kcrudup@lotus.com (Kenny Crudup LID-A0794) (07/03/90)
In article <5879@videovax.tv.tek.com> bill@videovax.tv.tek.com (William K. McFadden) writes: >If you really want to use batteries, definitely use the high capacity type. >The best deal I've seen for high cap. nicads >is Radio Shack, believe it or not. Their 4.0 AH D cell is $7, which is a lot >cheaper than any other source I have seen. Don't waste your money on the >consumer type 1.2 AH cells. They're cheaper, but they just don't add up when >you consider capacity (and a boom box needs a lot of that). Due to some aggressive shopping, I now have 30 (yup, thirty :-) D-cell Ni-Cads and a 7-hour charger. I also have a boom box that takes 10 of these suckers and uses (draws? wastes?) 45W (out the AC line; the batts probably draw 80% of that, so let's assume ~2.4 amps). I haven't made any real-time current checks, but they last a couple hours before I'm into another set. I'd like to go with hi-cap Ni-Cads, but the chargers I've seen for them tend to be expensive (to do a set of 10, I'd need 3 4-cell chargers at $50/pop). I think the expense is due to the fact that they charge in about 1hr, and there is some overcharge protection circuitry built in. Can I charge hi-cap Ni-Cads in a regular (this one is a 7- vs. 14- hour charger, though) charger, my only tradeoff being time? Also, I was able to get 30 D-cells at fire sale prices, and only spent $50 for them all. What is the best price seen for hi-caps? Are all makers hi-caps the same? Can I assume that all the ones I have now (GE and Rat Shack) are only 1.2 A/H? -- Kenneth R. Crudup, Lotus Development Corp. Contractor, NASD/QA system V 1 Rogers Street 6381D, Cambridge, MA 02142. (617) 693 4111. kcrudup@roxbury.lotus.com, ...!lotus!roxbury!kcrudup "I'll even use a word that don't mean nothin', like 'loopid'."
bill@videovax.tv.tek.com (William K. McFadden) (07/06/90)
In article <1990Jul3.150651.29985@lotus.com> kcrudup@lotus.UUCP (Kenny Crudup LID-A0794) writes: >Can I charge hi-cap Ni-Cads in a regular (this one is a 7- vs. 14- hour >charger, though) charger, my only tradeoff being time? Also, I was able to >get 30 D-cells at fire sale prices, and only spent $50 for them all. What >is the best price seen for hi-caps? Are all makers hi-caps the same? >Can I assume that all the ones I have now (GE and Rat Shack) are only >1.2 A/H? There really is no limit on how slowly you can charge nicad cells, except for the internal leakage rate. You can charge 4 AH cells on a charger made for 1.2 AH cells. It just takes proportionally longer. Actually, you can make your own charger really cheap. The 15 hour charging rate for nicads is C/10, where C is the capacity in amp-hours. So a 4AH battery would charge at 400 mA. To make a charger that does this, build a DC supply from a 12VAC transformer, rectifier, and filter capacitor. This will produce about 18 VDC. Next, get an LM317 voltage regulator and connect its Vin terminal to the positive output of your 18V supply. Connect a 3 ohm, 1 watt resistor between the Vout and ADJ terminals. Now, hook up enough battery holders in series for the number of cells you have, and connect the positive end to the ADJ pin on the LM317 and the negative end to the negative output of your power supply. You may need a heat sink on the LM317 if it gets too hot. You can get all the parts execpt the 3 ohm resistor at Radio Shack. This charger will supply a constant 400 mA, regardless of the number of cells you have. It takes 15 hours for a complete charge. If you want a different charging current, choose R = 1.2/I (e.g., a consumer type 1.2 AH battery would need a 10 ohm, 1/4 watt resistor-- P = I**2*R). If you want to get fancy, you could put in a rotary switch and several resistors to select the charging current based in the cell size. The common cells are N, 150 mAH; AAA, 180 mAH; AA, 500 mAH; SC & consumer C and D, 1.2 AH; high capacity C , 1.8 AH; high capacity D, 4.0 AH; F (not very common), 7.0 AH. I have used this circuit many times, and it works great. -- Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077 bill@videovax.tv.tek.com, {hplabs,uw-beaver,decvax}!tektronix!videovax!bill Phone: (503) 627-6920 "The biggest difference between developing a missle component and a toy is the 'cost constraint.'" -- John Anderson, Engineer, TI