One of the things you quickly learn when working in a large datacenter – Label The Cables!! Unfortunately, roughly once a year we had to buy a new rechargeable battery for the label printer. The machine would barely work with a dead battery. When the original wall wart burnt out, we replaced it with a cheap unit from Radio Shack, which worked fine if the battery was charged. With a dead battery, many of the labels would fail to print properly.
One day, I got fed up with the machine and ripped the battery module apart to identify the problem. Rechargeable should not die so fast… Surprise, surprise, the batteries were directly in parallel with the charger and the load, with no battery charging circuitry. If the printer is not running, the wall wart just keeps charging the batteries – and killing them. An evening of tinkering, and I was able to build a workable solution.
My primary goal was to get the label printer back in working order as soon as possible and looking much the same as it was before I touched it. Since I did not have on hand what is needed for a decent charging circuit, nor rechargeable batteries that would fit into the battery module, I opted for using the largest capacitors that would fit. The wall wart did not have enough power to drive the label printer pins by itself. With the help of the large capacitor bank, a simple resistor to limit capacitor charge rate and a small diode for full speed discharge, the machine can print perfect labels all day long.
All of this fit nicely on a simple protoboard and with a little bit of hotglue fit perfectly in the vacated battery box.
The end result speaks for itself. We have been using this ‘upgraded’ label printer for over a year now without any issues. We can’t walk around the datacenter printing labels indiscriminately, but we didn’t do that before either.