SLA Battery Charger

I recently came into possession of a used 12V SLA (sealed lead acid) battery.  This would finally allow me to go portable with my Rockmite 40M QRP rig.  I knew it would eventually need re-charging so I started looking for a charger circuit w/o all the bells and whistles.  I found one pictured below and documented at this website (555 battery charger).

The LM317 is configured as a constant current source providing ~500mA.  The 555 timer (with its two internal comparators) along with two external trim pots allows for the setting of two trip points: when a battery is hooked up and its voltage is 11.5V or lower the charger does its thing until 14.2V is reached, when it subsequently shuts off. Very clean and elegant design.  The addition of a couple of status LEDs and a reset switch to force a charging operation completed the project (see below).

Charger encased in plastic enclosure

Some of the component values were changed but the basic design was followed.  I did have to modify the 2N3904 biasing circuit by lowering the voltage at the base using a voltage divider.  Without this mod the 555 output never went high enough to turn off the transistor.  As shown below I decided to go with point-to-point wiring.

On a side note, I did learn something interesting about the 7805 voltage regulator.  A capacitor (.1uF) hooked to the output is absolutely needed even though the spec sheet says its "not needed for stability".  I found out otherwise.  Without it the chip was unable to maintain a constant 5V when the input voltage changed.