> The supply will have a 12V 7Ah gel cell to act as a mini UPS. So the
> input voltage will be from 12 to 14 volts.
>
> I want to get a regulated supply of 12V +- 5% @ 4A.


Be aware that a 12v gel cell "floated" off the main battery by a diode will
be undercharged when the main battery is charged. The extra 0.6v or so can
make quite a difference to cell life. This comment is based on practical
experience with 100m + car based systems using this arrangement and a
standard silicon diode.  If this is one off or small volume
then use of a low voltage drop diode can be useful eg Schottky diode. If you
don't mind having to replace the gel cell every year or 2 this may not be
too much of a problem.

A suitably low Rdson FET driven hard on is also very good BUT as the next
few sentences show, is surprisingly hard to use properly here. . Getting FET
drive voltage for an N Channel FET can be a pain as it has to be positive
ABOVE battery positive. Note that if a FET is used the gel cell will try and
drive the car via the FET body diode when the car battery is low under eg
cranking conditions.  A solution is to use the FET "backwards" so that the
body diode conducts to charge the gel cell and the on FET also conducts in
the same direction. You STILL have to bias an N channel FET with the gate
positive above battery even though you have reversed Drain & Source so
biasing is just as annoying. A P channel FET will solve this but their Rdson
is usually higher for otherwise similar specs. Use a Schottky diode :-).

A small boost converter sitting on the battery rail will solve these
problems. The 12v is boosted by about 2 volts or less to allow for diode
drop and low battery voltage at times. Wattage required is low as it only
supplies about 2 volts to add to the 12 volts - not the whole 13 volts or so
that you need.



        Russell McMahon


.

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