Alexandre Domingos F. Souza wrote:
>
> >A typical cheap circuit is a LM317 set to about 15 volts and a series
> >resistor going to the battery.  When the terminal voltage gets to
> >14.8volts, the '317 is switched to 13.8 and the battery then is in float
> >mode. If you don't need float mode, just take the battery off the charger.
>
>         Only this? But how the 317 is switched? No zener or comparator to change the output value?


Hi Alexandre, I have a few home made chargers for the
12v gell cells we use on tools in the workshop and for
charging customer's batteries.

Allow for the 12v gel battery to be fully charged at 14.2v.
If it's a 4Ah battery, you want to trickle it at about
10%, ie 400mA.

So imagine this simple circuit, (you can use a 317 or
even a 7815 regulator):

15.2v (regulated voltage)
|
2.5 ohm resistor (400mA @ 1v)
|
14.2v (charged battery) + terminal


So when the battery is charged it will safely trickle
charge and it can be left overnight. (10% current is
safe to leave charging).
Also, when the battery is discharged the circuit looks
like this:

15.2v (regulated voltage)
|
2.5 ohm resistor (3.28A @ 8.2v
|
7v (very empty battery)

So it will charge the battery quickly. In reality
my supplies are regulated at 1A max (using 7815)
so the empty battery is charged at 1A max.
I use 10W resistor for the series resistor, with:

(always 15v regulated 1A supply)
2.7 ohm (for 4Ah and 7Ah batteries)
5 ohm (for 2Ah batteries)

These chargers have been use for years and are
simple, cheap and great for hands free and overnight
use. And good for my dumb apprentices to use. ;o)
-Roman

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