Automatic SLA Battery Charger
This project is my build of the Sealed Lead Acid Battery Charger featured in the May 2001 issue of QST by Bob Lewis, AA4PB. Unlike simpler chargers, AA4PB's design does not maintain a fixed-voltage, trickle current float on the batteries. It applies charging cycles only when the voltage is below a level set by a voltage comparator. LEDs indicate when the charging cycle is on (green) or off (red). Even in a fully discharged state, the battery is brought up to full voltage by cycling the charge current. It works very well, and it's a nice project. I keep my stock of 7 AH gel cells charged with it.
FAR circuits has a board for this project, but, as usual, I did my own board using EAGLE CAD and toner transfer. You may notice on the last photo, the relatively close up picture of the PC board, that the two main filter capacitors aren't exactly vertical. I should have used a larger footprint in EAGLE for those, a fairly common mistake for me back when I started using it back in 2002. This circuit was originally one of my first PC board schematic capture projects. When I decided to build it a few months later, I had learned vastly more about EAGLE, and the original board layout and size were not very good. I designed a new board layout, but I forgot to go back and check the component sizes against the catalog dimensions of the caps. The result is the caps putting the squeeze on a couple of vertically-mounted resistors.
The transformer I chose is a Stancor SW-616, which I bought from Hosfelt Electronics. The price was just under 10 dollars, and it's slightly over-specified for the project, with ratings of 16 Volts at 800 mAmps. You could probably adapt a surplus unregulated wall-wart, typically a couple of bucks at the local GoodWill or other similar store. Some of the other parts may not be in your junk box, but they are obtainable...I typically buy from Mouser and Digi-Key. Make sure you get a 2-lead bicolor LED, not a 3-lead one. RadioShack carries both, if I recall correctly. There is actually a circuit to convert a 3-lead to a 2-lead function, but it involves a couple of extra diodes, and isn't really worth the trouble.
I used the case from an old ATX computer power supply, and it works out well, with plenty of room inside. I modified some of the existing case openings using a RadioShack nibbling tool, which cuts the steel of the case "like buttah". I made use of a few parts, such as the power switch, although I can't recall if it came from this particular power supply enclosure. It may have come from a PC case. I have used these old power supply enclosures for several projects.
I didn't really deviate from Bob's instructions, and, consequently, it works just as he describes. It's kind of mesmerizing to watch as it goes through it's sequence of checking the voltage and charging. As Bob mentions in his article, he designed it to charge a battery while in use with an Elecraft transceiver. I use several SLA batteries, and I have been rotating them in service on my workbench and field ops. Below are several photos of the completed charger. Click on the images to enlarge them.
