On Tue, 16 Mar 2010 12:31:21 +1300, "ivp" said:

> I figure I could make a constant current supply from an LM317
> (or is that just current limited ?) and a constant voltage supply from
> a 7805 or 7812 baseline
> 
> Following the diagram, for a 12V SLA, a current is applied to the
> battery until the terminals measure 14.4V (6 x 2.4V), at which point
> the constant current is replaced with a 14.4V constant voltage

That method of charging is OK, will not give you the longest service
life. It will dry out batteries.

If you put some hysteresis in there so it stops when it hits 14.x and
then resumes charging again at 13.x you will get twice as much life out
of the battery.

It's pretty easy to achieve, you can use an LM317, three transistors, a
rectifier-type diode and a few resistors.

Basically put the big diode in series with the input of the LM317 with a
low value resistor shunting it. This detects current. Put a transistor
across it too, so you get a current is on/ current is off signal.
Program the LM317 with an extra resistor so you can get two possible
voltages, and wire up a couple of transistors and resistors so your
current on/current off signal switches the voltage high when there
current is on and voltage low when current is off. A small capacitor can
be useful to keep it from getting into weird oscillation.

You should put an LED or two in there too for a light show.

I can sketch you something when I have time, but you probably see what
I'm describing.

Cheerful regards,

Bob 

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