pull the cover
> off your PC and stick an amp meter on the 12v rail
> and check it out. Design from the back forward. :o)
>
> > The nominal system input voltage will be 13.8V. It should provide enough
> > headroom for normal use.
>
> Not so. 13.8 is an "on charge" voltage. The moment
> the car stops, your 13.8 sinks quickly back to a
> typical battery "full" voltage of about 12.8v.
>
> > It's the 12V minimum input voltage that I'm trying to attack.
>
> I don't think it's a problem. Assuming 12.8v in your
> gell battery when the car stops, and 2A used at 12v,
> you need a low dropout regulator good for 2A at 0.3v
> drop, and you have a headroom of 0.8v-0.3v=0.5v.
> So your 12.8v can drop to 12.3v before the 12v out
> is affected. I would maybe choose a regulated 11.6v
> rail, as this gives you another 0.4v headroom, and
> a large enough battery to supply you for 10 minutes
> at 2A and not drop 0.9v.
>
> Remember this will be the 12v battery only, if you
> need a 5v supply to run the PC at some 20A or more,
> that will involve a whole 'nother battery and PSU...
> -Roman
>
> > Thanks for the encouragement.
> Anytime! :o)
>
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