Awhile back Mr. Nossaman asked me about making a battery eliminator for a cordless drill. Now, I always thought that the beauty of a cordless drill was that, well, it doesn't have a cord. Now I've often said that progress was okay for a while, but it's just gone on too long. We'll have to cut Ron some slack. He loves to remove and replace screws all day long but his memory doesn't keep his batteries charged. Hey, I'm at least 20 years his senior and I remembered something he said he'd forgotten the other day. Can't for the life of me remember what it was though. Anyway, most of us have a drill or two that the batteries won't hold a charge. And here's a few facts about batteries etc. Batteries, (as well as other power sources) have an internal or source impedance. This means that the internal impedance is in series with the load impedance. A fully charged battery has a low impedance and as it discharges the impedance rises and the resultant voltage to the load decreases. If you don't already know about ohms law, it is the formula describing the relationship of voltage current and resistance. Current equals the voltage divided by resistance. Power equals the current times the voltage. Etc. I have a cordless driver (panduit). It has two "c" cells. Nickel Chrome rechargeable. Voltage is 1.2 volts per cell. The resistance of the motor is about 2 ohms. I have two Skil pistol grip drivers with three "c" cells. The motor also measure about 2 ohms. They are geared down so they are slow but powerful. I have a Makita 7.2 volt drill driver. It measures 1 ohm but I only have a resolution of one digit. I don't have a very accurate method to measure low resistance. I also have a 9.6 volt Makita. It is a drill driver with adjustable slip clutch , variable speed and a high and low gear ratio. It measures the same resistance. It would appear from my measurements that the current of each motor is similar, but the voltage is higher. Remember that if current is constant, doubling the voltage doubles the power. A motor of this type, called a universal motor, draws max current at start, develops a back counter voltage as it speeds up (reducing the current) and then draws increasing current as the speed is reduced by loading. I scratch the date of purchase on new batteries and just noticed that my 9.6 volt was purchased in 1996. I went out and bought me a new one. About $30. To those that posolutely and absitively insist on tethering themselves to an umbilical cord--Have I got a deal for you!!! Go out and buy yourself a new computer, take the power supply out of the old one (I found 2 on the street during spring clean up week) and one of them worked (the power supply that is). Now a typical computer power supply puts out 5 volts at 25 amps and 12 volts at 9 amps. I could run all five of my drivers plus a couple more on that one power supply and it wouldn't even get hot. At 5 volts that 2.4volt driver really kicks butt. Just don't stall it for very long. The motor will get warm. The several cables that come out of the supply have 5 volts (red and black) and 12 volts (yellow and black). Regards to all Carl Meyer Assoc. PTG Santa Clara, California cmpiano@attbi.com
This PTG archive page provided courtesy of Moy Piano Service, LLC