|A Simple Float Charger
for Gel Cell Batteries
Craig LaBarge, WB3GCK
Sometimes, I just get the urge to build something. When the urge hit again recently one weekend, I scrounged through my junk box and cobbled together a very simple but effective float charger. I wanted a small, light, and reliable charger to charge my gel cell batteries when I take my QRP gear on the road.
The heart of this charger is a simple variable-voltage regulator circuit based on the LM317 regulator. No rocket science here. The combination of R3a and R3b in the adjustment circuit was selected to give me the adjustment range I was looking for while making use of available parts.
What's a little different here, though, is the portion of the circuit that indicates when the battery terminal voltage is within the proper range for float charging a gel cell. The LED begins to light when the output voltage reaches 13.5 volts. D2 is a 2-lead, bi-color LED. This device changes colors depending on the polarity of the voltage applied to it. It turns out that the device has a "dead zone" of over 3 volts; that is, for voltages between 0 and -1.8 volts and 0 and +1.8 volts (approximately) the LED will not be lit. This circuit takes advantage of that fact and uses the LED's threshold as a crude voltage indicator. U2 is a 5-volt regulator which provides a reference voltage. R6 is used to set the voltage at which D2 will begin to light (with the green color).
Construction is not critical. I built mine Manhattan style on a small piece of copper clad PC board material. The tab of U1 needs to be isolated from ground, so I used an exacto knife to cut a groove and create a small, isolated section on the board. For D2, make sure the flat side of the LED is connected to R6. This will ensure that the LED lights green when in use. I have found the green color to be more visible than the red, in this application.
Up to now, I haven't mentioned a power source. I have two "wall wart" transformers. One is rated at 14VDC at 350ma and the other is rated at 14VDC at 700ma. Both work equally well as a power source for this charger. The 700ma wall wart was salvaged from a cordless telephone that died and the 350ma wall wart was purchased from All Electronics. Both transformers have open-circuit voltages in the neighborhood of 18 or 19 volts. I would recommend you use one with a current rating of less than one amp due to the current handling limitations of the LM317T voltage regulator.
Once the charger has been constructed and a suitable wall wart transformer has been connected to the input, it's time to adjust it. The following adjustments are made with no load attached to the output of the charger:
To use the charger, connect your wall wart to the input and plug it in. The LED should be lit. Next, attached the output to the gel cell battery to be charged. If the battery needs a fair amount of charging, the LED will go out. As the battery terminal voltage starts to reach the float charging range, the LED will begin to light. The LED should increase somewhat in brilliance as the battery terminal voltage comes up to the float voltage setting (13.7 volts). Don't expect the LED to light very brightly; it will only light up dimly, at best. At this point, you can leave the battery on the charger indefinitely. The battery will only draw the amount of current it needs and no more.
C1: 0.1 ufd, 50V ceramic disk capacitor
D1: 1N5391 (Mouser #583-1N5391) Any silicon diode with a current rating of 1.5 amp or better and a PRV rating of 50 volts or more should be fine.
D2: 2-lead, bi-color LED (red/green) (Mouser #604-L57EGW)
R1, R4: 270-ohm, 1/4-watt
R2: 2.2K-ohm, 1/4-watt
R3a, R3b: 680-ohm 1/4 watt resistor in parallel with a 4.7K-ohm trimmer resistor (Radio Shack #271-281). Use a 500-ohm trimmer, if you have one.
R5, R7, R8: 180-ohm, 1/2-watt
R6: 1K-ohm trimmer resistor (Radio Shack #271-280)
U1: LM317T adjustable voltage regulator (Radio Shack #276-1778)
U2: 78L05, 5-volt regulator
Disclaimer: Always follow the battery manufacturer's guidelines for charging and handling.