This project is designed to help alleviate the issue faced by many amateur radio operators around the world when the power goes out. How do we maintain communications during such times, which are quite often the times we need it most? Handheld radios are nice, but they suffer from very limited range, and even more limited battery life. The most distance a ham with an HT can expect to see is about 5 miles (8km) unless the terrain is unusually flat. If there are repeaters nearby, this can be sufficient. Maybe. Then there's the issue of battery life. A typical handheld radio will last several hours if it's only being used to receive. However, a user can only expect to get about 20 minutes out of it if they're actually transmitting with it.
The other option many hams who have them consider the mobile station in their car as their back-up station. This can work, but is less than ideal for a number of reasons, including but not limited to: extreme temperatures, no bathroom facilities, uncomfortable operating position, no writing space, limited radio gear, etc.
My solution is to simply add a battery charging and maintaining system in series with the existing power system in the amateur radio operator's station. Many hams use Anderson Powerpole connectors, so I have chosen to use them in this project. The operator can easily substitute the connector of his or her choice in order to fit their individual need. The in-line charger (I need to think of a name for this thing) can be inserted between the existing power supply unit and the radio or radios. Here's a diagram for those who are interested:
Installation is simple, and now the ham has a reliable means of powering his or her communications gear. The charger itself consists of a pair of boost converters with a PB137 chip in between. The first boost converter provides 16v for the PB137, the second provides 13.8v for the radios until the battery voltage drops below approximately 10v. I may rearrange things, but that is the current layout.
Update: The boost converters are no longer part of this project, as they do not produce DC clean enough for my high standards. Also, the idea of boosting 13.8v to 17v so I could get 13.7v makes no sense.
On to the build notes:
A good friend of mine recently gave me 2 pairs of sealed lead-acid batteries and a charger. My initial intention was to just throw a Powerpole connector onto the battery cables and plug them into my Rigrunner power strip when needed, and leave them on the charger the rest of the time. Then I started to look over the charger and my wheels started turning. The charger was nothing more than an LM317 based variable power supply set to 13.7 volts. While this is a perfectly acceptable method of charging these batteries according to the research I've done, I wanted a way to integrate the charging system into my existing 12v power supply (which actually puts out 13.8v).
I then stumbled upon a reference to the PB137 chip buried at the bottom of an archived forum post. That post also mentioned that the chips were over $7 at Digikey. Yikes! Worth the price, but so much for having a few spares on hand... Then I actually found them on Digikey and realized just how old that forum post was. They are currently only $1.27! I guess I can afford to get a few spares after all! I even ordered a few for my friend that gave me the batteries.
After studying the datasheet for a while, I realized what a gem these little chips are. They will properly charge and float 12 volt SLA batteries, have built-in current limiting, prevent back-charging, and are pin-compatible with 78xx series linear regulators! Well that just saved me from having to design a much more complex solution, but I'm going to try doing that anyway because I'm weird that way. (I later discovered that the "built-in" current limiting does not actually function well enough to be relied on.)