[Soekris] small battery backup??

[Bernd Walter]

On Tue, Oct 26, 2004 at 07:03:16AM -0400, Vince Vielhaber wrote:
> Or get a local ham operator to build it for you.   Also ask him/her to
> add the necessary decoupling caps.  Definitely needs one on that 7812.
> Between the two, I think I'd go with Poul's design.  It can provide more
> current whereas that 7812 in the second circuit would get pretty hot, if
> it worked at all, trying to power a soekris and a radio card.

PHK's circuit is good for simple design, but you need more for long life
of the battery and maximum capacity.

You need underloading protection, because lead-acid batteries tend to
wear quite fast if underloaded - and you never know when power comes
back.
I do this with a self holding relais that is broken by undervoltage and
triggered by main power.

You need exact load voltage - vendors allow different values in the
range of 13-15V.
If you load with a lower value you don't load the battery 100% full and
waste capacity if you load with with a higher value batteries get too
warm and wear faster.
Usually battery vendors supply two load voltages - one that can be
pemanently applied for slow loading/holding and a higher one for quick
loading for a limited time.
I do this with a standart uC and some glue logic, but I think Maxim has
a few specialised controllers in their programm.

If you want to go with a simple design - you may want to think about
using a standard charger permanently attached.
Every RC modell supplier should have one in his shop and should be able
to do the cabeling for you.

-- 
B.Walter                   BWCT                http://www.bwct.de
bernd at bwct.de                                  info at bwct.de