"Bruce W...1" wrote:


You guys are getting unnecessarily complex me thinks.

Then you think wrong. Your own reference tells you to use
a regulated voltage for float charging.


Yes a voltage regulator would be nice but not really essential.

No, a voltage regulator is essential for float charging.

And
this might get stolen too. Besides, if I was going to go to that effort
I'd just buy a float charger (about $25).

And if you build a voltage regulator using the recommended
LM317 and one of the wall warts you already own, you'll spend
less than 5 dollars. If you don't have a suitable wall wart,
adding the cost of that will still leave you WAY under 25 bucks.


The car is in a garage in a good neighborhood but the owner of the
garage is on and extended vacation. The theives I'm worried about are
neighborhood children. Actually it's the homeowner's car, I'm just
doing them a favor, and saving myself from having to go there to start
the car once a week. So I'd rather not spend money when I have a
plethora of wall warts.

The car does have an alarm but I don't know how much current it pulls.

And "keep alive" current for the radio and microprocessors
and clock and ?? totals how much?


Case, I've got one wall wart which has a 17V open voltage. When I pull
it down to 12.0 volts the current is 120 mA. Might this work?

Yes - if the battery starts out fully charged, and if the
current drain on the battery is only a few mA, and if you
add an LM317, a resistor and a pot and a diode to make a
protected (by the diode) voltage regulator. See the datasheet
at http://www.national.com/ds/LM/LM117.pdf for a diagram
(minus the diode). Add a diode between the input and
output of the LM317, banded end toward the input.



I could increase the size of the transformer if needed. It's just that
I haven't yet figured out what's needed.

Lead-acid batteries have a rated current at which they can be float
charged without causing any damage.

No - not a current. You want a regulated voltage for float
charging.

This is the number I need, but have
not been able to find it.

Read the article you mentioned (below). It says:
"Next comes the Float Step. This is a regulated voltage
of not more than 13.4 volts and usually less than 1 amp
of current."


Here's a related article:
http://www.4unique.com/battery/battery_tutorial.htm