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>Posted on 28 Feb 1995 at 11:21:42 by DYER, BILL >Does anyone use NiCds in torches designed for Alkaline/disposable >batteries? Do you have bulb life problems? >I have adapted NiCds to fit my Q60s, by soldering a coin to the end >on one cell to connect with the spring in the end of the torch. >(because NiCds are insulated at the +ve end, up to the terminal). I know of many who use NiCds in their lights. In low powered lights it shouldn't affect the bulb life much. I would be careful soldering on the cells. If the rubber seal melts your cell is pretty much shot. Isn't it easier to remove a bit of the plastic "wrapping" ? (Incidentally I have no recollection of ever having had to do this). >I have been told that NiCd current is higher due to low internal resistence >so the actual Wattage of light is higher, and the bulbs' life is shorter. >(about 20 hours for Q60s). This is half right and half wrong. NiCds have a voltage of 1.25 Volts and for a good quality D cell an internal resistance of 0.03 ohms. A high quality alkaline has 1.53 Volts and 0.3 ohms resistance. So in low powered lights it is the alkaline which puts the most voltage on the bulb. But try to draw 2 Amps out of it and you'll find only 0.9 volts over it's terminals. With currents above 1 Amps you clearly loose a significant amount of voltage. The energy I x R_internal shows up as heat in the cell under question and is nothing but energy wasted from a scarce and limited supply. Due to the low internal resistance in the NiCd, they pretty much display negligible internal loss irrespective of the discharge current. I.e. we hardly loose any amount of energy as internal heat. Furher, due to their ability to maintain their voltage as they discharge, they keep the voltage on the bulb fairly constant. This is a huge advantage in that it ensures proper operation of halogen bulbs if the torch is so equipped. In general, by running bulbs at full steam we maximize the efficiency of the bulb. If the bulb dims it will start to output a disproportionate amount of energy in the infra-red which for a diver again is energy wasted. The only "compatible bulb" is a bulb which is rated for the voltage present at it's terminals. Your Q60 is specified at 7.8 Watts which means about 1.1 Amps at 7.2 Volts (assuming alkalines). With 6 NiCads you get more like 7.3 Volts on the bulb which seems just fine to me. Your bulb has a hot resistance of 7 ohms and a cold resistance then of about 0.6 ohms. At turn-on Ohms law gives: Alkalines: 6 x 1.5 Volts = I_inrush X (0.6 + 6 x 0.3); I_inrush= 3.75 Amps NiCads: 6 x 1.25 Volts = I_inrush X (0.6 + 6 x 0.03); I_inrush= 9.8 Amps
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